Diesel RPM?

[Lee Jerry]

Originally Posted by Wotname

Yes.

Assuming you prop is in the ball park, I would suggest running at 1500 is pretty close to ideal. See attached power / torque curves for the M-40. I have added the red lines.

So it is being loaded at ~19.2 hp from a max of 32 which is ~61% and running near the peak of the torque curve. If it was my boat, I'd be very happy.

That is the max output line, not a (the) propeller curve. For that engine at 1500 rpm, the prop will absorb (i.e. on the prop curve) about 5-6 hp. So ~17% MCR.

To check how close your prop is to ideal, run it at WOT in calm water with a CLEAN prop and bottom. If you can make 2600 rpm or better, the prop is suitable.

IMHO, the engine needs to reach rated speed (i.e. 2800 rpm) for the prop to be suitable. Anything less is over-propped.

Because I'm a belts and braces guy, I'd still run it at WOT for say 10 mins every 2 hours.

I agree with this concept, even if the details may vary - say >2400 rpm every 6-8 hr.

[boatpoker]

Originally Posted by Lee Jerry

IMHO, the engine needs to reach rated speed (i.e. 2800 rpm) for the prop to be suitable. Anything less is over-propped.
.

Slight over propping is not a bad thing.

[NBs]

Originally Posted by NBs

If your propeller is dimensioned correctly, then it seems optimal speed for your boat and the engine gets enough load.

NBs

Universal-40 /32hp On a clean hull, with a full throt, it should reach 2800 rmp, when propel is optimal.

[boatpoker]

Originally Posted by NBs

Universal-40 /32hp On a clean hull, with a full throt, it should reach 2800 rmp, when propel is optimal.

I disagree.
2800rpm in neutral is good. A 1/2" over propping will bring that down to 2500 - 2600 rpm at full throttle which will still get this vessel to marginally over hull speed.

In that state she should be able to run just under hull speed all day long at spec'd temperature and oil pressure which is what the OP is doing.

[Lee Jerry]

Originally Posted by boatpoker

I disagree.
2800rpm in neutral is good. A 1/2" over propping will bring that down to 2500 - 2600 rpm at full throttle which will still get this vessel to marginally over hull speed.

In that state she should be able to run just under hull speed all day long at spec'd temperature and oil pressure which is what the OP is doing.

If this engine is propped as you suggest (2500-2600 rpm @ WOT), then it will only produce ~30 hp, rather than the rated 32 hp. Why give up ~6% of your available power in the best case? It will only get worse as drag increases and the prop loads up more (weight, fouling, sea state, etc.) and the limit is reached at even lower rpm.

[Island Time O25]

I have Universal 5432 pushing 17,000# boat, >20,000# fully loaded and crewed with fixed 3 blade prop. Some years back replaced 50Amp alternator with 100Amp one. Sure it lowered specked hp output but other than that the engine runs fine. Those are tough old school workhorses.

[donradcliffe]

It all depends on how you use the boat. If you are happy with 5.6 knots, the engine run 1500 rpm for at least 12,000 hours. If you are a wanna get home fast on Sunday afternoon guy, put a prop on that will max out at 2600 rpm. That will burn more fuel, leak more oil, make more noise, and will tend to overheat when the cooling system gets a bit fouled. If you have limited tankage, cut the rpm down to 1100 and go 40% further. If you have a turbo, you will need to run the engine up periodically, but you already knew a turbo was not a good look unless you are the Sunday afternoon guy.

[Wotname]

Originally Posted by Lee Jerry

That is the max output line, not a (the) propeller curve. For that engine at 1500 rpm, the prop will absorb (i.e. on the prop curve) about 5-6 hp. So ~17% MCR.

I agree the prop curve is a better source but I couldn't find a reference for the prop curve for the M-40; can you post one?

IMHO, the engine needs to reach rated speed (i.e. 2800 rpm) for the prop to be suitable. Anything less is over-propped.

I agree that ideally the engine should make max rpm however if it is slightly over propped (i.e. can only make over 2600 but less than 2800), the OP will have a sufficiently suitable prop. His main question indicates he mostly runs at 1500 and it is unlikely he will want to fine tune the prop if is close enough.

[NBs]

Originally Posted by donradcliffe

It all depends on how you use the boat. If you are happy with 5.6 knots, the engine run 1500 rpm for at least 12,000 hours. If you are a wanna get home fast on Sunday afternoon guy, put a prop on that will max out at 2600 rpm. That will burn more fuel, leak more oil, make more noise, and will tend to overheat when the cooling system gets a bit fouled. If you have limited tankage, cut the rpm down to 1100 and go 40% further. If you have a turbo, you will need to run the engine up periodically, but you already knew a turbo was not a good look unless you are the Sunday afternoon guy.

The Marine turbo has only been in use for 100 years, apparently only in Sunday use. With Okei's dimensioned engine, propeller and service interval, the turbo is reliable and long-lasting if the exhaust pipe is built correctly.

Couple articel
https://www.sbmar.com/articles/what-a-marine-diesel-turbo-should-look-like-when-inspected/

https://www.sbmar.com/articles/new-turbo-vs-27000-hours-turbo/

Avoid turbo if you think an over prop is good or an oil and filter change is just finesse.

