Help with prop size

[natew]

Good day everyone,

Looking for some help in re-proping my trimaran. I have a 38' trimaran, spec displacement is around 9000lbs I do not know the actual displacement/weight. Currently I have a Universal M25 powering it with a 15x13 3 blade prop. In this configuration I get a maximum RPM of about 2100-2200 out of the boat at which point I get a good amount of black smoke as well so I usually back it off and cruise around 1900-2000 which gives me about 5.5 knotts on a good day. However, this engine should max out at 3000 with a cruise around 2600-2800. So, it is obvious that I am over proped.

I have read and run around some calculators and have come up with a "recommendation" of a 3 blade 15x10 prop. However, I was also wondering if a 14x12 prop would also work and if there would be any difference between the two? My goal here would be to raise the operating RPM of the engine to more within specs to utilize all of the power as well as to hopefully gain 1-2 knotts of cruising speed so I am not entirely sure which blade would be more effective and efficent at doing that.

Any suggestions?

[NCboatrx]

If your propeller is holding the engine back to 2100 RPM, you only have the HP developed by that engine at 2100, not the full HP that the engine can develop at 3,000.

Losing 3" of pitch to have a prop of 15"x10" three blade will help substantially and should get the engine operating where it should. This will also have the effect of increasing your boat speed a bit.

14"x12"x3 is not the same and wont give you as good a result.

[djmarchand]

Prop selection is as much an art as a science. Find a good prop shop and work with them. They will usually change it out or repitch it if they get it wrong.

David

[Pete the Cat]

Get and read "The Propeller Handbook" by David Gerr. If your engine cannot reach full RPM in forward gear and full power you are destroying your engine.

[Tynesider]

Does this help you to check?

Vicprop - Prop calculator for Displacement and semi-displacement hulls