How To Select The Best Power and Propeller Settings For Your Engine


In the last chapter I wrote about how important it is to run our engines regularly at higher power settings. But what exactly does that mean? How can we select the right RPM setting to make sure that our engines are not underloaded and thereby wrecked by glazing?

And any time we talk about the best power settings for our engine, we need to think about the propeller too.

The good news is, that if we get all this right, we can save a lot of fuel, emit less carbon, as well as save our engines from early demise.

Read on for how to do that.

First off, it would be nice and easy if we could just run our engines at say 75% of their wide open throttle setting (WOT) and know that that equated to 75% of maximum power output. But sadly it’s just not that simple.

Let’s look at the power graph (above) for the Perkins M92B that we have in our own Morgan’s Cloud to see why.

This engine has a rated WOT of 2400 RPM, so 75% of WOT would be 1800 RPM. And at 1800 RPM the net output curve shows about 56 kw (75 hp) and, since the engine is rated at 63 kw (85 hp), that would seem good, since 56 is 88% of WOT.

And, if we had a controllable pitch propeller (CPP), all would be good. In fact, if we had a CPP the whole underloading problem goes away at all power settings, because by adjusting the propeller pitch the operator can match the load from the propeller to the output of the engine.

However, with normal propellers, including folding and feathering ones, that pesky propeller matching problem, that Matt explained so well in earlier chapters, screws things up…big time.

To understand why, we only need to look at the propeller curve to see that the propeller is only loading the engine to 28 kw (37 hp) at 1800 RPM. That’s just 50% of the available power at that RPM and just 44% of WOT power.

And here’s the thing, even though we are not loading the engine heavily enough to really clean it up, it is still driving the boat at 7.6 knots, the top end of her efficient speed-to-length ratio.

Damn, this is just not working out well, since what we must do to achieve say 65% engine load—being conservative here—to clean up glazing is…let’s see: .65 x 63 kw=41 kw…look at the propeller curve…answer: 2050 RPM.

But at that RPM our fuel consumption has gone up to 10 litres (2.65 US gallons) an hour, and we are still only doing about 8.2 knots due to the approach of hull speed and the big drop off of efficiency when flat blade (folding and feathering props) reach too high a tip speed—so we’re burning lots more fuel and not going much faster…not good.

And this after we reduced the size of our engine in the last repower from the 120 hp engine that came with the boat!

And guess what? We did in fact glaze up our new engine in the first year after installing it, exacerbated by our 2011 Arctic voyage when we often ran at just 1400 RPM—a load of just 13 kw (17 hp) or 20% of WOT output—for long periods to extend range or move slowly in ice. And then we had a hell of a time and burnt a pile of fuel getting rid of that glaze by running for long periods at 2200 RPM as recommended by a very smart Perkins distributor.

So, the next time we hauled, we set the prop pitch a bit more coarse, which dropped our WOT to 2325 RPM (turns out we were just a tad under-propped before). And, while this is not perfect—nothing is without a CPP—and still requires us to run a lot of the time at 1800 RPM or more to avoid glazing, it did result in a substantial drop in our fuel use as well as a good jump in speed for the same RPMsame thing said a different way.

In fact, we could probably go one more click coarser on the prop, which would take WOT down to about 2200 but, if we did, we would need to be careful not to overload the engine (even more damaging than underloading) when punching into big breeze and sea, so I think we will stand pat.

So, to summarize, by:

  • reducing the engine hp when we re-powered,
  • selecting a slow-revving commercial continuous engine,
  • carefully selecting the right reduction gear ratio so that we could swing a relatively large-diameter prop,
  • taking the time to really understand the relationship between available output power from the engine and actual load from the prop;

we now have an installation that’s about as efficient as it’s going to get without resorting to advanced powertrain options like a CPP.

And we are treating our engine kindly by assuring that it’s properly loaded, which should contribute to a long and trouble-free life.

What About Your Boat?

This underloading problem may be way worse on your boat than on Morgan’s Cloud because, chances are, she is much more over-powered for her size and weight than our boat is now, and things may be even worse if your engine is rated recreational light duty as most yacht engines are (see the last chapter).

The first thing to do is to get some understanding of how bad the problem is, and the best way to do that is to get hold of your engine and propeller performance curves and start experimenting and analyzing as we did. But failing that, here are a couple of down and dirty tests, thanks to an interesting email conversation I had with engine guru Steve D’Antonio.

Check Oil Temperature

First test: Run your engine up to your normal cruising speed and use one of those infrared handheld temperature guns to check the oil temperature by targeting the middle of the oil pan (make sure you hit below the oil level).

The result should be at or above about 180F (82C). If it’s less than that, your engine is not properly loaded, and if it’s way less than that, you are heading for a big problem and a big repair bill, according to Steve.

One note, don’t be tempted to use your water temperature gauge. Since cooling water is in contact with the very hot exhaust manifold and block, it heats up quickly to about 180F (82C) and thereafter its temperature is controlled by a thermostat.

By the way, an even better test is to make sure your exhaust temperature is well over 230C (450F), but few boats are fitted with the required gauge to measure this.

Run Hard

Second test: A quick rule of thumb that Steve shared is to run up to WOT and then back off 10-15% on the revs to get to a power setting that will clean up your engine if you use it regularly.

So give it a try. If this is a setting you can live with, on at least an occasional basis for an hour or so, and it is close to your normal fast cruise, well probably all is well and good, but if you now find that you are burning more fuel than you are comfortable with, and digging a big hole in the water to boot, it’s time to think about increasing your propeller diameter, number of blades, and/or pitch.

Increasing any of these variables will increase the load the propeller places on the engine at all RPM settings, as well as decrease the maximum RPM that the engine can reach at WOT from the governor setting on the fuel injection pump, known as over-propping or over-wheeling.

For most boats propeller diameter is constrained by the installation, so you are left with pitch and number of blades to play with.


However, over-propping is not without its dangers:

  • If you advance the throttle to the point that the engine is trying to produce more power than the prop will allow, it will “lug”, which will kill it even more quickly than underloading.
  • You may not have enough thrust or have cavitation problems when punching into a head sea.
  • If the engine manufacturer finds out what you are doing, they may void your warranty.
  • If your favourite mechanic finds out, he or she may call you an idiot and stop speaking to you. Many professional mechanics are surprisingly ill-informed about the dangers of chronic underloading, probably because they work a lot with planing and semi-planing boats where underloading is not as much of a problem.

How Much?

Having said that, if you only over-prop by an amount that reduces WOT by 100-200 RPM, you are probably pretty safe and, in fact, many engine manufacturers are perfectly happy with this amount.

But what if you have a real high-rev screamer engine that is way too big for the boat (a surprisingly common situation these days)? Then you have a difficult call to make, since correcting the potential glazing situation and getting to an efficient fuel burn regime may require over-propping by enough to reduce WOT RPM by way more than 200 RPM, and in this case the dangers listed above come into play.