Turbo & commonrail ❤️

[boatpoker]

Originally Posted by Lee Jerry

If this engine is propped as you suggest (2500-2600 rpm @ WOT), then it will only produce ~30 hp, rather than the rated 32 hp. Why give up ~6% of your available power in the best case? It will only get worse as drag increases and the prop loads up more (weight, fouling, sea state, etc.) and the limit is reached at even lower rpm.

1971 Perkins HT6-354T. spec'd at 2800RPM
Photo taken at 20,000hrs. on that engine.

Run at 1600RPM with 1.5" over prop @ 1.6gph at 10nmph with temp and oil pressure within spec. in a 37' vessel with a 36.2" waterline

Nope, I have no problem with some over prop. altho' I did have to replace a lift pump once

[Lee Jerry]

Originally Posted by boatpoker

1971 Perkins HT6-354T. spec'd at 2800RPM
Photo taken at 20,000hrs. on that engine.

Run at 1600RPM with 1.5" over prop @ 1.6gph at 10nmph with temp and oil pressure within spec. in a 37' vessel with a 36.2" waterline

Nope, I have no problem with some over prop. altho' I did have to replace a lift pump once

And all that has absolutely nothing to do with the question I asked.

What do you think the benefit(s) is (are) of over-propping? If you could give a direct answer to that question, I can (most likely) explain why that is not the case and also give a handful of reasons for why it is bad (including the one you already ignored).

[Cheechako]

Originally Posted by massnspace

The diesel is a Universal M-40 (32 HP I believe), and the boat is a 32’ long by 10’ beam long keel cruiser that weighs 15,000 lbs.

Again. I cruise all day long at 5.7 knots when at 1500 RPM. Is that OK? Is the motor “under sufficient load”?

To me that could be a bit low, but you need to know your hull speed. What does the rpm max out at wide open in gear? Is it at hull speed when doing so? or does it reach hull speed at lower rpm? But I also believe you dont need to run a diesel as high as many seem to think.
Most engine prop combinations there is a sweet smooth spot where it also develops good horsepower on the curve and reaches near hull speed. That's about where I want to be set up for, then can go slower if I want.

I would say it's under sufficient load, if anything it could be loaded a bit much if you are reaching good speed at a low rpm, hard to say without a HP curve and hull speed.

[Lee Jerry]

Originally Posted by Wotname

I agree the prop curve is a better source but I couldn't find a reference for the prop curve for the M-40; can you post one?

The "actual" prop curve is a function of the prop characteristics, the hull resistance characteristics and environmental factors. The prop performance curves are defined by the thrust coefficient (Kt), torque coefficient (10Kq) and efficiency (eta). Hull resistance is the hull shape, but changes with weight and fouling. Wind and sea state change, as well as the heading with respect to them. Change any one of those things and you have a "new" prop curve. So there is no one prop curve (but rather an infinite number of them).

A "nominal prop curve" is used to simplify all of those unknowns. It is a basic exponential curve (for a displacement hull) - P = a*RPM^b. The exponent 'b' gives the shape to the curve and the "real" value is a function of all those variables above; typical values range from 2.5 - 3.0, with heavier/fatter at the higher end, lighter/finer lower. 'a' is then determined to hit the rated speed at rated power point. (Or, if you're trying quantify a "known" prop/trans/eng system, use the intersection of max RPM reached at WOT and the max power curve.) So if we use an exponent of 2.7, then:
a = 32/2800^2.7 = 1.577e-8

And the prop curve I propose is:
P = 1.577e-8 * RPM^2.7

I agree that ideally the engine should make max rpm however if it is slightly over propped (i.e. can only make over 2600 but less than 2800), the OP will have a sufficiently suitable prop. His main question indicates he mostly runs at 1500 and it is unlikely he will want to fine tune the prop if is close enough.

Well, I don't know what anyone else thinks is "close enough." But I'll ask you the same question: what is the benefit(s) of being over-propped? (Other than not buying a new prop.)

[boatpoker]

Originally Posted by Lee Jerry

And all that has absolutely nothing to do with the question I asked.

What do you think the benefit(s) is (are) of over-propping? If you could give a direct answer to that question, I can (most likely) explain why that is not the case and also give a handful of reasons for why it is bad (including the one you already ignored).

More thrust/control in tight quarter manouvers and lower fuel usage without hurting the engine at all.

[Lee Jerry]

Originally Posted by boatpoker

More thrust/control in tight quarter manouvers and lower fuel usage without hurting the engine at all.

Thanks for the direct answer. However, none of those are given as true, and are quite possibly not true. I'm curious if you have any proof or citations in support. The first two are most likely to be the same between the two props (to the level that one could notice or reasonable measure), with smaller but equal chances that either could be slightly better than the other (proper vs over-).

For example, the over-propped boat will probably have a slightly higher minimum speed in gear (at idle speed, depending on the engine design) - that doesn't seem like a good thing to me in tight quarters. I think it will also take an extra second or two to get good flow over the prop when switching gear as the higher pitch churns the water more. Again, not as good.

As for fuel consumption, there are several ways to address and I'm not sure which is best. How about: at a given boat speed, the over- will operate at a lower RPM, but (essentially) the same power and therefore (essentially) the same fuel consumption. Again, the details could make either prop slightly better. Basically, the resistance is the same and the propulsive coefficient is essentially the same, therefore the FC is the same.

And then there are the negatives: mainly the reduced range of performance as conditions become more demanding, but also reduced power available, reduced RPM range available, higher probability of cavitation (at all speeds),...