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Old 07-12-2007
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Horsepower - Gas vs. Diesel

I've been looking at a lot of boats recently, most of which are from the late 60's/early 70's, and as such many of them have the old Atomic 4 (30 HP). I've notice when those boats have been repowered with diesels, the horsepower on the new engines is decidedly less than 30. Is this due to the difference in torque - in other words, can a diesel with less horsepower make up the difference with torque?

I've been told that a good rule of thumb is 2HP for every thousand pounds of displacement.

Last edited by TSteele65; 07-12-2007 at 02:10 PM.
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Old 07-12-2007
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Is this due to the difference in torque - in other words, can a diesel with less horsepower make up the difference with torque?

YES
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The average semi truck only has around 350 hp and can motivate up to 80,000 lbs. down the road.
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So is my formula of 2HP/1000 lbs still workable, or can I put 10HP diesel in a 11K pound boat?
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The Atomic 4 wasn't usually chosen for having just the right horse power, but for fitting the right price range. If your boat was smaller you got extra power.

10HP in a 11K boat sounds pretty low. Do you plan to motor much, or just in and out of the marina. Trying to push through a few knots of current when coming back up the inlet could prove hard.
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A 30hp Atomic 4 has about 30 ft lbs of torque, and a 15hp Yanmar has almost 50.
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I have heard the 2 hp rule applied to diesels. My universal 16hp diesel works well for my 9,000 lb. boat. In answer to your question, I think a 10 hp diesel would be inadequate for an 11,000 lb boat, especially if you are sailing in areas with current, significant winds, etc.
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Quote:
Originally Posted by danjarch
Do you plan to motor much, or just in and out of the marina. Trying to push through a few knots of current when coming back up the inlet could prove hard.
Potentially a lot of motoring. I'm trying to figure out how much HP to get if I buy a boat with an A4 and have to repower in the near future.
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Your propably looking at about 20hp if you want a little comfort, but A4s can be rebuilt pretty cheap. Unless it's trashed.

Post the make and size of the boat, and some is sure to have repowered one before.
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Last edited by danjarch; 07-12-2007 at 02:50 PM.
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Bunch of candidates right now, but three are finalists:

Bristol 32
Alberg 30
Tartan 34C
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