Be Safe


The best way to guard against overloading (lugging), the biggest of these dangers, is to fit an exhaust temperature gauge and make sure that you never run the engine at higher exhaust temperatures than recommended by the manufacturer.

Great in theory, but I have found that some manufacturers are very cagey (who knows why) with this information.

By the way, the maximum exhaust temperature on our Perkins M92B, measured with a 100% load at the factory, is 598C (1108F). In practice, even when pushing hard into wind and sea and slightly over-propped, the maximum we see is 510C (950F). Having said that, be aware that different engines have different specifications, so I’m not sure how generally useful that information is. Eric Klem and Steve D’ Antonio both mention 400C (750F) as a good place to be, and that agrees with our fast-cruise reading.

You May Need a Lot

Back to how far you should go. Well, it depends…don’t you hate that? If you have a real high-revving screamer engine rated light-duty recreational, coupled with a reduction gear ratio of under 2:1—a distressingly common and very inefficient situation for a displacement boat—the answer is going to be a lot, particularly since flat-bladed propellers, like most of the feathering and folding propellers we fit to sailboats, become very inefficient at excessive tip speeds.

What I can tell you is that some years ago I had a conversation with Steve Dashew about this problem, in which he told me that extensive experimentation with his line of Sundeer boats, which were fitted with high-revving light-duty engines, resulted in adjusting the propellers to reduce WOT RPM by as much as 600 RPM (if memory serves) without ill effects. However, they were very careful to keep a close eye on the exhaust and back off at least 200 RPM if any smoke appeared.


And theory would seem to back this up. If you think about it, all Steve Dashew was doing was making the same adjustments with the propeller as the operator of a controllable pitch propeller (CPP) would do to match propeller load to engine output.

Having said that, you should be aware that some recreational engine manufacturers have so little faith in their creations that they specifically ban the use of CPPs, so that may not be an excuse they will accept.

When To Install a EGT

I would add that my recommendation would be to install an exhaust temperature gauge if you need to over-prop by more than say 200 RPM. Yes, they are expensive, but a hell of a lot cheeper than new engines. And given the amount of money you are going to save on fuel by solving a big time engine mismatch problem, an EGT will get paid for quickly.


The bottomline is that when it comes to the decision on significant over-propping, there are a lot of parameters to weigh, so you need to be really diligent about understanding what you are doing and what the trade-offs and dangers are.

On the other hand, improving the matching of your propeller and engine in this way can, when done right, increase the life of the engine by reducing glazing and, to boot, significantly reduce your fuel burn and carbon footprint.

Further Reading

This is a complex subject, so I strongly recommend that you carefully read or re-read the following chapters before over-propping your engine—thank you, Matt Marsh, AAC Engineering Correspondent:

The CPP chapter is particularly relevant because if you understand how a CPP works and how to set one you also understand what you are doing when you over-prop and how to avoid overloading.

By the way, I think many readers felt that those technical chapters from Matt were just an academic exercise. But, in fact, as this chapter shows, said reading can save us a bundle and reduce our carbon footprint—we must understand the theory to reap practical benefits without screwing something up.


If there is something that still does not make sense after reading the above chapters, please leave a comment.

Also, if you have over-propped your engine, it would be great to hear how it’s working for you.

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Odd Arne

Hi John,

Very useful information again, and thank you for bringing this subject up to a higher level. This understanding can save us money but it also gives many frustrations on how to do it right.

Also frustration regarding propellers, feathering propellers has very low drag, but when you see the flat surface of the blades you don´t need to be a rocket scientist to see that the prop is not the most efficient, but that is maybe another discussion since you put load on the engine with this propeller also.
Bore glazing occurs at low speed and light load operation and if you are lucky you can get rid of it as you did. With a correct run-in engine I think it´s not so easy to get bore glazing, -if you don´t change the routine you had the first (50-100?) hours.
Cylinder bore polishing is worse and you can get it after wrong run-in and long-term under-loading with incomplete fuel combustion and loss of lubrication. Then the microscopic peaks and valleys in the cylinder are gone.

jim clifford

Hi john,
This has been a very interesting series of articles on engine/propeller installation and optimization.
Over the years I have reached much the same conclusions as your articles as to what constitutes an efficient engine installation and have installed a Perkins M65 (45Kw @2600 rpm) engine driving a 23 inch feathering prop through a 3:1 reduction gearbox on our current 15 meter 17 tonne cruising yacht.
At commissioning I set the propeller pitch to the manufacturers recommendation and sea trials gave 9 knots at 2600 rpm (wide open throttle) which was a little better than expected top speed.
With these settings we were running at about 2000 rpm to achieve a cruising sped of 7 knots but noise levels were a bit on the high side . A couple of years ago I increased the propeller pitch to a level which only allows the engine to achieve 2450 rpm at wide open throttle. This has brought the cruising speed revs down to 1800 rpm and resulted in a much quieter installation. Fuel use has improved a little to 5 l/hr at 7 knots.
I have noticed two minor changes to the operational characteristics of the boat which may be significant if substantial over pitching were to be contemplated:
The boat speed at idle is now 3.5 knots which requires frequent shifts to neutral in slow speed maneuvers in marinas.
The prop walk in reverse has increased and definitely needs to be allowed for.

Another aspect of engine installation which needs to be considered if substantial over pitching is contemplated, is the strength of the drive train .
On a previous boat I changed from a fixed three blade prop to a controlled pitch prop after several thousand hours of problem free motoring and soon ran into problems with the gearbox clutch packs slipping. The extra power that the CPP was capable of adsorbing was more than the gearbox could transmit.
I suspect that some engine manufactures size their gearboxes to suit a typical propeller curve ie they can easily transmit the power that a typical propeller can adsorb but can only transmit full engine power at the seldom used wide open throttle revs.
From an engineering perspective a far more demanding criteria is to size the gearbox or prop shaft to transmit full engine power at the revs where the engine puts out maximum torque . This closely corresponds to the bulls eye in Matt’s fuel mapping diagram so any changes which we make to the engine/propeller system to move towards this ideal, will result in increased loads on the drive train.

Jim Clifford Wonambi 2

Alan S

John, note the modern generation of folding propellors, eg Varifold (as fitted to my yacht) and Flexofold have twisted blades and their performance is typically as good, if not better than a conventional fixed blade prop,. Also plenty of thrust going astern.

