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post #1 of 8 Old 05-06-2005 Thread Starter
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engine size

When looking for boats on web sites and at marinas, is there a quick way to know if the diesel engine hp is big enough to handle the boat? How would you estimate the hp needed for a boat for coastal and near offshore cruising as compared to inland cruising? Would this include number of prop blades?
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post #2 of 8 Old 05-07-2005
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engine size

I once read (I forget where) as a rule of thumb, a sailboat''s auxilliary engine should be about 1 HP for every 500#s of displacement.

Elsewhere I''ve read, two blades should be sufficient, but three blades will provide a smoother application of power, as well as more thrust when backing down. They also cause more prop-walk. The size and pitch of the prop are also critical, and need to be matched to achieve max hull speed while the engine''s running at its ideal cruising RPM without over-stressing it.

~ Happy trails and sails to you ~ _/) ~
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post #3 of 8 Old 05-09-2005
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engine size

I think it should be more like 1hp for every 1000 pounds of displacement or 2hp for every ton otherwise we would be in the same class as stinkboaters a ninethousand pound disp. would have 18 hp well maybe that wouldn''t be all that bad my boat has a2gm most days thats enough.
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post #4 of 8 Old 05-09-2005
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engine size

To get a displacement boat up to hull speed in calm weather it takes next to nothing so your 2 horsepower per ton would be fine,however in bad weather and heavy current4 horse per ton like 928FRENZY has stated is recommended by many designers.
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post #5 of 8 Old 05-09-2005
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engine size

It also depends on your intentions for the motor. If you are only using it to get in and out of a harbor, 2hp/ton should work. But like 928F and dman have said, if you are looking to be able to make time while cruising to weather into a seaway, 4hp/ton is minimum. Add to that the loads from alternator/refer/etc and you may want 4.5 - 5/ton.

Prop pitch and sizing is CRITICAL in a repower situation. You need to take into account displ. waterline, gear reduction, etc. There are formulae to figure it out, but it is best left to the pro''s AND have second and third opinions done. Nothing beats a three blade fixed prop for motoring, but it will add drag while sailing.

On my 37'' 13,500lb sloop, I went from a 14hp 2cyl Volvo to a Yanmar 3GM30F 27hp. Once the proper prop was put on (in my case a 16" 2 blade Martec folding) the difference was staggering! I could now make time motoring into steep 6'' - 8'' waves and 20Kts+ winds. I would have been stuck in the harbor with the old set-up. I was no longer stopped dead with every wave hit.
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post #6 of 8 Old 05-09-2005
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engine size

Of course, there are exceptions to every rule.

My Nauticat 33 motorsailer displaces 9 tons. With her Lehman 90 hp, iron heart beating beneath the pilothouse sole, it''s comforting to know we can exceed hull speed when needed . . . or desired. A 22" three blade prop on a 40 mm shaft is assurance that the running gear has been designed to accommodate the torque of that engine.
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post #7 of 8 Old 05-13-2005
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engine size

When I was building my boat, I did all the math and came up with about 30HP requirement, being conservative. It''s a bit beamy and heavy, but at only 32 feet, 30HP in diesel should be fine.

It was a cold and brutal Boston winter and I was living under the hull not 20 feet from the waters edge. I took a much needed break and went to Bonaire to get a diving ticket and some much needed warmth. While I was there, I ran into a guy who''d been cruising the Caribbean for many years and he gave me some valuable advise: "Whatever you think you need on paper, double."

He went on to tell me of many situations where trade winds and current conspired to stop sailboats in their tracks in channels and harbor mouths when all other weather conditions were ideal for sailing. I took his word for it. I also opted for a nice fat three bladed prop and have had no regrets. I am deaf to the silly arguments about the increased drag of the three blade, because it only counts when you don''t have enough wind to make hull speed. Once you''re there, the two tenths of a knot of additional drag is moot.
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post #8 of 8 Old 05-13-2005
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engine size

Displacement is important, but hull shape is a big deal, too. A long skinny boat will take a lot less HP to push than a short fat one with the same displacement.
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