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Hi Matt,
Just curious what engine the fuel island plot is from if you don’t mind sharing? It seems to be an engine that is optimized for fairly high speed operation as the torque curve is relatively flat at the higher speeds and the area of maximum efficiency occurs at fairly high rpm. One thing that I find interesting is to compare the plots from mechanical injection and electronic injection engines. Probably the most interesting plot I have ever seen is for a 1.9L VW tdi engine.
I am looking forward to the discussion on matching propellers.
Eric
Alas, I couldn’t get any manufacturer to give me a fuel map with permission to publish it in a public article. So the example one is made up. The figures are all “in the ballpark” of real 100 kW class, 4 litre high-speed industrial/marine diesels, but do not correspond to any specific model.
Yeah, it can be hard enough to get a curve for private use. When I was doing the hybrid spreadsheet, I settled for including 2 curves that were publicly available on the internet. I have access to a few more but I know that some people would be mad if I ever posted them publicly. Two things that jump out at me in looking at these is that the relationship between the torque curve and the island plot is pretty consistent so you can get a decent idea without having the full plot which is good because the plots are otherwise remarkably different. I wish that I had one for our Westerbeke 38B four but I don’t.
Thanks.
Eric
Good data, but in the real world, I’m not going to change out my 1994 Westerbeke 38B or the transmission with only 950 hours on them, unless I have a major failure. So I’m stuck with her for now, and realize that it is inefficient, but still workable. Certainly good to know that low rpms can be as bad or worse than too high rpms. It’s clear from my use of the hour meter to calculate gals used per hour, that 2500-2800 rpm is this engine and propeller’s sweet spot.
Hi Matt
Fantastic article
Without wishing to be pedantic and or drag you off topic
I was always taught, BMEP x surface area of piston x stroke = Torque
I appreciate we have just defined displacement, but it then leads to a discussion about “short” stroke and “long” stroke motors and different torque and power characteristics.
With regards to fuel burn Caterpillar, used to issue service and rebuild intervals based on fuel and burnt.
Best Regards
Bob M
Hi Matt (and Eric),
What, if any, effect does the availability of computer controlled injection engines have on any of this? In other words do these engines have any ability to improve the matching between the load from the prop and fuel they are burning at less than WOT?
If I’m jumping the gun and that’s coming in a later chapter, just say so and I will wait to read it there.
I think it’s fair to say that while computer control gives you some additional control over the fuel map, it can’t change the inherent design of the engine.
Electronically timed injection, variable valve timing, and other high-tech features can reduce the specific fuel consumption in the optimal operating regime, they can and they can make the optimal operating regime larger.
But they can’t change the fact that some conditions, like low load at high RPM, will always be suboptimal.
Hi John,
Hopefully I am not jumping the gun by chiming in here. As engines have gone from mechanical injection to common rail systems with a few iterations in between, there have been many other changes as well such as turbocharging, valves and designing for emissions regulations. These other changes have also had significant effects but some generalizations can be made about the specific effects of electronic injection.
The major difference in injection systems comes in the control of them. Older mechanical injection systems operated at much lower pressure, had much less precise injection events and had fixed timing. By getting much better control over the injection events, the engine can be tuned to run well over a wider range of rpms. The change is not drastic but it does help. This can be seen in the shape of the torque curve and also in the fuel island plot (or whatever you want to call it). In addition to efficiency, by using multiple injection events per combustion stroke that are carefully timed, the engine noise and emissions can be reduced.
If you want to see examples of plots for other engines, the engine efficiency tab in the hybrid spreadsheet shows 2. What is remarkable is just how different they are and how different they are from the one that Matt shows.
Eric
Take a bow Matt,
You never fail to make “way-over-my-head” stuff really interesting and tantalisingly within reach! Being seated firmly in the back row of your AAC marine science classes, may I ask some dumb questions please:
You suggest choosing a propeller slightly greater in pitch to improve an engine’s fuel map, could be a good thing. Does this mean we could achieve the same thing by having a correctly pitched propeller with a slightly under powered engine, for a given displacement and hull form?
