Engine RPM's low, wrong pitch?

[a64pilot]

The generators that I am familiar with that have engines that are run at higher RPM in other applications, are de-rated. Meaning for example a 24 HP engine at 3600 is a 14 HP engine at 1800 RPM. Since they are only trying to extract less HP out of the same engine at lower RPM, the effect is longer engine life.
However if you leave the max fuel flow stop at the same flow rate for the higher HP, yet load the engine down so much that it's RPM decays excessively, then you will likely damage it if you continue to operate it that way.
A great many people have the experience with engines that they can tell from the sound when an engine begins to labor, and they won't advance the throttle past that point, they can safely operate an engine overpropped, cause they are derating it.
You of course realize and accept that you have less HP to work with, your OK.

The biggest reason the fuel burn is less on an overpropped boat is the HP is reduced, your not significantly increasing the Brake Specific Fuel Consumption of an engine by lugging it, there are some frictional losses, but they are almost un-measurable.
I know cause I have run those test flights with aircraft equipped with constant speed props, as much as we would like to think we can out smart the engine manufacturer it sort of takes X amount of fuel to produce X amount of HP

[david knowles]

diesels like to run and they achieve rated horsepower at rated rpm the way to wear them out is to lug them and not allow them to get to rated rpm. give you motor clean air, fuel and oil and Rs and they will be happy

[Cheechako]

As I stated earlier, it's all about the HP your boat needs. Diesel engines do not come in infinite models and HP. So if a yacht needs 18 HP the builder may put in a 25 HP engine, or may skimp and put in an 18 HP engine. In most engines today the 18 HP engine will need to run at high rpm to make the HP required. Thus it's recommended to be run at max rpm.
"maximum torque is at 2400 rpm and maximum HP at 3600. 3820 is the maximum 'high idle', no load speed." So if this engine has adequate HP for the boat at 2400 rpm, no problem running it at that speed.


-Truckers idle their diesel for many hours on end. They get very long life.
-Large boat diesels are sold at different HP and rpm rating for the SAME ENGINE. I know, I used to specify them sometimes for build. So if you go to say the MANN diesel builder for a 800 hp engine, it may be rated at 3000 rpm for a pleasure craft intermittent use. 2600 rpm for commercial use. and 2400 rpm for 24/7 use. The same engine. Why? because they want it to last longer in 24/7 use.
Of course there are a multitude of variables, in deciding what rpm you can run yours at. Just running lower or higher rpm isn't the only thing.
BTW, both my Yanmar diesels in my cat failed at less than 2500 hours. They were set up to run near max rpm. Piston failure. A small engine rated at HP it only produced at very high rpm.... that way they can call it "lightweight".

[jimbunyard]

Originally Posted by Cheechako

As I stated earlier, it's all about the HP your boat needs. Diesel engines do not come in infinite models and HP. So if a yacht needs 18 HP the builder may put in a 25 HP engine, or may skimp and put in an 18 HP engine. In most engines today the 18 HP engine will need to run at high rpm to make the HP required. Thus it's recommended to be run at max rpm.
"maximum torque is at 2400 rpm and maximum HP at 3600. 3820 is the maximum 'high idle', no load speed." So if this engine has adequate HP for the boat at 2400 rpm, no problem running it at that speed.


-Truckers idle their diesel for many hours on end. They get very long life.
-Large boat diesels are sold at different HP and rpm rating for the SAME ENGINE. I know, I used to specify them sometimes for build. So if you go to say the MANN diesel builder for a 800 hp engine, it may be rated at 3000 rpm for a pleasure craft intermittent use. 2600 rpm for commercial use. and 2400 rpm for 24/7 use. The same engine. Why? because they want it to last longer in 24/7 use.
Of course there are a multitude of variables, in deciding what rpm you can run yours at. Just running lower or higher rpm isn't the only thing.
BTW, both my Yanmar diesels in my cat failed at less than 2500 hours. They were set up to run near max rpm. Piston failure. A small engine rated at HP it only produced at very high rpm.... that way they can call it "lightweight".

In reverse order...

While you may have actually gotten an 'anti-Granny' (it'd be nice to know what engine model we're talking about here...), where, even though she lived a healthy macrobiotic life, wound up kicking off at sixty, from your somewhat sketchy synopsis it sounds like your short-lived engines were the victim of the very policy you appear to recommend.

At least if I interpret what you said correctly...

Again, the generally accepted way of propping a boat to insure maximum efficiency (in this case defined as longest life and best fuel economy) is to use a prop that allows the engine (with a clean hull and prop and full load) to achieve a certain percentage of rpm over the rated maximum, in-gear rpm, and then running the engine in the proper rpm range for it's duty cycle rating.

So, that you "set up (your engines) to run near max rpm", could mean that you were either over-propped or under-propped; the problem is that no manufacturer recommends that you run their propulsion engines at max RPM constantly, regardless of the duty cycle, and in most cases doing so will void their warranties.


