Over proping

Davidasailor26 · Jun 17, 2021

Doesn’t answer your question, but you might want to make sure your tach is accurate before making any changes. Better to check with an optical tach on the crankshaft pulley than relying on an old alternator driven tach. If it’s not overheating or smoking I’d start there. But I’ll defer to others on what the other engine problems may be.

Maine Sail · Jun 17, 2021

First confirm your tach is accurate by using a digital photo tachometer.. Westerbeke/Universal & Yanmar will all void warranty if an engine is over-propped.

This is for the Universal M-25XPB but the recommendation is the same on all Universal/Westerbeke engines based on the individual engines max rated RPM..

M-25XPB
"Universal recommends a propeller that will allow the engine to turn 3000 RPM underway at full throttle."

In conversation with Joe J. at Westerbeke he was adamant that any Westerbeke or Universal engine should be within at least 100 RPM of max rated, in smooth current free water, but preferably less than 50 RPM of max rated. Spot on is preferred, but not easy to hit.

LakeOntario270 · Jun 17, 2021

It's a 35 year old engine. I wouldn't worry about it.

kloudie1 · Jun 17, 2021

Some mechanical stuff to consider:
A diesel is set up from the factory to deliver fuel enough to make it reach rated RPM at full throttle with full load (max horsepower)
If you don't run at full throttle (very much) with an overpropped condition, you are not causing any damage to the engine..
This is the Yanmar 3GMF power curve with an ideal propeller curve on the chart.. An overpropped condition moves the propeller curve to the left (if his tacho is correct) to the point where it would intersect the power curve at 2600 RPM instead of 3400 RPM. At all rpm below 2600, his engine would be making below the continuously rated power as shown by the propeller curve. It also shows that he would be "giving away" about 4 horsepower (20% in this example) that might have kept him off a reef ! Another thing to consider is that if he tried to make the engine go faster than the 2600, he'd start seeing black smoke from unburned fuel because he governor would be trying to speed up the engine by adding more fuel but the low RPM (and several other factors) would be keeping it from being able to ingest enough air to burn the fuel.. essentially, the engine can't make more power than the rated curve. I think (my opinion) that as long as he doesn't run in a rpm band that produces black smoke, there is no damage.. My opinion is that the manufacturer wants a perfect prop in order to keep the engine and injectors clean and not carboned up.. and to keep unburned fuel from diluting the oil on the cylinder walls and increasing wear.

Maine Sail · Jun 17, 2021

Pagan Baby said:
Thanks for the info. I’ll check the tach output before I do anything else. Not sure how much luck I would have with any kind of warranty claim on a 35 y.o. engine

None, but it is just there to let you know how seriously the engine manufacturers consider correct propping to be.

sail sfbay · Jun 17, 2021

Pagan Baby said:
I’m wondering what are the long term effects of overproping? The engine on my 86 Cape Dory will only do about 2600 rpm at wide open throttle (WOT), it should be turning at 3000+............... I’ll look into either getting a new prop or this one reworked at next years haul out.......

Seems to me that inability to over rev the engine is better than over revving it. Engine rpms and pitch are inversely related. Increasing the pitch will decrease engine rpms and decreasing the pitch will increase engine rpms. Going back to the question of performance at wide open throttle (WOT), if your engine is under revving, consider a propeller with less pitch. A new propeller can be specified with a diameter and pitch to match your hp at WOT. This should provide more top end speed at the expense of some low speed thrust. Also your propeller can be changed out in the water, so there is no need to wait for a haul-out.

Over proping

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