Background to my question – we own a Beneteau 473 of some 12 tonnes and 14.3 metres which has a naturally aspirated Volvo 55hp. Although it wasn’t our decision (bought 2nd hand), we have never felt under-powered, and we achieve excellent fuel economy. Our cruising speed is around 7.5 knots. But most other 473 I have seen advertised had 75hp diesels, and later “run-out” models even had 110hp. So could the old adage “a good big player will beat a good small player” be wrong for marine diesels? Has the industry been overselling us, heaven forbid?
Cheers,
Rob
Hi Rob,
You should take the bow! You nailed it. Most sailboats are incredibility over-engined, which results in poor fuel economy and short engine life.
More here.
Hi John, I hadn’t seen your earlier articles (still working my way slowly through the back catalogue). Seems we can add propeller tip and hull speed to life’s “gotchas”, right up there with death and taxes.
You hit the nail on the head, Rob.
Consider a yacht with a 40-foot waterline and a comfortable cruise speed under power of 7 knots, using 40 hp to achieve that speed. A motor rated at 55 hp would sound about right, cruising at a comfortable 73% of rated output.
If the salesman wants this boat to be able to reach 9 knots under power, the builder might have to fit a 100 hp motor. Now we’re using only 40% of the engine’s rated output at cruise, and we’re operating well into the lower-right region where specific fuel consumption goes up – and where glazing and carbon buildup become important problems.
Assuming both engines have correctly matched propellers and gearboxes, the 100 hp might be nice to have on that one day you need to make an emergency mad dash for the harbour…. but the smaller motor will be cheaper, longer-lived, more fuel efficient and easier to live with most of the time.
Hi Matt,
I my experience with sailboats offshore you can’t even use that 100hp for the emergency mad dash to the harbour. The reason being that if that mad dash is upwind—if it’s downwind hoist some sail—most sailboats will pitch too much to get that power into the water—the prop will just cavitate.
The problem is the mast, which increases the pitching problem when motoring up wind much more than most sailors realize. (This is another reason that motor-sailors make little sense to me.)
On MC (56′ and 25 tons) we have still can’t use our full 87HP to motor upwind in any sort of a sea—close, but not quite—and we have a carbon mast, which reduces pitching hugely.
My guess is that most 40 foot boats would be better off with about 40 HP.
Sorry for the late reply here, I have been off the grid for a few days. I wanted to add a few thoughts on engine sizing to what has already been said here. As has been said, many people want to have the ability to put down unusable amounts of power. In many cases, you hardly increase the boat speed at all and you can have large negative effects on efficiency.
The distinction that I want to make is between usable power and engine size. The plot that Matt shows scales reasonably well across engine sizes and if you notice the units, they are really efficiency units, not direct power or fuel consumption. If you were only to operate the engine at a single load and everything else was equal, you would choose an engine that put you in the efficiency island at this one load which in this case would probably be at 2200 rpm or so. By choosing an engine that runs at this speed and gives you your desired output, you would be choosing a much larger engine than you would if you simply looked at the maximum output. In a sailboat, I like to have some reserve so you can’t size exactly this way but going with a slightly bigger engine that is “overpropped” can yield some efficiency boost if you are really careful about specifying things. This can definitely be taken to excess and really only makes sense for people who generally use close to the maximum horsepower they want. There are many other problems as well such as size, weight and cost.
Engine fuel maps generally result in approximately constant cylinder pressure across the rpm band at WOT. If efficiency was constant (it isn’t), then the torque curve would be flat and horsepower would be linear with rpm. For engine designers, the easiest way to get more power out of an engine is to run it faster. Since you get about the same amount of usable work out of each combustion stroke, if you can do more of these events in a given amount of time, then you have more power. Therefore, there is a lot of incentive for engine manufacturers to make higher revving engines as they are smaller and less expensive. The unfortunate thing is that the efficiency falls off at higher rpms for a few reasons so it is not a great place to operate for long periods of time. By changing your prop loading, you are effectively changing the maximum rpm of the engine.