Which brings us to, "Large boat diesels are sold at different HP and rpm rating for the SAME ENGINE."

Yes, and it's typically called 'Duty cycle rating' or something like that. It doesn't have much (if anything) to do with prop sizing, the procedure is the same, as described above; the prop is sized to match the RPM designated by the duty rating; i.e. we don't expect to see the same prop on a harbor tug as we'd see on a canyon runner, even though they have the same engines.

""maximum torque is at 2400 rpm and maximum HP at 3600. 3820 is the maximum 'high idle', no load speed." So if this engine has adequate HP for the boat at 2400 rpm, no problem running it at that speed."

Agreed, as long as the engine is propped correctly, as described above, to reach a little more than max rated rpm, and then run in the manufacturer's specified operating range.

But if you're saying that an engine designed with an operating range of 2900-3200 rpm, and a maximum of 3600, can be operated at a propeller-governed 2400 rpm maximum with out accelerated wear, you are mistaken.

[Cheechako]

Originally Posted by jimbunyard

In reverse order...

While you may have actually gotten an 'anti-Granny' (it'd be nice to know what engine model we're talking about here...), where, even though she lived a healthy macrobiotic life, wound up kicking off at sixty, from your somewhat sketchy synopsis it sounds like your short-lived engines were the victim of the very policy you appear to recommend.

At least if I interpret what you said correctly...

Again, the generally accepted way of propping a boat to insure maximum efficiency (in this case defined as longest life and best fuel economy) is to use a prop that allows the engine (with a clean hull and prop and full load) to achieve a certain percentage of rpm over the rated maximum, in-gear rpm, and then running the engine in the proper rpm range for it's duty cycle rating.

So, that you "set up (your engines) to run near max rpm", could mean that you were either over-propped or under-propped; the problem is that no manufacturer recommends that you run their propulsion engines at max RPM constantly, regardless of the duty cycle, and in most cases doing so will void their warranties.


Which brings us to, "Large boat diesels are sold at different HP and rpm rating for the SAME ENGINE."

Yes, and it's typically called 'Duty cycle rating' or something like that. It doesn't have much (if anything) to do with prop sizing, the procedure is the same, as described above; the prop is sized to match the RPM designated by the duty rating; i.e. we don't expect to see the same prop on a harbor tug as we'd see on a canyon runner, even though they have the same engines.

""maximum torque is at 2400 rpm and maximum HP at 3600. 3820 is the maximum 'high idle', no load speed." So if this engine has adequate HP for the boat at 2400 rpm, no problem running it at that speed."

Agreed, as long as the engine is propped correctly, as described above, to reach a little more than max rated rpm, and then run in the manufacturer's specified operating range.

But if you're saying that an engine designed with an operating range of 2900-3200 rpm, and a maximum of 3600, can be operated at a propeller-governed 2400 rpm maximum with out accelerated wear, you are mistaken.

As mentioned, the failed engines were propped for high recommended rpm.
If you prop an engine to reach hull speed at slightly below max rpm, like maybe 3400 rpm for a 3800 rpm engine, then running it at 2400 rpm would make no sense anyway as you wouldn't reach hull speed. Here's just one example of ratings for the same engine at different rpm.

[a64pilot]

Originally Posted by Cheechako

If you prop an engine to reach hull speed at slightly below max rpm, like maybe 3400 rpm for a 3800 rpm engine, then running it at 2400 rpm would make no sense anyway as you wouldn't reach hull speed.

Hull speed for my boat is 7.7 kts, she will hit I believe it is 8 kts full throttle and at 3350 RPM on the tach, she will turn 3500 RPM in neutral, supposed to be 3600, I think the tach is a little off.
However she cruises at 7 kts at 2200 RPM, 6.5 at 2,000.
You don't have to cruise at hull speed, I can but its noisy as heck and I think I burn twice as much fuel, so why would I want to double my fuel burn by increasing speed by .7 kts?
Running at a lower RPM for decreased noise and vibration, increased engine life and doubling your fuel range makes a lot of sense to me.

[Cadence]

Originally Posted by a64pilot

Hull speed for my boat is 7.7 kts, she will hit I believe it is 8 kts full throttle and at 3350 RPM on the tach, she will turn 3500 RPM in neutral, supposed to be 3600, I think the tach is a little off.
However she cruises at 7 kts at 2200 RPM, 6.5 at 2,000.
You don't have to cruise at hull speed, I can but its noisy as heck and I think I burn twice as much fuel, so why would I want to double my fuel burn by increasing speed by .7 kts?
Running at a lower RPM for decreased noise and vibration, increased engine life and doubling your fuel range makes a lot of sense to me.

