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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel
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Old 05-11-2012
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Question repowering

What size motor should I use? I have a bristol cutter and am thinking about a 20-28hp kubota. Any thoughts?
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Re: repowering

Quote:
Originally Posted by lennyv View Post
What size motor should I use? I have a bristol cutter and am thinking about a 20-28hp kubota. Any thoughts?
Whatever the HP is on the boat now is what you should be looking for. If you go larger it may help when powering against the tide but that's about it.
Hull speed is hull speed & if your current power plant gets you to hull speed...

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Old 05-11-2012
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Re: repowering

What misfits said. Also, if you go larger you're probably looking at having to buy a new prop as well as other peripherals that were designed for a smaller hp engine (exhaust hose, batter7 cables, engine mount, etc.).
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Old 05-11-2012
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Re: repowering

Absolutely the easiest Re & Re will be to replace like with like. Motor mounts, shaft alignment, prop pitch etc will all match. Problem arises when that motor is no longer available, then look for the closes match in terms of fit.

Moving mounts is not usually a big deal, but motors often have different mounting angles and/or limitations, different gearboxes have different exit angles, and most engine boxes leave little room for shifting things around to line up again.

Know the overall dimensions you need to fit into, V drive vs straight drive, overall height and width of the engines under consideration, spacing of the engine beds (easier to make it narrower than to go the other way). Even the weights can differ considerably for similar horsepower, though on a BCC that's not going to be a big issue.

The other thing to look at closely is the gear ratios. Changing that significantly will almost guarantee the need for a new or modified propellor. There is no 'standard'.

Good luck.
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Re: repowering

Agree with going with what you have. The biggest problem with sailboat diesels is that they don't get run at full e.g. 80% power enough. Overpowering can create problems. Power is best matched with hull speed calculations. Geer's Propeller handbook has a good discussion on this subject. Agree with the issues adding complexity and cost by having a different fit, prop size and pitch, possibly shaft size etc etc.
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Old 06-26-2012
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Re: repowering

A rule of thumb is 1.5 to 2 HP per 1000 lbs displacement.
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