Dick Stevenson

Hi John,
A very nice article and well, thought out. You have brought together and balanced an impressive confluence of inter-dependent variables. A question and a comment.
What lead you to determine that you had glazing in the first year of your new engine?
You asked about others experience over-propping. We did so on our last engine/drive train in roughly the way you describe: conservatively, and were very pleased with the results which were in line with what you experienced. It was hard to parse out exact results as we were looking for more “punch” in adverse conditions and had gone from a 17 inch 3 blade Max Prop to a 19 inch 4 blade. We now have 2 years on a new engine and to date have stayed within the warranty limits of the engine. I just re-pitched one degree which should be about a 200 rpm decrease at WOT.
My best, Dick Stevenson, s/v Alchemy

Dick Stevenson

Hi John
With respect to fitting a larger diameter prop: most recommendations for prop to hull clearance figures (such as the ones in Dave Gerr’s excellent prop book) are overly conservative. At least they were in my case. I believe they tend to give clearances based on the prop being below a flat hull bottom, such as found on many/most power boats. Sailboats generally have a significant “V” in the hull configuration above their prop which minimizes the exposed hull to the forces from the prop and its wash. It also helps to have a good prop guy backing you up such as Chris at Darglow in the UK. He was prepared to shave my Max Prop blades and re-balance (not really a big deal as I understand it) were there any problems.
My best, Dick Stevenson, s/v Alchemy

Dick Stevenson

Hi again,
People may be interested to hear that a Yanmar 4jh5e (a 3000p rpm engine) has a running in (breaking in) sked as follows: first 10 hours = moderate usage, nothing extreme. 10 to 50 hrs. = full power range with most time spent at 70% duty and every ½ hour a 10 minute period at 80% and a 4-5 minute period of WOT. No light duty for more than 30 minutes. We felt fortunate that we were in “delivery” mode and doing a lot of motoring, but even so the above regimen was still a pain. I suspect most sailboat usage (in and out of marinas/anchorages) would make the honoring of the above quite difficult and that many owners do not bother for that reason as well as it may feel “abusive” to the engine to push it hard in this way.
My best, Dick

Dick Stevenson

Hi John,
Yes, they specify an RPM level to run at (interestingly, by directing the user to run not at a specific RPM but at a certain number of RPMs below WOT) and then they say what approximate duty they are aiming toward.

Eric Klem

Hi John,

I think your recommendation is very reasonable. In my opinion, people get way too hung up on the max rpm rating of their engine and base way too much off of that. Engine designers are typically limited by cylinder pressure over most of the operating speed range and they determine their fuel map based on that. Therefore, displacement will essentially give an engine’s torque rating and then taking the max rpm, you can estimate the power rating. As a designer, the easiest variable to tweak in order to increase power output without changing displacement is max rpm. This isn’t entirely free power as the efficiency typically falls off at the high rpm range but it doesn’t require any physical design changes, just governor changes until you run into things like inadequate cooling, valve float, etc.

Since the cylinder pressure at full throttle is the same at max rpm or a few hundred revs short, I have no problem at all increasing prop pitch to do this. For that matter, if you look at a lot of engine’s available, the only difference is the governor setting. As an example, the Beta 38 (3600rpm, 38hp) is simply a faster running Beta 35 (2800rpm, 35hp). By increasing the rev limiter by 29%, they got an extra 9% power. There is absolutely no reason why an owner of the 38 shouldn’t “overprop” so that they max out at 2800rpm where the engine makes 35hp if they never need more than 35hp. Note: at some point you can definitely overdo it and it depends engine to engine how much the designers have made the engine protect itself versus how much they rely on the user. Journal bearings rely on a film of oil to keep the shaft “floating” and speed is very important to this so if you drop the speed enough and keep the same torque/loads, you will eventually damage these. For the same amount of power, your engine is pumping a lot less air at lower rpm so your EGT’s are higher for the same power output.

One of the variables that has been alluded to many times is power density which can mean many things. An engine running at high power/displacement will generally run warmer although in some cases, torque/displacement is a better indicator as it is more independent of rpm. For example, your Perkins is about 19.5hp/L at wide open while the Beta 38 I mentioned is closer to 25.3 hp/L which is a significant difference caused by the higher speed of the Beta, the Perkins actually has a higher torque/displacement value. The other thing that matters is the surface area to volume ratio of the cylinders. Larger engines have a lower ratio meaning that they lose less heat to the cylinder walls per unit time, an important factor when determining cylinder temps.

Finally, one note on EGT’s. The probe location is very important to the reading, the closer to the cylinder, the more accurate it is. This is especially true with turbos which are thermodynamic devices so they lower the exhaust temps often by ~200F. For accuracy, a pre-turbo measurement is best but many prefer post turbo for higher reliability in the case of a broken probe or a leaky attachment and then apply a correction factor.



hi folks-

Anyone have a good suggestion for a source for an EGT unit? I’d like to fit one to my ol’ Perkins and see what’s going on in there at various rpm. Seems to be simple enough that it’s a no-brainer… but it’s not something you see ‘on the shelf’, so to speak. I’ve done a little surfing on it – lots of options for the offroad and competition truck modders. I suppose there’s no reason those units won’t work on my wee 65hp 4 banger – temp is temp, right? just need to make sure there’s clearance for the probe in the exhaust manifold where the hole is drilled I presume. Anyone done this?


Eric Klem

Hi Ben,

I have installed a few from Isspro and never had a problem with one. I have not done any real research on this so I can’t speak to what is actually best.

How you install it can be a bit of a trick. One way as John mentions is to have a collar welded onto your exhaust. Some exhaust manifolds actually have thick enough walls to drill and tap. Either way, it is definitely a bit nerve wracking as you do it. If you do a pre turbo install, it is best to remove the turbo so you can clean the pipe properly as otherwise the metal shavings will go through it when you start up. It probably wouldn’t be a bad idea to carry the appropriate size pipe plug to replace the sensor in case you ever have a problem.

As you search for an EGT gauge, it may help to change the term to pyrometer, that is what I have heard it called more commonly in the performance diesel world.


Bill Wakefield

Hi Ben,

A pyrometer is also on our list for this year for our Ford Lehman SP90 engine. [This will help us apply the lessons from this excellent series from John…]

While I prefer analog gauges for such readouts [like our tach- reducing the need to think/interpret the reading…] I am intrigued by the various digital gauge offerings from CruzPro. [NZ]

Most of their gauges have internal, user settable alarms, and integrate to a NMEA 0183 network- if desired, and will fit well into our existing instrument cluster.

I don’t yet have first-hand experience with their offerings, but my research so far reveals solid, positive reviews- for years. [18 years in business…]

Therefore, several of their offerings will find a home in our instrument cluster as new capabilities are added [e.g., pyrometer] and existing gauges are replaced. [e.g., fuel gauge that you calibrate to calculate live consumption data, etc.]

Here is the link to their pyrometer offerings in case this is of interest:

Cheers! Bill

Ed Finn

There is a lot of re hash on this motoring issue…Gee I thought this was a website devoted to sailing rather then motoring…. Just kidding, I think every cruiser motors more then they would like, and the engine is an important part of almost every sailboat.
You aksed in your original post about examples of over propping. Well here is my experience.

