Engine or propeller?

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richardjacobs

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I have a 35ft long keel yacht, with a Lister Petter 30hp diesel engine. The engine is rated to deliver maximum power at 3000rpm. We recently discovered that it tops out at 2500rpm under way. The engine revs freely to 3200rpm out of gear. The bottom of the boat is clean. We have a FeatherStream propeller. Unfortunately I don’t know whether anything has changed recently, or when the old fixed propeller was replaced with the FeatherStream, as we didn’t try full throttle. The calculations for the Featherstream came up with the same answer as for the old fixed prop.

We get 6.7kts through the water at 2500rpm, which seems reasonable. When we push the throttle open wider, absolutely nothing happens. No change in noise, no visible smoke.

The local mechanic thinks it is a prop problem. The prop supplier suggests that it’s an engine problem, but can supply a kit to adjust the pitch.

I’m not particularly worried about top speed, but I don’t want to lose power, or ignore a fuel problem that might get worse.

Any suggestions would be appreciated.
 
From the info supplied it actually not possible to tell... It can be either the prop (too big or too much pitch) or the engine not being able to deliver its full power (fuel starvation most likely cause). The "free revs" are of no significance and as the possible overload is relatively small, the absence of smoke doesn't mean much either. I would put my money on the prop, as it is statistically more likely. I'd suggest to consult the engine manufacturer about the recommended prop specs (they should be in the user manual anyway) and take it from that point.
 
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Could be a 'fouled' prop. The symptoms are the same as I get after a year in the water. However, I know that (as a baseline) with a clean prop I can reach almost max engine revs.

Are FeatherStream propellors fixed or folding? I have an Autoprop which only needs a small amount of fouling to reduce the max revs.

Get your wet suit out and go down and give the prop a clean ...... :) :)

Alan
 
jiris said:
From the info supplied it actually not possible to tell... It can be either the prop (too big or too much pitch) or the engine not being able to deliver its full power (fuel starvation most likely cause). The "free revs" are of no significance and as the possible overload is relatively small, the absence of smoke doesn't mean much either. I would put my money on the prop, as it is statistically more likely. I'd suggest to consult the engine manufacturer about the recommended prop specs (they should be in the user manual anyway) and take it from that point.
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Inability to rev without any smoke = Lack of fuel?
 
To use the last 500 rpm , which may increase the speed a little, you need to take off an inch or so pitch using the new stops Darglow will supply. Assuming the bottom is clean and there is no fouling on the prop the restriction is probably due to the boat being heavier than anticipated.

Although it is unlikely to be an engine problem it is worth checking that the speed control lever is actually opening the fuel rack correctly. Also check what the rated maximum revs is as getting 3200rpm without load suggests the governor is set for that speed. sight of the power curve for the engine would be useful to see how much power you are not accessing.
 
Also worth checking that your tacho is reading correctly. A £10 reflector type meter as sold on eBay will sort that out. I had a very similar issue. No smoke, good speed just seemingly low rpm but the tacho was miles out at the top end
 
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Chaktoura said:
Also worth checking that your tacho is reading correctly. A £10 reflector type meter as sold on eBay will sort that out. I had a very similar issue. No smoke, good speed just seemingly low rpm but the tacho was miles out at the top end
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That is worth doing as it removes one uncertainty, but revving to 3200 in neutral doesn't sound too far out.
It's also worth checking the gearbox ratio is what you think it is.
How old is the engine?
It's worth checking the exhaust elbow.
How does the engine behave when motoring into some waves? Does it hold its RPM or get knocked back?
Are the speeds through the water at 1500RPM and 2000 RPM consistent with the prop and gearbox being what you think?
What will it rev to in a bollard pull test?
 
Thank you for these various suggestions. They seem to cover the full range, so we'll just need to work through them. It's the fuel question that I've been worrying about, as that's seems more likely to get worse at an awkward moment. Where would you look, after changing the filters?

To answer some of the other questions:
- The prop is clean.
- Throttle control continues to move on engine, after rpm tops out.
- Engine is 15 years old (LPW3), with 800 hours on it. (The fuel tank is also 15 years old and inconveniently has almost 200l of diesel in it)
- RPM hold in waves, of course boat speed does not.
- Speed through the water at lower RPM seem sensible to me (4.7kts @ 1600rpm, 5.6kts @ 2000rpm)
- Revs to 2200rpm when tied to the pontoon.
 
Being "over propped" does allow for "motor sailing"
my engine is rated @ 37.5hp @ 3k rpm
my MaxProp is set to give me a max of 2850 rpm & 7.5knots, with a clean bottom & prop , i cruise @ 2k rpm & approx 6.8 knots (but dependent on time of the season).
I wouldnt over worry about what you have @ present
 
richardjacobs said:
Thank you for these various suggestions. They seem to cover the full range, so we'll just need to work through them. It's the fuel question that I've been worrying about, as that's seems more likely to get worse at an awkward moment. Where would you look, after changing the filters?

To answer some of the other questions:
- The prop is clean.
- Throttle control continues to move on engine, after rpm tops out.
- Engine is 15 years old (LPW3), with 800 hours on it. (The fuel tank is also 15 years old and inconveniently has almost 200l of diesel in it)
- RPM hold in waves, of course boat speed does not.
- Speed through the water at lower RPM seem sensible to me (4.7kts @ 1600rpm, 5.6kts @ 2000rpm)
- Revs to 2200rpm when tied to the pontoon.
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That doesn't sound over propped to me.
It sounds like the governor might be slightly off optimum adjustment.
Or maybe the data sheet we're looking at is not quite right for the engine...
But the engine seems to basically be doing a good job of powering the boat.
Assuming you don't have overheating, smoke or fuel consumption issues it seems pretty good really.

Now you have some baseline data, it should be easy to see if anything changes.
 
I had similar problems and found a valve seat burnt out, a blocked fuel system (priming bulb issue), blocked fuel filters (that looked clean) and injectors not working correctly. A compression test would be a good start together with a quick check that fuel is flowing to the filters and a new set of filters.

Yoda
 
The suggestion to use an external calibrated tachometer is most sound.

One other factor that has not been aired is your draft and trim, how is your waterline in comparison to design WL and are you overly trimmed by the stern at all. Either of these will increase shaft torque.
 
NormanB said:
The suggestion to use an external calibrated tachometer is most sound.

One other factor that has not been aired is your draft and trim, how is your waterline in comparison to design WL and are you overly trimmed by the stern at all. Either of these will increase shaft torque.
Click to expand...

But since the engine is not smoking, and maintains the same RPM into waves, it would appear not to be load limited?
 
Not worrying about it is certainly a tempting option, especially as I've got no reason to think that anything has changed recently.

Thank you again for the suggestions.

While there's no visible smoke. The transom does get dirty more than I'd like, even at lower speeds.
 
I cannot determine at a distance but with an accurate tachometer and assembling the other salient factors should help in forming a judgement.
 
In a bollard test try max revs in reverse - the prop pitch is often lower and it may spin faster. You will then know if it’s prop or engine.
 
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