Eric
Hi Eric,
Great comment, thank you. I thought I understood that area well, but I’m even more clear on it now.
Matt’s articles and your comments between them have got me to the point (finally) where I really get the theory behind what I have seen on our boat where going from a 120 HP 2800 RPM engine to a 87 HP 2400 engine that we have slightly over propped at 2350 WOT has yielded such a large fuel consumption and range benefit.
Interestingly the new engine has almost exactly the same max torque as the old.
Eric and John: This is precisely the reason I chose to install a Variprop, a design that allows in-water pitch changing and different pitches for reverse and forward rotation. When I bought the prop, it was some years prior to replacing our engine (I do everything out of order when it comes to boat gear). All I could discern was the rough displacement of the boat (prior to loading for passagemaking, so I added one tonne!) and my desire to have a “sweet spot” of maximum fuel economy at between 4.5 to 5.0 SOG in flat water. This is turning out to be about 1,900-2,000 RPM, indicating I might wish to alter the current pitch somewhat. But I intend to “prove” this empirically through fuel consumption recording, but the main point is that I will see the real world fuel map from the prop end, not the engine end.
That RPM, by the way, plus the four blades on the prop, gives plenty of power left to expend when I needed it. I have done some roaring emergency stops and I think we’ll be OK!
Hi Marc,
Just to clarify, while the variprop will let you change pitch, it does not do that dynamically under load, (as far as I understand) and therefore it will not solve the basic problem that this post is about, and more than a properly specified fixed prop or a Maxprop would.
As far as I know, the only dynamically re-pitching propeller on the market is the Autoprop, although PYI inform me that Maxprop are working on one.
No, that is correct…it does not vary pitch “dynamically”, but my point was (in our application) our “motorsailing” RPM is largely fixed, or will be, at the point of greatest fuel economy for a rather arbitrary “we will get there when we get there” speed through flat seas…otherwise we would be sailing! So the pitch will be adjusted pragmatically to find that sweet spot. I have a friend with an AutoProp on a steel ketch with roughly our dimensionals, and while it’s a thing of beauty, we wanted a slightly different approach that would allow differential pitching for forward and reverse. I didn’t know about the MaxProp initiative, so thanks for that.
Hi Marc,
While you are right that you can over pitch to get the sweet spot in flat seas at cruise in theory, you will then not be able to reach full revs at WOT. Worse still, I think you will find that if your engine manufacturer finds out, they will, as Matt says, void your warrantee because of the risk of lugging.
Further, no matter how you set the prop, you will be very inefficient while motorsailing. That’s just the way it is for all of us with props that don’t dynamically repitch, and the only way to fix that is to have a prop that can be made more course while motorsailing. If you were to set your prop to be efficient when motorsailing (very course prop setting) you would damage the engine in very short order due to lugging when not motorsailing.
None of this is to say that you should not overprop a bit—as we do on our boat (100RPM)—but I’m guessing that you will not be anywhere near the sweet spot for efficiency, as Matt explains in the post, unless you overprop a lot, and that has the issues I cover above.
John, thank you for the further explanation. As I’m not going to swap out this prop, I will merely aim for the “slightly overpropped” zone. We really do intend to sail as much as we can (which is where the feathering comes in; I have a Gori folder on my other boat and was mightily impressed by the difference between our previous fixed prop and the Gori). I will be happy if we are at 0.8 U.S. gallon/hr, given the boat, loaded, will weigh about 16 tonnes. I’m less interested in the ultimate speed…unless we need to stop quickly, and the engine can lug all it wants for the five seconds I need to hit full reverse. Thanks again for the clarifications.
Hi Mark,
Happy to help. This stuff is right confusing and I’m only just getting my head really around it after three re-powers and endless tweaking.
One of the key things I have learned is that one must be careful about motorsailing too much because in that configuration if the sails are doing anything at all, and they are usually doing more than you think, then the engine is horribly under loaded and glazes up very quickly.
One thing you can do is to run the engine at WOT, or close too it, for 30 minutes after motorsailing.