I have a 35 ft IP. With a modified full keel, that is nameplated at 17500 lb.
The prop is a three blade Michigan wheel fixed prop 17×15″
The engine is a yanmar HM 35F nameplated at 30 hp (no not 35hp)
The gearbox is a Kanzaki KBW10, ratio 2.83
The prop and engine are original from 1990
There is 4400 hrs on the engine
It burns no oil, none.
Yes, there is a spotty stain on the transom
I cruise all day at 2400 rpm making 6.0 knots and my maximum rpm in gear WOT is 2800 rpm. The labeled redline , and owners manual show 3400 rpm redline, which I cannot reproduce.
My fuel consumption is 3 liters per hour, 2KM per liter.
Their is only one factory thermostat available for this engine , and the glycol coolant runs at 145 F , no higher. I dont know what the exhaust gas temperature is.
Its a great little engine, no excesive vibration, minimal noise, burns just a little fuel, and no lube oil at all, I love it!
I give it a little hug every time I see it!
It seems to me based on your verbage about engines that it is over popped..
And should suffer all those things ” that the flesh is ‘er to”
But you can’t argue with success, 25 seasons and still working fine.

Moving on, I think the engine is well past its prime, but its working great, albeit a little over propped for my boat. I think I’m going to let it run to failure….as I am disinclined to spend any serious money on repairing a 26 yr old engine.

Most operators, and even some Engineers say, ” dont fix it if its not broken” .
That is usually a good policy.
If / when I have to replace the engine, then I think I would keep the same fixed 3 blade prop, but increase the horsepower to a 35hp Beta or that 39 hp Yanmar mentioned above. I think ypu reviewed both those engines for the AT 40. Funny, I previously researched both those engines, and thought either would be a good choice for me.

Ed Finn

Why would you prefer the British Marinized Beta ( Kabota)
over one of Yanmars. The 3 ym ae. Making 30 hp at 3200 rpm
Or the Yanmar 3jhme? Making 39 hp and a lot of torque and redlining at 3000 rpm
Am I detecting an anti Yanmar bias here?
If you dont like Yanmar then its OK, Anyone can have there preferences, and you are entitled to them, I am just asking why you feel this way….

Ed Finn

Forgot to mention
At 2400 rpm I am using close to all the torque available.

Marc Dacey

All this is very helpful, given that we are still in our Beta 60 break-in period and we have a feathering prop (a Variprop D-107) for which we can change the pitch in the water should we discover the issues described here.

Speaking of which, the issue of gearbox wear we decided to address with a hydraulic ZF 25 A with a reduction ratio of 1.93, which I certainly hope is sufficient, and an AquaDrive CV joint on a welded thrust bearing to mitigate both wear and noise. So far, so good, but I’m going to take notes this season to confirm that. The EGT installation, though fraught, makes a great deal of sense to me.


Thanks for the links folks – very useful. And a great discussion!

…I’m going to take the leap and put in a pyrometer. first thing is to see if I can get an accurate wall thickness reading on the exhaust manifold of my old perkins 4-154 using my audiogage. If there’s enough wall there I’ll drill and tap… holding my breath the whole time!



Hi John,

Both articles great and very informative.
I do understand that a continuous duty engine should be loaded at 75% since it is permitted (by manufacturer) to go as far as 100% load factor for unlimited hours. But typical sailboat engines such as Nanni diesels are sold under M4 rating and on their brochures they limit the load factor to 40% of rated horsepower i.e the cruising speed should be such that the engine is not loaded more than 40% and this is reflected in their warranty given for a new engine. Limitations on hours under full load exist.
So in this case how do you think the engine should be run for maximum longevity and without voiding the warranty?


James Sarrett

Hi John,

From an engineering standpoint, in general (I know, quite a contradiction!), turbochargers are almost an clear benefit. They take what is otherwise waste [exhaust] heat and turn it in to useful work [air density increase]. I don’t know about the specifics of the Nanni and Beta engine controls, but at least *in theory* adding a turbo to a steady-state engine will only improve it’s efficiency. That’s why every tractor and gen-set made in modern times has one, and they are typically continuous duty engines. It can be a problem though if you put the *wrong* turbo on the engine. you can end up with a turbo that doesn’t help at your operating point, i.e. the big one on a passenger car that only spools up at high RPM. It’s very possible that the Nanni is in that regime, but I can’t say for certain. My gut says that on the kind of low RPM, mid throttle that sailboats IMVHO operate at, you’d like a much smaller turbo than the equivalent (for the displacement) automobile.

Eric Klem

Hi Nikolas,

My take on this is that it is a function of putting many different output ratings on the same block which is the reason why I struggle with absolute rules of thumb around % load. Our concern is over cylinder temperature which in turn effects several other things including oil temperature. An engine manufacturer can take the same engine block that output 100hp and had ideal temperatures at 80hp and make it a 200hp engine by changing the governor and the fuel plate/map. If gearing is equivalent, the engine will still run those ideal temperatures at 80hp but that is now only 40% load because we changed the denominator is a somewhat arbitrary way. I think that the best thing you can do is watch things carefully and monitor temps if possible. In general, I would not try to exceed the engine ratings unless you are monitoring EGT, oil and coolant temps carefully. The ratings are necessarily conservative but typically not overly so.


David Popken

So, I’m wondering what two alternators and engine driven refrigeration adds to the the total loading? I’m guessing that it’s akin to de-pitching the prop. So, if I wanted to work the engine a little harder, it would be with the refrigeration engaged at something less than WOT. Sabre 38 MKI at 15,600 displacement, Westerbeke 38B with only 975 hrs on it, Max Prop at some PO determined pitch. My fuel consumption is .78 gal/hr at 2800 rpm. No smoke, no oil burning. This is not empirical, but I’m feeling like it’s a pretty solid setup.

David Popken

Hi John,

Without getting too technical, I feel like my setup is good. I do have an infrared temp gun, so at some point, I’ll check engine oil temps. The great thing about this discussion is now I’m AWARE that running the engine at anchor, or at low rpms for long periods is not good. On our 22 month trip (10 days sailing followed by 2 months back in Texas working and so on) from New Jersey to Texas, we brought aboard one of the worst things a cruiser can have in many people’s minds, a schedule. One thing the schedule did was to force me to keep the rpms higher when we were motoring and needing to get to a certain place by a certain time. So, unbeknownst to me, I was giving the engine what it really wanted! Thanks again.

Eric Klem

Hi David,

We have the same engine but in a CS36T. Yes, alternators and refrigeration can add significant load. For example, a 70A alternator will require a maximum of around 2.7 hp (this is using the old 50% efficiency rule of thumb that includes belt efficiency, I have not checked recently to make sure it is still valid). Personally, I like to set my prop like these loads are not there and then know how to disconnect the loads without ruining alternator diodes should you ever need that max power. This is the way we are set up and while I have run a few minutes at close to max power here and there, I have never had to disconnect the load although it is quite easy. Some very conservative people don’t like to set pitch this way as it doesn’t use worst case assumptions but I feel it is the best overall compromise as I have never actually needed full power.


