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 RPM—same 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.
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.
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.
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.
A DIY CPP
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.
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:
- Understanding An Engine Fuel Map
- Propeller Efficiency
- Controllable Pitch Propellers
- The tip speed problem explained, complete with calculator.
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.