I think I may have implied something I didn’t mean with the term “motorsailing”, John. We actually intend to motor (with perhaps only a staysail to steady us if needed, and bare poles if not) when we absolutely must. We will sail as long as we don’t wallow or slat, as we hope to be going nowhere fast, which is the case for any boat of our type, anyway. The plan is to motor at 75%-80% of WOT should we need to up anchor to get out to sea for the necessary pumping out…and then to come back, alternators spinning and batteries topped when we return. But for almost all passagemaking, I hope to merely sail. As I said, if you want speed, get on a plane!
Anyway, it’s a big topic and thank you for tabling it.
Hi Matt
Two question concerning the propeller curve formula in the footnote.
I assume that k in that formula is a constant specific for a propeller, probably mostly related to pitch. Is that right?
I just changed my propeller from a fixed pich 2-blade folding prop to an autoprop. Since this is an automatic pich propeller I assume that the propeller curve will look different. Would it be reasonable to assume that n will be nearer 1 so the curve is more liniar?
The Bruntons Autoprop (which we briefly mention about two chapters from now, stay tuned…) is a pretty clever little gadget. Its purpose, essentially, is to straighten out the propeller curve (the blue one in the charts discussed above). The zero point and the maximum-power point are unchanged from the equivalent fixed-pitch prop, but in between, it adjusts its pitch so that torque is (almost) a linear function of RPM. Thus, it tends to avoid the inefficient lower-right corner of the fuel map.
As for “k”, it’s just a scaling constant for this rule of thumb; you can think of it as combining diameter, pitch, cup, rake, disc area ratio, etc. This rule isn’t meant for prop selection, it’s just to describe the approximate relationship between torque and shaft speed when you’re assessing an engine spec sheet.
John,
Thanks for the clarification regarding motorsailing being potentially “bad” for our engines. Your suggestion to run the engine at or near WOT for 30 minutes after makes sense. I suppose it would also apply to a situation where for safety or other reasons, while motoring only, one had to under rev for a period of time, then if possible, one should go to WOT for awhile before shutting down. We have a Maxprop that was originally set by the PO, but obviously could be re-pitched on the next haulout. I am intrigued by the notion of over pitching. Also, is there some neat empirical way to judge engine health beyond a compression test and oil health?
There the exhaust gases temperature sensor, which I recall John discussing here within the last year or so. It’s a nice bit of kit that could save your engine.
Hi David,
A sure sign of a healthy engine is a clean exhaust. Any smoke at all indicates issues. Also an oily sheen on the water from the exhaust can often be the first sign of cylinder glazing.
And yes, you are right, it is a good idea to run your engine hard after any prolonged period of slow running.
Hi Matt et al,
We have recently removed our old generator and are planning not to replace it (for a whole bunch of reasons) by using our new 130 amp Mastervolt alternator with smart regulator and 4 new 135 W solar panels. Our contingency plan is to use our main engine to charge the batteries if the solar isn’t providing what we need, say in winter. From re-reading your article Matt, this would be fine on passage, but at anchor is more problematic. The new alternator at full load takes 5hp from our Volvo 55hp engine. We have a manual cut-off function (should we need full power to the prop when motoring), and I can hear the engine load up as I bring the alternator on-line, and vice verse.
I have also experimented with engaging reverse gear to around 1500 rpm (max is about 2800) to add further to the load at anchor, and Bonnie Lass settles nicely, pointing about 15 degrees to starboard of normal, with the chain rode stretching out steadily. Since we don’t run a freezer and our electrical load per day at anchor is modest (less than 100 amp hrs), can anyone see an issue with running the engine in reverse for an hour or so on the occasions we need to? Any other ideas?
Thanks,
Rob
Hi Rob,
Since you will not be doing this often or for long (no freezer), I would not worry about running in reverse to load the engine. At 1500 you will still be underloaded, even anchored, so the benefit is not that great, when balanced against fuel use and wear on the drive train.