David Popken

Hi Eric,

I do have the data that gives me the MaxProp details. I should give that a good look and maybe talk to them as well. As John says, at 37hp and 38 ft, I’m not really over powered, so I feel pretty good about my setup.

Ed Finn

Your comments of April 18th are confusing me.
Here they are again,
“”Also begs a question. In the manual do they specify an RPM setting to achieve 70% duty? If not, I suspect that may owners get tripped up by thinking that 75% duty equates to 75% of max RPM”
Well, What do you mean when you use the term ‘power’ or 75% power??
Wikipidia, and other texts define power as
Work over time, and units of measure of power are
Watts, kilowatts, and ‘ horsepower ‘ . So it seems ot me that
horsepower hp os synonomous with ‘power’
I think I understand your meaning of duty cycle and 75% power to be
75% of maximum ‘ torque’. Or perhaps I’m really confused.
So, Can you explain (or tell me where you covered it previously)
Power , 75% power, duty cycle, and torque
Perhaps some of our engineering support team , can help me out?
PS , overppropped and lugging, are well up there on my list of fuzzy terms too!

Jeff Totman

Interesting discussion about overpropping. We bought our Tayana 47 and the Yanmar 4JH2-DTE had been grossly overpropped to the point where it would only turn 2400RPM at WOT, about 1000RPM less than specified and the prior owner had run it that way for years. At this speed the turbo wasn’t even operating. It also had other problems so to regain confidence in it, I sent it off to Mack Boring to be remanufactured. After consulting with PYI, we set the Max-Prop so that it now turns 3500RPM as it’s supposed to be able to. This engine is an 88hp turbo diesel and our boat actually weighs 46,000lb, but we can still cruise at 7+ knots at about 2600RPM so I am running into the problem that’s already been discussed here where I’m tempted to accept that “comfortable” cruise RPM rather than pushing it up to 2800-3000RPM. If I do that for even a portion of the day, I run it at higher RPM for awhile before shutting it down. I’ve thought of increasing the prop pitch slightly but worry that with a turbo engine, the lower RPM’s might mean the turbo doesn’t hardly ever get used and will get dirty and gummed up.

Also, ever since the remanufacture (and possibly before), only about 150 hours ago, there seems to be more black smoke than normal and whenever it gets up over about 3000RPM, it’s very noticeable. It’s certainly not overpropped and since it has done this regularly even during the break-in period, I don’t understand what could be causing it. I realize this is slightly off topic but I’d love to know what’s going on with it if anyone has seen this before.

But my primary question is in regards to how deliberately slightly overpropping by about 200RPM’s might affect the health of my turbo. Is there a different consideration for turbo engines as opposed to normally aspirated ones?


Great article series, worth subscription for next year!
– love the pic of your engine panel, that’s what they should all look like (though I’d add a bilge pump ‘running’ indicator too )


Marc Dacey

In light of the propeller discussion to date, I would like to query the collective wisdom, as I’ve obtained some real-world numbers.

We have a four-bladed VariProp of dimensions 19″ and a (factory) pitch of 15″. We have a Beta 60 naturally aspirated four-cylinder diesel with a 2,700 RPM full load limit with a 1.97:1 ZF25 hydraulic transmission. The boat is currently light load at 29,000 lbs. in the slings and a waterline of 31 feet on a modified full (box-type) keel.

Yesterday, in very light winds and near-flat seas, I drove around 7.2 knots at 2,100 RPM. I had about an inch of throttle left but could not raise the RPM. Engine temperatures and pressures were normal (the temperature was slightly below 180F, but the lake was cold!)

Now, 7.1-7.2 is faster than I would usually care to go for reasons of fuel economy. I would be happy with “motor all day at 5 knots” if I could do it at under one gallon per hour fuel burn. So it strikes me that I am overpropped and should flatten out to a pitch of 13 or 14. My prop allows different reverse and forward pitches, and I am well-pleased with the stopping and backing down power at 15, so this would simply be a change to the forward pitch to get the RPM up to avoid damage to the engine. My fuel map indicates that at 2,100 RPM, I am at about 42 BHP out of a theoretical 56 BHP at 2,700. That’s actually not bad having re-read Matt’s “fuel map” piece, but I’m trying to cover all my bases. Certainly if I drop to 1,600 to 1,700 RPM, the fuel map suggest my consumption lowers considerably to under three litres/hour…and I’m still doing 5 knots.

Does this appear to be the right strategy? I’m still in the break-in period and intend on running the engine properly. An additional wrinkle is that the boat will shortly have about a ton of batteries, additional tankage and gear added to it. Any suggestions would be helpful, given the variables.

Marc Dacey

Good comments, John. The problem may in fact resolve itself when a) the boat is fully loaded with water tanks, a third fuel (day) tank, its imminently arriving six L-16s batteries, and whatever gear we choose to load. B) would be a slight depitching, and C) would be that EGT to determine we are in the zone for an adequate load. I’m not convinced that 60 horses (read: 56 BHP) is ultimately going to prove too big, given I have yet to put in a right-sized alternator (it’s just the small-case stock one at the moment) and we are well off our lines to the point we have lead ingots for trim in the bow. I did, however, want to start off on the right foot, and a slight depitch and an EGT is a good place to start, especially as I am modifying the exhaust system to a port and starboard “North Sea” type this summer. Thanks for the comments: the greatest value of this site is the exchange of ideas and the filling in of the blanks of knowledge that can arise from self-study.

Paddy Glenny

Hi John
I read this post a week or so ago and it set alarm bells ringing in my mind.
I have only been sailing for three months so forgive me if some of the things I say are ill informed.
I have a 1985 45ft Gulfstar CC factory fitted with a Perkins 4.108 which was fully reconditioned by the previous owner and had 400 hrs on when I bought the boat and now has 750. It burns no oil (but annoyingly leaks a bit but the engine access is so bad I cannot see the other side of the engine and thus cannot yet identify the source of the leak) and runs very sweetly.
Until your article I had no idea I was killing her with kindness- I have racked up most of those 350 hrs over the 3 months cruising here in the Med, at about 1800 rpm at a speed of about 5kts. At this usage she uses 2.4 l/hr which I think is pretty low consumption.
After reading your article I took her out on a calm-ish sea and did a bit of testing.
To measure the oil temp I fixed a digital thermometer probe to the outside bottom of the sump (best I could think of). The boat has a water temp guage on the dash (which I suspect is under reading dramatically, as you will see).
Appt wind of 10kts off the port bow.
At 2500rpm, water temp read 56c and oil 76c (after 30 mins of running). Speed 6.5kts
At 2800 water read 66c, oil 79c, speed 6.8kts
At 3000, water 70c, oil 82c (yay!), speed 7.3kts (I think the slightly non-linearness of the speed figures would be accounted for by changing wind conditions).
Truthfully I chickened out at about 3300 as it sounded super loud and as if she would blow up at any minute (no thank you!) But I blipped her to 3500 and she eould have gone further if I had had the balls to push her but I didn’t.

