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kms 05-06-2005 03:10 PM

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?

928frenzy 05-07-2005 03:29 AM

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 ~ _/) ~

KenD 05-09-2005 03:32 AM

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.

dman 05-09-2005 04:33 AM

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.

Silmaril 05-09-2005 11:50 AM

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.

TrueBlue 05-09-2005 02:51 PM

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.

FalconEddie 05-13-2005 10:07 AM

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.

dkory 05-13-2005 07:38 PM

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.

Fantail 12-21-2015 01:16 AM

Re: engine size
 
I know this thread is 10 years old but I've read a couple threads like this and still haven't found an answer to my similar but totally different question. Just learned to sail a couple months ago, I'm loving it and there's no turning back so I want to learn as much as I can. I would love to have someone here educate me a bit more on the subject. Here's the question...

Why don't sailboats have more powerful engines? I'm a car guy so yeah, horsepower is a big thing. But 20-50hp in boat? Ya gotta be kidding! I do have numerous friends who are boat people but they are all motorboat fans. One even has a racing boat (lake boat). So these friends talk big about their power, even the guys who don't race. So why can't a sailboat have more power for motoring when the wind is nill?

eherlihy 12-21-2015 08:42 AM

Re: engine size
 
Unlike car engines, which today use aluminum blocks, sailboat engine blocks are predominantly made from cast iron. Larger horsepower engines have larger displacements, and/or turbo chargers, which means they would weigh more, and require more space. That means less capacity for you and your stuff, and less storage for your stuff aboard. Finally, a larger engine would consume more fuel, and require larger fuel tanks to achieve the same range.

My boat, which is 35' LOA and displaces 12Klbs, has a (29 year old!) Universal M25, which produces 21HP (on a good day at WOT). The boat will motor at 6.5kts with no current or wind. The engine weighs 350lbs, and consumes 1/3-1/2 gallon of diesel per hour. When combined with my 35 gallon tank, this gives me a range of 680-450nm depending on how hard I push it...

People that wanted horsepower tended to buy Margregor 26s, or the Tatoo 26 (or the new Tatoo 22!).
http://www.russellsmarine.com/mac/im...orpowering.jpg


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