Hull speed is such a nebulous no.. You have the right idea when the engine sounds like it is beginning to labor you're there. Throw the theoretical no. out the window, save fuel and wear and tear.

[a64pilot]

I can tell you at eight kts, I am making quite a large wake, and I know where the energy to roll those big waves comes from too. Little Yanmar never strains, just revs, but to me it sounds like chainsaws mating.
Too much like flying an Ultralight. Ever fly one of those? After 10 min you start hoping for an engine failure
I'll run it up every now and again to blow it out, but I'd never leave it there, the noise is obnoxious and to me goes against every reason I have the boat.
I'd really love an engine that I could motor along at 1,000 RPM or so

[Cadence]

Originally Posted by a64pilot

I can tell you at eight kts, I am making quite a large wake, and I know where the energy to roll those big waves comes from too. Little Yanmar never strains, just revs, but to me it sounds like chainsaws mating.
Too much like flying an Ultralight. Ever fly one of those? After 10 min you start hoping for an engine failure
I'll run it up every now and again to blow it out, but I'd never leave it there, the noise is obnoxious and to me goes against every reason I have the boat.
I'd really love an engine that I could motor along at 1,000 RPM or so

At 8 knots she is probably squatting and throwing a large stern wave? I guess those that wish to go fast should buy a planning power boat.

Would like to have flown an ultralight but I am still alive.

[Suijin]

Originally Posted by a64pilot

Thankfully you have an adjustable prop, and I thought a maxprop could be adjusted without pulling the boat?

I have a "classic" MaxProp and adjusting the pitch in-water would be exceedingly difficult. I essentially have to take the prop apart in order to change pitch, and reassembling it without dropping a piece is one of those tasks where you wish you had three hands. Doing it underwater would be challenging to say the least, and I'd not do it unless it was in clear shallow water with a sandy bottom, lol.

MaxProp has changed the design with various models; more recent models may be easier to adjust.

[Suijin]

Originally Posted by a64pilot

However she cruises at 7 kts at 2200 RPM, 6.5 at 2,000.

That's impressive for a 38' full keel. What engine do you have and what prop?

I have a more modern underbody, a 44 hp Universal (although probably no longer 44 hp), and a 16" Maxprop, and I cruise at 6 kts at 2400 RPM. Maybe I need to change something lol.

[Perna]

I think it is fairly simple. You have engines which produce 21 hp at a rated speed of 3600 rpm. They will produce nothing like that power at 2200 rpm. In a real blow I would want all the power I can get, maybe to get me out of trouble. I would not want only 60-70% of the power I paid for, hence I would adjust the angle of the prop to be able to get more or less the rated speed when necessary.

I don’t have to run at rated power but it is a safety factor.

[a64pilot]

Originally Posted by Suijin

That's impressive for a 38' full keel. What engine do you have and what prop?

I have a more modern underbody, a 44 hp Universal (although probably no longer 44 hp), and a 16" Maxprop, and I cruise at 6 kts at 2400 RPM. Maybe I need to change something lol.

Yanmar 4-JHE, which is 44 HP at 3600 intermittent, 40 HP at 3500 RPM continuous, I'm propped for 3350 max, so I never get close to 44 HP.
It's an Autoprop which self pitches and acts like your overpropped at lower RPM, yet decreases pitch to let the RPM come up higher under power.
An Autoprop would be ideal I think on a Cat as when you needed the power it will let the motor turn up, but will overpitch at partial throttle.
Why I say ideal for a Cat is I think with only one engine running, your pushing a lot more boat with only one engine (half the thrust) as when you have both going, so I think the way the Autoprop self pitches, would be ideal.

However I wouldn't want to have to pay for two of the things

[mrybas]

Originally Posted by Suijin

I have a "classic" MaxProp and adjusting the pitch in-water would be exceedingly difficult. I essentially have to take the prop apart in order to change pitch, and reassembling it without dropping a piece is one of those tasks where you wish you had three hands. Doing it underwater would be challenging to say the least, and I'd not do it unless it was in clear shallow water with a sandy bottom, lol.

MaxProp has changed the design with various models; more recent models may be easier to adjust.

I re-pitched the port side from 22 degrees to 18 degrees today while free diving with the help of another cruiser in the San Blas Islands, Panama.
Starboard side tomorrow, then a test ride🤞

[mrybas]

Originally Posted by mrybas

I re-pitched the port side from 22 degrees to 18 degrees today while free diving with the help of another cruiser in the San Blas Islands, Panama.
Starboard side tomorrow, then a test ride🤞

I went for a test run. Both props pitched at 18 degrees (used to be 22) Max rpm was 2600 at about 8 knots. I think I'm going to change the pitch again. Should I try 14 or 16 degrees?
14 degrees should get me to 3100 rpm. Is there any disadvantage to running the props pitched so low (that's the lowest setting on the Max props). I guess these props are probably to large diameter?