So, my take on this is I am under propped. Do you think so?
I have no idea of the spec of the prop but will swim down there today if I knew what I should be measuring, although in my mind I picture measuring the diameter as very difficult with rudder, propshaft etc stopping me putting a tape measure squarely across the prop. Also, with a three bladed prop, how does one actually measure the dia in situ?
A related point is that this boat has a pretty fearsome propkick. To moor stern to, you have to start a long way away as the stern heads off hard to port for a long time, no matter how you treat the wheel. Shameful as it is, I have now started tweaking the bow with the bow thruster as this is the only way I have found of getting her on target in less than about 100m….
As an aside, I have, distressingly, had to give up on the idea of a Spade anchor for now…the S140 steel here in Greece is….€1287. I just ain’t got it!

Ed Finn

Whats the max rpm rating of that old Perkins?
Id run her at a comfortable 2500 to 2800rpm.
With my existing prop, and not worry to much about the theory and
The Perkins is an oldie but a goodie.

Paddy Glenny

Well thank you both John and Dave for your replies.
I’ll research the costs of repropping as John suggests and see if I can afford it or not….but I imagine it ain’t going to be peanuts..
I just checked and yes, the wot is 4000!!
For an old school diesel, that is truly a screamer…and honestly not something I am very keen on.
For obvious reasons the last thing I want is this engine to blow up and she sure purrs happily at 1800…but I guess for the most part those days are over then!
I’ll start using her at 2500 to 3000 for now and get busy researching the prop options.
Btw, if the wot is 4000, to load her to 85%, should I be running her at 3400?

Paddy Glenny

Oh, and I forgot to ask, how do I identify the prop dimensions of my current prop?

Ed Finn

To measure the diameter of a three blade prob…
You can only measure the radius on one blade , then double it to get the
Diameter. So measure from the center hub/ center of the drive shaft to the outside edge ofone blade, if that number is 9″ then your diameter is 18″ …simple
The pitch is not as easy, perhaps you will find it written on the hub of the prop…
Do you know what rpm rthe previous owner ran her at? Id take that into consideration, even though the motor has been rebuilt…
And I think I’d be comfortable running her at 2500 rpm, and expect my fuel consumption to almost double…

Paddy Glenny

Once it warms up a bit today, I’m going to take a wire brush and a tape measure (guess I’ll be writing that off!) then dive down to the prop to measure the radius and see if the pitch is stamped on it sonewhere.
I looked at the Maxprop site- wow, they make some sexy props- and I’ll get in touch with the Greek dealer as soon as I know what to ask for. I am also interested in the fact they say with one of their folding props I can say goodbye to propwalk…as I said before, this boat has savage propwalk that I would be thrilled to eliminate.
Almost €1300 for a Spade anchor, god-alone-knows how much for a Maxprop….this is a capital intensive way of life!

Dick Stevenson

Hi Paddy,
You will not get rid of prop walk, but you will tame it considerably with a Max prop. I personally like a bit of prop walk as it helps in marina maneuvers. You are new to your boat and sailing, so give yourself some time to make some of these bigger decisions work well for you. For ex. Max props also come in 4 blade and 5 blade varieties as well as the more common 3 blade. These might be nice if you motor sail a lot or if the size prop you can swing is limited. By the way, (to avoid unnecessary confusion) I believe that if you are talking Max prop, you are looking at a feathering prop rather than the folding prop you mentioned. Similar results, but they work differently and have different capacities.
Good luck and keep us posted, Dick Stevenson, s/v Alchemy

Paddy Glenny

Thanks for your info Dick, much appreciated. I can see the advantage of propwalk for turning on a dime in a marina but mine is so severe that when mooring stern to, as is pretty well universal here in the Northern Ionian, I have the following proceedure to contend with:
No lazy lines and often depths of around 10 metres so we have to drop the anchor about 50 m distant from the quay. I usually ask my wife to start off with,say, 30m of catenery then I begin to back up. No matter how gently I do it and of course with a straight rudder to begin with, the stern resolutely heads off to port and I cannot get here to respond to the helm until we have covered around 25m so by now I am way off course…then before making the final approach we have to know for sure that the anchor has set. If it does set we are now at a standstill so as my wife pays out more rode I have to get the beast back on course to snuck into the available slip, which is damn near impossible…and naturally there are always half a dozen people standing on the quay watching and you feel such a twit because your boat is basically uncontrollable, given the need to set the anchor. One cannot, either, start the whole process further downstream to starboard, as it were, to compensate for the propwalk as you would then be dropping your anchor over your future neighbour’s chain.
And we have had it several times where we have needed 3 or 4 attempts before the anchor sets…and my wife, being Italian, is not the most naturally patient woman in the world…then throw a 15kt wind on the sb beam into the mix as well and you will understand why quite often we just give up and head off to find a quiet bay to simply anchor!
A lot of these difficulties stem from the crap abilities of my cqr anchor. I have been discussing this issue with John and know well that I need a Spade S140 but the Greek dealer wants almost €1300 for one and we are under-capitalised live abords so for now I’ll have to put up with the cqr, it seems.
Re the prop, I probably used the wrong adjective. I believe a folding prop is the sort that kind folds up when not being driven so as to reduce drag, and a feathering prop has some form of adjustable pitch, no? I thought I read somewhere on the Maxprop site that with their prop you can kiss propwalk goodbye…but I can’t that bit looking again this morning so I think I must have imagined it.
What I don’t understand though, is just how does one of their props even reduce propwalk if the pitch cannot be altered when actually in use, but only by diving down to the prop with a few Allen keys etc in hand?
In truth, I don’t even understand what causes propwalk at all except I am guessing it is some sort of torque reaction to the thrust of the prop ie one of Newtons laws perhaps?
Anyway, enough of my prattling for now..

Paddy Glenny

I just dived down to the prop and the radius is 210mm which equates to a diameter of 16 1/2 inches…do they come in half inch increments?
But I couldn’t see anything about pitch stamped anywhere, even though to be completely truthful I didn’t hang around very long as I find it pretty dark and spooky down there.
In general terms with a Perkins 4.108, and 45 ft Gulfstar, is a 16.5 inch prop in the ballpark or a bit small? The only thing I can say about the pitch is it is very obvious, the blades aren’t remotely flat, as it were…

Ed Finn

Please allow me to give some more input…
Reflecting back on your temp and pressure recordings above…
Those temps looks funny, they should be constant, throughout the entire operating range, thats the purpose of the thermostat.
Did you check your thermostat? Are you sure you have a thermostat? Perhaps it got left aside at the rebuild…
Can you get a thermostat that opens – functions- at a higher temperature ?
That would get your engine water – glycol coolant- and your base oil temperature up. But Im not even sure that is necessary.
Moving on, I think if there is something wrong with your engine , you cant really fix the engine by spending several thousands of dollars or Euros on buying and installing a fancy prop. You still have the same old 35 year old engine.

