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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #1  
Old 08-05-2003
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Enough power?

I''m in the preliminary stages of purchasing a 33 ft.boat.The boat seems to be ideal for my needs, but I''ve got a concern. The boat,which displaces ~10,900 lbs.,orginally had an Atomic 4-30hp gas engine.She has been repowered with an 18hp Yanmar.I''ve not actually taken the boat out yet, so I don''t know how well the Yanmar performs.Is the 18hp Yanmar enough of a powerhouse to handle the 10,900lbs.? Do smaller hp diesels generate as much torque as larger gas powered engines? Does anyone know what formula is used to calculate hp needs per lbs.of displacement?
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Old 08-05-2003
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Enough power?

I owned a 30'' C&C with a 18hp yanmar diesel and she powered along just fine with the engine. certianly in excess of 6 knots. If your boat has alot of free board or a full keel or something else that causes alot of drag that might be an issue. Horsepower is horsepower but gas engines are rated at peak hp and diesel at continuous. It is my understanding that gas hp is often overstated because of this. (IE a lower HP diesel may be equal in power to a higher HP gas engine)

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Old 08-05-2003
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Enough power?

Hamiam,

Thanks for the information.It doesn''t sound like I really have anything to worry about.
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Old 08-05-2003
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Enough power?

Stede,

FWIW, it has always been my understanding that you need 2 - 2.5 hp per 1000lbs. Based on that formula 22hp would be the minimum for your vessel. If you are talking about strictly lake or bay sailing you might be ok, but if you are fighing strong currents and strong winds you might be under-powered.

bob-m
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Old 08-05-2003
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Enough power?

Bob-m,

Thanks for the information.The 2 to 2.5 hp per 1000 lb. general rule makes sense to me.I thought some more about the issue this afternoon,and decided to call a Yanmar rep.His comments concerning the hp for the boat were just about identical to yours.He mentioned that for the displacement value I gave him,his book listed an 18 hp,and a 27 hp.The 18 hp was considered "minimal", and the 27 hp was considered maximum.Apparently most people when repowering this particular boat opted for the 18 hp(cheaper),but he went on to say that if the boat was going to be used for serious coastal/offshore work he would recommend the 27hp even though it would be somewhat over-kill.That comment, and the ones you guys have provided is making me re-examine purchasing the boat.I''ve learned from storms I''ve been in that minimal hp rating means minimal forward motion. Thanks again.
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Old 08-06-2003
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Enough power?

I have an O''Day 30 which disp 11,000#. I have the old 2QM15. It moves the boat along at 6.9-7.0 knots in calmer water. when the wind and or waves pick up I find that its too rough a ride to try to keep up this speed. and back off to 6.0 to 6.5 or in some cases even less. I think that the cost to repower to an even bigger engine than the 20hp isn''t cost effective. you do have a sailboat and not a powerboat. It is very important that it be proped correctly or you will overload the engine or rev like crazy and get nowhere.
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Old 08-06-2003
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Enough power?

capnim02,

Thanks for the reply. I hadn''t ever really given much thought to the hp rating of diesels on sailboats until I started seriously looking to upgrade to a larger boat.My 26 footer has a 8hp Volvo-Penta. To me, it''s always seemed kind of small and squirrely,but yet it always got the job done.However,recently I got caught offshore in a storm at the Marquesas and the engine definately seemed under powered.Granted the conditions were extreme,but it made me think how much more effective the engine would''ve been if it had of been a 12hp rather than a 8hp.My 26 footer has a displacement of 4300 lbs. Going by the rule of thumb that Bob-M mentioned,(2 to 2.5 hp per 1000 lb.) the minimum size engine for the boat should be around 8 hp,and the maximum would be around 11 hp.By researching different boats while looking to buy,it appears to me that most sailboat mfg.''s tend to go with the minimum size required engines,rather than the maximum. I guess this is o.k.I don''t have enough first hand experience with the issue to have an opposite opinion. I''m just trying to make sure that I don''t purchase a boat that will be under powered for what I intend to use her for-offshore work. I also thought the same thing as you mentioned as I''ve been contemplating this issue."It''s a sailboat,not a powerboat." How concerned should I really be about this? Yet, I''ve sailed long enough to know that there have been many times that hearing the rumble of the ole diesel cranking up and knowing it has the power to get me out of a tight fix has been very reassuring.Thanks again for the info.
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Enough power?

Hi Stede

Good luck you the boat. My info is: 22,000 lbs, 38ft LOA, 31 LWL, 47 HP. Thus 2.1 hp per 1000 lbs disp.

47 HP gives me ALL the power I need. It can push the boat past its hull speed and I have power to spare. I make 6 knots at around 2200 rpm.

I DO think it is important to have an engine that can power you through adverse currents, waves, and storms. Very important. 18 seems only about 10-15% less than what might be ideal...and as you know, you can push a diesel more than a gas engine.

Is there an owners group you are pose this Q to? What is the boat?

All the best

John
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Enough power?

JohnDrake,

You wrote, "Good luck you the boat."

What is that? Are you hitting that tequila and rum with the four women already?!

Your friend,
Duane
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Old 08-06-2003
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Enough power?

oops!!! that must have come from my Italian grandparents! :O))))...or I was typing way too fasastiubtpjiobiuhv
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