Engine RPM's low, wrong pitch?

[Just Another Sa]

Originally Posted by mrybas

Calculated slip is .37
What does that mean?

It means reducing pitch is fine and sensible solution, if the prop really is over pitched. No reason for you to do anything for gear ratio or diameter, which is good news because that would have been more costly.

[Cadence]

Originally Posted by mrybas

Calculated slip is .37
What does that mean?

I'm guessing .37 is very optimistic? In the case of the 22inche pitch it means only advancing .63 percent of 22inches per revolution. To many variables to accurately calculate slip. Maybe on flat water between two locks and no wind a pretty good shot at it may be gotten with a particular boat and prop.. Even at that it is a SWAG at what RPM?

[Just Another Sa]

Originally Posted by mrybas

How much damage/ wear and tear does it cause running the engines while miss proped?

If you very seldom use more torque from the engine than it is designed to put out at max throttle with recommended RPM, there is hardly any wear worth mentioning.

Since it is very unlikely you know how much torque you are using by any instrument reading, engine specs need to be read to find torque curve from there. Power is mostly irrelevant, since that is already low even with max throttle in such revs.
There might be different versions for that engine, so I'm not sure the numbers I find match your engine, but this is what I found:
15,4 kW / 3600 RPM at max power means 40,85 Nm of torque at that RPM.
And max torque 47 Nm at 2600 RPM
Without the curve I can't know how much torque the engine can put out at 2200 RPM, but it is unlikely to put out much more than 40,85 Nm if even that, meaning the stuff in parenthesis below is irrelevant in case of your engine:
(If you do use substantially more torque continuously, it is tough for the bearings of crank shaft and shortens their life expectancy. This might or might not be an issue, depending how they are dimensioned. This is only relevant for those engines that can put out substantially more torque at lower revs than at max power, like 50% ... 100% more.)

Any black smoke coming out in continuous use is also a problem. Valves might collect unburned stuff and not close well after some time. This requires servicing (cleaning them) but mostly do no permanent damage.

If that is done at high power level (your engine can not do so with max RPM of only 2200) overheating internal engine parts can cause more serious issues due to lubrication oil running too hot.

It seams in your case you have an option to do nothing for your prop, and just continue using it. Your fuel efficiency is probably high, but your boat is lacking in power in high windspeeds upwind in a seaway if not sailing. But only if the reason for low max RPM really is over pitching, otherwise correct the problem what ever it is.

[a64pilot]

Low RPM and high fuel in a Diesel almost always means high EGT's, that can burn valves etc.
I had a pyrometer in my truck and could demonstrate that, I'd start pulling a small hill running heavy and EGT would start to climb, it would get up to close to 1200 F, I'd add throttle forcing a downshift (automatic transmission) and RPM would increase and EGT would drop instantly to 1000 F or less. More RPM = more air, more air = cooler EGT's
Aluminum alloy pistons melt at about 1400 F and the heat stress was not good for them.
A boat especially one overpropped is on a hill that never stops

[Cheechako]

Whether it's harmful or not depends on what HP your boat needs and the HP curve of your engines. If the bottom/props are clean, It sounds like you are over pitched though. I would shoot higher but I wouldn't pitch for anywhere near 3800 rpm. Maybe 3200.

[a64pilot]

Originally Posted by Cheechako

Whether it's harmful or not depends on what HP your boat needs and the HP curve of your engines. If the bottom/props are clean, It sounds like you are over pitched though. I would shoot higher but I wouldn't pitch for anywhere near 3800 rpm. Maybe 3200.

Don't forget he is a Cat, and those guys often run on one engine?

[Cheechako]

Originally Posted by a64pilot

Don't forget he is a Cat, and those guys often run on one engine?

Good point, so another factor in the mix should be: "How do you use your engines"! My cat had maxprops and Yanmar engines. It was pitched to do 3200+ rpm. I hated that rpm... damn cat sounded like I was driving an airplane at that rpm.
I only used one engine when motorsailing.... the leeward engine at high idle would point me up an additional 15 degrees or so.

[barnakiel]

The first thing is to check the book value 3800 or whatever AGAINST no load (neutral, relenti) max attainable revs. These should be very close. If they are not, it is neither pitch nor the train. This would imply fuel, air, exhaust or pump problems.

If the engine revs up fine at neutral, only then you look at the train - friction, prop size, pitch, fouling, etc.

If you have folding or feathering props make very sure they mechanically operate correctly and smoothly.

b.

[Cadence]

Originally Posted by a64pilot

Low RPM and high fuel in a Diesel almost always means high EGT's, that can burn valves etc.
I had a pyrometer in my truck and could demonstrate that, I'd start pulling a small hill running heavy and EGT would start to climb, it would get up to close to 1200 F, I'd add throttle forcing a downshift (automatic transmission) and RPM would increase and EGT would drop instantly to 1000 F or less. More RPM = more air, more air = cooler EGT's
Aluminum alloy pistons melt at about 1400 F and the heat stress was not good for them.
A boat especially one overpropped is on a hill that never stops

Yep, a boat is always like going up hill all the time. His pitch is probably only determining the grade and you can't down shift when it loads down, which sounds like the situation?