Perhaps you step back and let the dust settle on this one.
Run the engine at 2500 and see how things work out. That in itself is a big change, and actually could cause engine failure!,
Actually your safest bet is to do nothing, run it at 1800.
The engine is 35 years old , and recently rebuilt, thats a dicy proposition at best. And you have been running it sucessfully , idling along at 1800rpm.
Buying a new prop and considerable expense, then loading up that old engine to close to max power, torque, rpm or hp. (whatever) can exceed what that old engine is actually capable of. I wouldnt expect and old engine to function as a new engine, and a new prop.
The engine is not new from the factory, its rebuilt, its 35 years old. It may be just a good as a new engine, and maybe – probably – not.
Now, I’m not suggesting that the theory of diesel engines props, effciency,
And all that technology is bogus, its certainly not. But I would be reluctant to try to make an old engine perform like new…
Some figures of speech that come to mind:
If its not broken don’t fix it.
Stop throwing good money after bad.
You cant make a silk purse out of a sow’ s ear.
In closing , Please know that my intent here is to help and not ridicule or belittle anyone. I just feel that if I were in your position, I wouldn’t spend a lot of money trying to fix an apparent problem that I didnt know I had until I read about it on the internet a few days ago , regardless of the source, or how technically sound the info.
……I appologize in advance if this is not what someone wants to hear..
Ed Finn

Paddy Glenny

Thank you all. I have much to mull over…

Marc Dacey

Paddy, unfortunately the best answers come from a holistic appreciation of not only the entire drivetrain from engine to prop, but aspects of how you yourself drive, rudder size and angle and hull shape. Fortunately, all these things are knowable and technique comes with time.

I agree with the sentiment that an old rebuilt diesel such as you have should be throughly explored as it stands before getting into the esoterica of a potentially better prop. If you can find a quiet place off a beach to practice anchor drops and backing down, you may hit on a less entertaining (for the observers) method of getting into that Med mooring situation. It could be as simple as more throttle to back down straighter, and then going to neutral to coast back in a straight line, allowing for contrary winds and currents. Prop walk in my experience is most noticeable at low RPMs; we tend to coast in and give a brief blast in reverse to kick the stern into the dockside and to (nearly) stop, as rudders don’t work at zero knots, anyway. When we re-engined and re-propped, I did a lot of close quarters maneuvering next to a buoy first and then against a 100 foot stretch of sea wall. The peculiarities were made manifest before we had to dock for real.


So I will bring this thread back to life as I just joined AAC. We recently repowered our classic Tartan 40 with a Beta 50. Definitely overpowered. But since the engine box protrudes into cabin, noise reduction was a big objective so I wanted a low revving cruise speed. The 43 and the 50 are same block and dimensions.
So I overpropped with a flexofold. I meant to go 200 RPM lower than 2800, but it ended up topping out at 2550 light, and 2450 with full cruising load. So reading this makes me feel better about not getting a lower pitched set of blades for my flexofold to satisfy “warranty”. Essentially I now have a 43 hp 2400 redline engine. Cruise at 7.1 knots at 1800 RPMs which is really smooth and quiet. I’ve calculated HP needed to drive hull (theoretical) and then modeled 5 and 10% gear losses, alternator loads to see if engine is theoretically overloaded based on prop curve.
It looks like with 5% gear losses and 1-2 hp for alternator (usual light charging) I always stay below the power curve (using conservative industrial ISO 8665 rating). Only if I model the worst case- 10% gear and bearing losses, and 4 hp for alternator- do I find that at 1950 RPM my hp needed to make speed starts to exceed theoretical engine output.
2 questions for John and others:
1. Is this method of calculating safe overprop engine loads based on theoretical HP valid?
2. You had mentioned fuel pump and fuel mapping earlier. Other than that my engine is not governed at 2400 RPM, is there something else that would be changed if my engine were actually detuned in a shop to 2400 RPM/43 hp? Ie is there different fuel timing/delivery/etc?

If anyone is interested I can post my spreadsheet with calculations of HP/RPM/Speed compared with actual speed and RPM

Eric Klem

Hi Mike,

Regarding your first question, you started in the right place by doing the analysis and then you move to checking the results once everything is put together. For a given rpm, if the torque required to turn the propeller is greater than the torque that the engine can produce, you will have a torque mismatch which leads to an angular acceleration meaning that the engine speed will decrease. This decrease continues until you reach an rpm where the engine output torque is equal to the torque required to turn the propeller and then things will stabilize there. If you are really punching into heavy weather, the propeller required torque versus rpm is not constant so the rpm of the system will not stay constant but rather hunt around a bit as the load changes. This is also somewhat dependent on how your governor is setup, whether for amount of fuel or rpm with boats being designed to hold an rpm and vehicles being fuel delivery based. In your case, the rpm where the torques are equal is 2450 on the engine. I highly doubt that in the real world you have any points below this rotational speed where your load exceeds your possible output as you wouldn’t be able to get beyond that speed. Since you found that you had that from a theoretical standpoint, I would take a look at the hull drag numbers you used which should be roughly exponential and is the biggest reason why available torque and required torque only actually coincide at one point.

People talk a lot about overloading engines and I think that it kind of misses the point. For example the one thing that I really didn’t like about Nigel Calder’s video was when he talked about matching the propeller load and engine at the governed rpm and then said that if you added alternator load the engine would be overloaded and damaged. This assumes that magically the engine would start producing more power/torque at that rpm which it couldn’t, your rpm would drop until torques were balanced again. What you really want to know is what rpm range can your run the engine at wide open throttle without shortening its life unacceptably. The people that designed the engine will have looked at cylinder pressure (this determines mechanical loads), exhaust gas temperature (don’t melt those pistons and valves), plain bearing film thickness (low speed can’t handle as much load), emissions, etc. Once they have determined what they deem safe, they put that into the engine through a fuel map, fuel plate, AFC, etc. The question then becomes whether they limit the output so that wide open throttle is safe at all rpm or not and unfortunately I can’t speak for what the different manufacturers do. At the very least, most of them give an rpm range around the governed rpm that they say is acceptable to operate to the duty cycle related to their M rating. For engines where there is another version that is just governed down, I would have no problem propping the engine as if it were the other version.

