Horsepower - Gas vs. Diesel

TSteele65 · #1 (**permalink**) 07-12-2007

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.

USCGRET1990 · #2 (**permalink**) 07-12-2007

Is this due to the difference in torque - in other words, can a diesel with less horsepower make up the difference with torque?

YES

USCGRET1990 · #3 (**permalink**) 07-12-2007

The average semi truck only has around 350 hp and can motivate up to 80,000 lbs. down the road.

TSteele65 · #4 (**permalink**) 07-12-2007

So is my formula of 2HP/1000 lbs still workable, or can I put 10HP diesel in a 11K pound boat?

danjarch · #5 (**permalink**) 07-12-2007

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.

knotaloud · #6 (**permalink**) 07-12-2007

A 30hp Atomic 4 has about 30 ft lbs of torque, and a 15hp Yanmar has almost 50.

FrankLanger · #7 (**permalink**) 07-12-2007

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.
Frank.

TSteele65 · #8 (**permalink**) 07-12-2007

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.

danjarch · #9 (**permalink**) 07-12-2007

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.

TSteele65 · #10 (**permalink**) 07-12-2007

Bunch of candidates right now, but three are finalists:

Bristol 32
Alberg 30
Tartan 34C