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Old 30-09-2013, 09:41   #1
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Join Date: Jun 2012
Location: Ventura, CA
Boat: Horstman Tristar 38
Posts: 200
Help with prop size

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?

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Old 01-10-2013, 06:03   #2
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Join Date: Aug 2008
Location: North Carolina
Posts: 237
Re: Help with prop size

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.

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Old 01-10-2013, 10:17   #3
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Join Date: Dec 2008
Location: Oriental, NC
Boat: Mainship Pilot 34
Posts: 1,429
Re: Help with prop size

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.

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Old 01-10-2013, 10:42   #4
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Join Date: Nov 2006
Location: Maine and California
Boat: Tartan 37 "Velera"
Posts: 338
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.
Ray Durkee
S/V Velera
Tartan 37
Castine, Maine
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Old 09-10-2013, 04:04   #5
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Join Date: Oct 2010
Location: Northumberland UK
Boat: Colvic Watson
Posts: 93
Images: 12
Re: Help with prop size

Does this help you to check?

Vicprop - Prop calculator for Displacement and semi-displacement hulls

Archivist for the Colvic Watson Owners Group
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