In addition to the post that John linked to on your second question, if you are really curious and if you can get ahold of the parts diagrams, you can compare key parts such as the fuel limiter in the injection pump (guessing it is a fuel plate in this engine but not sure), the injector nozzles and the cam. You can also look at the workshop manual and see what they have you set the timing too as I suspect that they are not running different timing gears with different timing marks. If I had to guess with the 2 power levels being not that different is it is likely they changed the governed rpm and the timing but nothing else.



Eric and John- this is such helpful information. It validates my approach and I will look into the parts approach to see how this engine is detuned. I am pretty sure I’d have to search industrial equipment- this very Kubota block is in a lot of Bobcat skid steers, ironically including the one that our marina owns.

But it sounds like an exhaust pyrometer is about the only way to use data to figure this out if I want to be precise about knowing where engine is optimally loaded vs under/overloaded. Also when conditions such as payload and bottom growth, wind and sea state change the load for speed. For now I am quite confident my slower speed cruise isn’t overloading engine in the 1850 RPM and even upwind in 15 knots I still get >6 knots. But I probably would like to know what happens above 2000-2100 RPM which would be a fast cruise or get out of trouble speed….
I think I read that you have to tap into manifold for a transducer placement, and can’t get by in the first part of exhaust elbow possibly before the water is injected? Also is there a general temp range for small diesels or is it really manufacturer specific?

Philip Chen

Hi John,
Thanks for this series. I know that this is an old article, but it has me wondering:
You have made a very compelling argument for not buying excessive hp. The conclusion then seems to be find an engine that will just get you to hull speed, or with a bit left over and overpropping a bit. (I’ve inferred)

I’m wondering if you’d consider an article or reply that explains why underpowering is not ideal.

For example. My boat takes 28 hp to reach a hull speed of 7.34. At half that horsepower, it can still reach 5.9 in windless, flat water. What would be the harm in using a 14hp without overpropping (so that the engine does not bog into wind/waves) or using an AutoProp. Would that not keep the engine more properly loaded and be more fuel efficient in general, especially compared with boats that are regularly run at half-throttle to conserve fuel?

If beating into a chop, you’ve already argued that everyone should be motorsailing. So what circumstances are there where you absolutely need maximum hull speed in the absence of the wind to use your sails? Are there comcenrs of damaging or overloading a smaller wngine if you’ve decreased the pitch to compensate?

I ask because I will need to repower within a few years. Although the cost does not increase dramatically as engine power increases if purchased from a dealer, and the installation will cost approximately the same, there seems to be new but second hand engines available regularly on Kijiji from boat building projects that are abandoned, but they are usually in the 13-25 hp range, ?because of the typical size of amateur projects. If there is a good reason to not go that route, I’d like to know, but if it is perfectly viable so long as the system takes into consideration the limitations, it could be a way to manage the cost of a repowering.


Michael Albert

Hi everyone-
Bumping this thread 1 week into a cruise with my new EGT installed on my Beta 50 on a Tartan 40. Very validating that I overpropped deliberately. The 50 is same size as the 43 but by overpropping I can now extract the power I need at lower RPMs which are much quieter. I give up top HP which is too much for my boat anyway. So I have given up about 350 rpm of top end power and now have 2450 redline. At 1800 rpm which is 7 knots I barely get to 500F on EGT. 1900 gets me 7.2-7.3 and digs a nice wake and gets me to 550-600. I had never dared go to 2000 rpm but I get nearly 7.5 knots and about 650 on EGT. Going above 2200 and the EGT starts to approach 850-900. And that’s super overpowered. So I have a good cruise setting at 1850-1900 which seems to just load the engine enough. I was worried I had overpropped too much but in fact the engine is more lightly loaded than I thought. So the pyrometer is a great addition and didn’t cost that much. Great thread- I bet lots of folks out there would be shocked to know how much they are underloading their engines. In fact I think that’s why my old Yanmar 4JHE which came with boat smoked and left a fuel sheen in the water- I think I the old owner liked to run it at 2000rpm (of 3600 redline) and 6 knots which I think never really loaded the engine.

Per & Meri Lovfald


Love the website! Your article on “Power and Propeller Settings For Your Engine” is one of the most concise well written articles we’ve read about the issues with sailboat diesel auxiliaries. We recently purchased a Pearson 530 engined with the same engine you are currently using, a Perkins M92. The boat currently has a 2 bladed fixed prop 23×14 and we are worried about the engine issues you describe in the article. Through reading your article and doing some research we are looking at purchasing a H6 Autoprop (566mm). To us the Autoprop is a hybrid. It attempts to address the below issues:

1) Propeller feathers reducing drag while sailing.
2) Blades have twist reducing tip loss at higher RPM’s.
3) Substantially reduced prop walk in reverse due to unique design (blades use same leading edge in forward and reverse).
4) Automatically varies pitch depending on operating conditions, maintaining maximum efficiency at all times resulting in higher speeds and greater range with reduced RPM and fuel burn.

Three questions for you John,
1) What is the make and size of your prop with your current pitch setting?
2) Curious your thoughts if you have them on the Autoprop?
3) What is the make and where did you get your engine instrument panel?

Our boat – 1981 Pearson 530
Engine – Perkins M92 with 2600 hours
Transmission – Borg Warner Velvet Drive (1.91:1 reduction ratio)
Driveshaft – 1.75”

Thanks in advance for your response,
Per & Merissa Lovfald
S/V Sojourner

Marc Dacey

I am qute pleased with our four-bladed VariProp. another feathering prop that has (to us) the advantage of setting different pitch settings in forward and reverse. Using John’s sound logic, we flattened the pitch in forward in order to reach very close to our diesel’s rated RPM in forward, which put us in the desired fuel economy zone at 75% of maximum RPM. Keeping a coarser pitch in reverse, however, gave us stopping and maneuvering power for the few seconds we typical use reverse on a heavy displacement vessel with no bow thruster. I also favour John’s use of an exhaust temperature sensor and may have to alter our helm panel to have one to confirm that our pitch settings are as good as we can get them for fuel economy and engine efficiency.

Terence Thatcher

First, thanks for all this information. It helped me immensely as I made the decision to replace the Perkins 4-108 on my Morgan 382 with a Beta 35. The Perkins was always more engine than the vessel needed. I am also moving to a MaxProp. My Perkins has probably never been run over 2400 to 2500 rpm in 43 years and definitely never run over 2300 in the last 25 years of my ownership. WOT is 4000. Despite my decision to repower, I have no evidence the low rpms have caused glazing or any other problems. The engine turns a 16×11 prop with a 1.88/1 reduction. Why would it be that I and the previous owner have not damaged this engine running so far below its 75% load? On exhaust temperature: I would like to have a gauge with the new engine, but I am not sure I trust the Beta exhaust elbow to accommodate the necessary drilling of a hole. Just to check occasionally, could one use one of the infrared thermometers you suggest using on the oil pan? It is not hard on my installation to reach in to point at the exhaust. Thanks again.