What I would do is make sure that you regularly run the engine hard, at or close to full throttle, while underway for at least an hour. That will tend to keep the rings seated and will scrape off the gazing that will form when you are charging.
More on the generator/engine trade off here: https://www.morganscloud.com/2014/04/24/do-you-need-a-generator/
Well, there’s always a reason to upanchor a liveaboard, and that’s to pump out beyond the statutory horizon. So I would consider running the engine to charge in that context of blasting out at relatively high RPMs to the legal pump overboard distance if you need to charge the banks in the absence of sunshine sufficient to keep the amps flowing. Barring that, if you risk losing “your spot” (although even with a big holding tank, you’ll have to up-anchor at some point), you might consider either a wind gen of the fixed or temporary (hoisted) kind, and/or a Honda genset, taking all necessary precautions of course to keep the exhaust out of the boat. You don’t state the size of your house bank in the context of 100 Ah/day draw down, but it would be a rare situation indeed to have full overcast and no wind for enough days to merit starting the engine to make amps.
We will have a very similar “no genset” setup, on the basis of a bigger bank (750-11oo Ah) in order to store the power we can make with identical solar (4 x 135 W in a cambered arch array) and a 400 W wind genny, and a pair of 90 amp alternators. Again, I can see reasons to motor out for a few hours at least once a week to deal with the human factor, so to speak, and that’s when I would charge batteries, make water, and make ice cubes…just pick a relatively flat day, or motor sail. Easier on the ground tackle.
Hi Rob,
Another take on this with basically the same conclusion as John.
Light loading of an engine can be a problem when you don’t get up to temp. This is a much bigger issue for engines in over the road trucks than it is for boats in general as they often idle all night. These engines employ a few techniques to combat low idling temperatures, some of which apply to boats. The most basic technique is to bring the idle up to “high idle”. Since your goal is battery charging, make sure to bring your rpm up high enough for the alternator to throw full current and so that your temp gauge is at normal running temp. The next technique they use is some form of exhaust backpressure, usually through the application of an exhaust brake or closing the vanes in a variable geometry turbo. I would not play with backpressure on a boat partly because you would need extra hardware and partly because there is more to go wrong. Exactly what you need to do is very engine specific, some engines can idle all day long and not have a problem and others have problems even at high idle. If you see signs of raw fuel in the exhaust or the temp gauge is not up, you should increase engine speed or put a load on it.
Given your solar setup, I expect that you will be fine the vast majority of the time provided that you have no panel shading and a decent sized battery bank. We have a single 140W panel with a tiltable mount on an MPPT controller which can just keep up with our ~75Ah draw from late spring to early fall. Very occasionally, we need to use the engine at anchor to charge but this is pretty rare except for the end of this season when our failing battery bank didn’t have enough reserve to get us through cloudy stretches and short days.
You may want to think about what time of day you want to charge. If your batteries are relatively full, you won’t be able to use the full potential output of your alternator as you will be voltage limited meaning that you have to run the engine for longer for the same amount of energy input to the batteries. From the standpoint of getting the most charge per unit of engine run time, charging when the batteries are low is best. This usually means charging in the morning and having to guess at your solar output for the day. Solar is much better at topping off batteries as its output is lower but for a longer period of time. If you have a big enough battery bank and don’t like guessing, you can play the game of charging right before you go to bed and only bringing the batteries up high enough to keep you above 50% SOC all night. Assuming you have lead acid batteries, you should get them up to full charge pretty often.
From a safety standpoint, I don’t really like leaving the engine in gear and not being there. On a previous boat where the engine took way too long to come up to temperature, I would wait 1 minute after startup and then engage reverse for about 10 minutes before going to high idle so that the engine got to temp relatively quickly. Idling large engines up to temperature can be really bad for them but putting enormous loads on a cold engine isn’t great either so some middle ground makes sense. If you do decide to use reverse a lot, check your transmission manual and make sure they don’t recommend against it.
Eric
Hi Eric,
Lot’s of great recommendations, thank you. I particularly like your tip on the timing of the generator run in relation to the battery state of charge solar panel charging.