[mrybas]

Originally Posted by Cheechako

Whether it's harmful or not depends on what HP your boat needs and the HP curve of your engines. If the bottom/props are clean, It sounds like you are over pitched though. I would shoot higher but I wouldn't pitch for anywhere near 3800 rpm. Maybe 3200.

Why wouldn't I pitch the prop to the manufacturers max RPM?

[captain58sailin]

I don't know if you have turbo chargers or not on the engines. I am unfamiliar with this particular make, if you do have turbos, you will want to watch your pyrometer, if you are over taxing the engines the pyro will run up. If normally aspirated, the old timers rule of thumb around here is if you are getting max rated rpm underway and you push up against the dock and run up to max rpm you should get a 200 rpm drop, which would mean you are propped about right. Barring that, I found this site that can help you calculate your pitch and diameter needs. Vicprop - Propeller Calculator , I have used them on my full displacement monohull and came up with the same numbers as the prop pros here at their prop shop.

[jimbunyard]

It is very likely that you can adjust the starboard engine control for maximum throw at either the engine or the control, or both, so you can leave your cutting tools in the toolbox.

It is best to prop your boat to reach designed rpm, for both longevity and fuel economy. According to the horsepower curves, available here

http://www.google.com/url?sa=t&rct=j...Sk86WcgY0y2hDQ

maximum torque is at 2400 rpm and maximum HP at 3600. 3820 is the maximum 'high idle', no load speed.

Most diesel engine 'professionals' recommend that you prop so that your engine can reach 50-100 rpm over maximum in-gear speed (3600 in your case) in ideal conditions with clean hull and prop, mainly to allow for future loading and other 'non-ideal' conditions. It would seem to me that in a catamaran one would be especially inclined to underprop a little, given the likelihood of running with one engine...

I suppose there are instances when one would deliberately over-prop, say in an application where the correct prop size resulted in the engine having to be run in a particularly annoying rpm range, resulting in excessive noise and/or vibration, but these cases should be rare.

Since you have the luxury of an adjustable pitch, I would take advantage of it and set the system up so that it reaches designed rpm, the industry standard practice, and if you're not happy you can always 'overprop' it to your heart's content later...

[Steelcat]

Check that the props are perfectly clean.
The best efficiency and engine life is usually at rpm of maximum Torque

At this rpm you need to be at about 80% of the maximum power produced at those revs.

If max torque is at 2600 rpm then with clean props/hull you should run to at least 2800 rpm but you should always operate about 10% below these revs.

Most marine 3000 rpm engines are also available as 1500 or 1800 rpm generators so can be run for long periods at these revs.

You will not cause any damage to engines if you reduce throttle position so engine rpm reduces by about 10% from maximum possible.

[Just Another Sa]

Originally Posted by mrybas

Why wouldn't I pitch the prop to the manufacturers max RPM?
Attachment 141365

Added fuel consumption when motor sailing and noise are only reasons not to pitch the prop to the manufacturers max RPM. If neither of those is a problem for you, go ahead and pitch the prop to the manufacturers max RPM = 3600 under load, there will be no problems for doing so other than added noise and fuel consumption when motor sailing.

[jimbunyard]

Originally Posted by Steelcat

Check that the props are perfectly clean.
The best efficiency and engine life is usually at rpm of maximum Torque

At this rpm you need to be at about 80% of the maximum power produced at those revs.

If max torque is at 2600 rpm then with clean props/hull you should run to at least 2800 rpm but you should always operate about 10% below these revs.

Most marine 3000 rpm engines are also available as 1500 or 1800 rpm generators so can be run for long periods at these revs.

You will not cause any damage to engines if you reduce throttle position so engine rpm reduces by about 10% from maximum possible.

Not exactly sure what you're saying here, but it seems you're suggesting that the OP props to achieve max 2800 and then runs his engine at 2600 in the interests of 'efficiency and engine life', based on the fact that (many) generator engines run at 1500 or 1800 rpm.

Such advice, unless he motorsails all the time, will likely result in much shorter engine life and likely poorer fuel economy to boot.

A generator engine is not a propulsion engine; they have different roles to play. A propulsion engine must provide smooth, adequate power over it's rpm range, a generator engine goes from 0 to wide open, full power output, in an instant. There are differences in these engines that can be, individually or in combination, cam profile, injection timing, injector calibration, cam gear timing or governor.

So, while it may be best to operate the engine at 2600 for "best efficiency and engine life" (indeed, the page from the operators manual the OP supplies shows the recommended cruising rpm to be 2880-3240), the engine is best propped to achieve max rpm and then run at the lower cruising speed to achieve longest engine life.

(And yes, just like Granny who smoked two packs a day and lived to be a hundred and four, there are exceptions to the rule, but the usual experience is lugging an engine constantly results in significantly shorter engine life...)