Thanks everyone,
Really helpful comments and suggestions as always on this site – very cool.
For completeness, we have the equivalent of an 800 Amp hr house bank. A mobile generator would be ideal for our needs, but we don’t like the anti-social element associated with their use in quiet anchorages.
In summary, I feel happy our occasional use at anchor should be fine then, if we watch the temperature gauge on the cooling water following Erik’s regime, run the engine close to full stick for an hour as per John’s advice, and better still “head out” to lay down a Marc!
cheers, Rob
Much as it may be illogical, I have a philosophical issue with running the engine without moving. Experience also suggests that a Honda 2000 running on a vibration dampening pad may be quieter than running a 40-60 HP diesel, at least from the point of view of trying to nap below decks on a nearby boat. I keep expecting them to leave…and they don’t!
Matt, a couple of questions regarding BMEP, EGTs and related issues raised here. Because the technology is essentially identical for truck diesels and marine diesels, I think I have a handle on where to tap a hole for an EGT probe.
1) Do you have a preferred brand for the typical 40-75 HP marine diesel?
2) Do you think a 1/8th inch probe would be a better choice given the comparatively skinny manifold outlet of such diesels (at least, compared to that of a truck or pickup diesel)? Not only would I prefer the smallest probe possible, I don’t want said probe to impede exhaust flow more than necessary. In fact, if I could avoid the hole and get a contact probe for the outside of the short pipe between the manifold and the mixing elbow, that would be my preference. If such devices exist, I’m not seeing them readily,
3) Do you think it is necessary to also know the manifold pressure in the small marine diesel application? I am seeing some “dual dial” probes where this can be achieved, but I’m not sure it’s necessary to ballpark the conclusions one can get from just RPM vs. EGT.
Thanks. Ever onward in the voyage of learning!
Wait, I spoke too soon. I think I just saw something called a “clampable thermocouple”. Any thoughts on the accuracy of this versus a tap ‘n’ drill probe?
Hi Marc,
This will not be nearly as accurate as it measures the outer wall temp of the manifold. In a water cooled manifold, there will be an enormous difference and with an air cooled one, the temp will be closer to that of the exhaust gases but it will be lower.
Personally, I would not feel the need for an exhaust pressure gauge. Exhaust pressure is important but all it tells you is how much air your engine is pumping and how much restriction your exhaust has both of which you easily design for while being pretty conservative. Where you may run into issues is if you run underwater exhausts.
Eric
Hi Mark,
VDO makes a EGT. Installing it is really not that big a deal. It comes with a small threaded boss that must be welded to the exhaust elbow, but any machine shop should be able to do that for a few bucks if you remove the elbow from the manifold and take it to them. The probe is way too small to cause any restriction, so no worries there. The gauge comes with instructions on where to place it for an accurate reading. I guess what I’m saying is it’s not worth reinventing the wheel on this one.
I used to think that EGTs were only for the anal retentive (like me) but the more I learn about getting the best out of an engine, the more I think they are worth the trouble and expense, particularly if your boat is overpowered, and I’m near-certain your boat is.
Quite probably, although as I noted, I have at least one and a half tonnes of further loading to do, which will provide a clearer picture. I do appreciate the insights, however.
One thing that is interesting I am finding in speaking with local sailors I consider knowledgeable (for one thing, they monitor RPM and fuel burns!) is that over-propping and consequently reaching hull speed in the lower part of the 2000 RPM band, rather than closer to the max RPM of 2700-3500 RPM seems quite common to the point where we’ve collectively wondered if overpropping to give impressive torque for maneuvering isn’t the “industry standard”: distances here are short and the wind can be fickle in the summers, so the penalty for burning fuel and lugging the engine somewhat versus the feeling of control when revving in tight docking situations may play a role.
Of course, offshore in the Doldrums the opposite is true and maximum fuel economy (and diesel preservation) is paramount. I think I’ll take one inch of pitch from my prop the next time I haul and get an EGT installed for a further data point. I would be pleased to get to even 2,400 RPM (300 below max. RPM) thereby.
Hi Marc,
I swing back and forth between amusement and pain when I hear many people’s comments about torque and I fear that your marina friends may have fallen into one of the classic torque comment traps. When it comes down to it, torque is just that, a measure of twisting force but it tells you nothing about how much useful work you can do. Horsepower is a measure of how much work you can do per unit of time. If you think about stopping a boat, the boat has a certain amount of kinetic energy and it takes an equal amount of energy to stop the boat. The more energy per unit of time you can do, the quicker the boat will stop. Because torque doesn’t tell you anything about time unless you also give an rpm, it will not tell you how quickly a boat will stop.
I find the easiest way to understand torque to be using a car analogy. Pretend that we have a car where fourth gear leaves you at 4000rpm and fifth gear puts you at 2500rpm when traveling 60 mph. At 4000rpm, your car produces 100hp and 131 ft lbs of torque while at 2500 rpm, your car produces 80 hp and and 168 ft lbs of torque. For maximum acceleration, which gear should you use? The answer is 4th gear running 4000rpm. Why? You can simply look at the power numbers and see that the engine produces more power at the higher rpm (I am using a car here and not a boat because boats are load limited by their propellers). With more power, you put in more energy per unit of time which means that after a given amount of time, you have more kinetic energy or more speed. Alternatively, you can solve it by torque. We know that fifth gear has 62.5% the gear reduction of 4th gear as we know our road speed and our engine rpm so when we correct for this, we find that the equivalent torque is 80% of the fourth gear torque or exactly what we already knew from looking at horsepower.
I did this example very quickly but for anyone looking to really understand horsepower and torque, I encourage you to actually spreadsheet out your car’s driveline. The end goal is to get the most force between the tire and pavement propelling you forwards since force is equal to mass times acceleration. Starting at the engine, you take your rpm and torque, multiply both by your transmission ratio, multiply both by your rear end ratio and then your tire diameter giving you the speed of the wheel and the force propelling you forwards. What you will find is that torque by itself is a meaningless number to acceleration without knowing rpm which actually means you know power (hp=torque*rpm/5252 in english units). The higher the speed the torque occurs at, the better because you can gear down more and turn it into more force propelling the car forwards. One way to think about it is torque is a measure of how much cylinder pressure you have and horsepower is a measure of both this pressure and how often that cylinder fires.
Hopefully this helps. And Marc, I am not trying to pick on you but I fear your dockmates may be mistaken about torque as stopping power is all about putting power into the water and max power is almost always at a higher rpm than max torque.
Eric
Hi Eric,
Thanks very much for yet another really great explanation of a much misunderstood subject. I know that I, for one, am much clearer on the issue than I was before reading your comment.
I would also add a quote from…Erik Klem that really helped me get a better grip on the subject.
Hopefully I have quoted you correctly?
Of course, maximum work/fuel at max torque only gets done if the engine is fitted with a way to match torque to the load of the propeller (CPP or the like). I think I have that right?
Eric, thank you for your thoughtful and comprehensive reply. I’m aware I’m using “torque” in not the specialist sense but the generalist sense of a force multiplier, as in the leverage advantage of, say, a long wrench.
A good 2000 rpm “shot” of engine power in reverse with a four-bladed prop at its current pitch both stops our heavy boat at a speed slightly less than dead slow (because I go into neutral to bleed off speed as I dock) but kicks the boat’s stern toward the dock thanks to favourable prop walk. I realize this has less to do with torque than it does just raw power as expressed with water being moved against the direction of travel.
Different prop pitch values in forward and reverse are as close as I can get in the absence of investing in a controllable pitch prop, which is indeed the ideal solution. Had I known of it when I was shopping for a new prop for a new engine, I would have considered it.
Hi Mal19
Yes, there are some big benefits to the Autoprop. That said, historically there have also been some reliability issues. Not sure what recent experience has been. The other issue is that the Autoprop requires a substantial shaft lock on any boat with a hydraulically activated transmission. And shaft locks are a royal pain in the neck.