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What would happen if I went diesel 300hp

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Old 03-22-2008, 02:54 PM
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Default What would happen if I went diesel 300hp

I found a matched set of low hour 300hp nearly 600 lbs torque diesels but I am told they only rev in the 2800-3000 rpm range at WOT. They would cost less than my two rebuilds of existing gas motors and I could sell the ones I have and the gears to pay for tank clean outs and other mods needed.

The boat is a deep vee cruiser int he 15,000 lb range with anrneson drives. I know the drives can hanfle 1000 HP each and since they are surface piercing I can have whatever size props I need on it (diameter and pitch)....but.....what should I expect for performance possibilities?

Swamp - still fishing around
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Old 03-22-2008, 11:01 PM
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Originally Posted by Swamplizard View Post
I found a matched set of low hour 300hp nearly 600 lbs torque diesels but I am told they only rev in the 2800-3000 rpm range at WOT. They would cost less than my two rebuilds of existing gas motors and I could sell the ones I have and the gears to pay for tank clean outs and other mods needed.

The boat is a deep vee cruiser int he 15,000 lb range with anrneson drives. I know the drives can hanfle 1000 HP each and since they are surface piercing I can have whatever size props I need on it (diameter and pitch)....but.....what should I expect for performance possibilities?

Swamp - still fishing around
What exact make and model are the engines?
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Old 03-24-2008, 01:03 AM
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How much power were your gas motors making?

If more than 300, you're going to take a hit in top end at the least, maybe degrading performance everywhere.

If they were similar in rated power, you can get approximately equal** performance if you re-gear to get your prop speed back. Prop speed is going to be key for this type of repower.

**maybe better, maybe worse, depending on the exact power curve of both engines. It may not be easy to get a definite answer on this.

As you stated, changing props may be required, it depends on whether or not the right gear ratio is available.
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Old 03-24-2008, 01:51 AM
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If more than 300, you're going to take a hit in top end at the least, maybe degrading performance everywhere.

Not exactly... once you figure in the torque curve of a diesel engine, whatever was in there gas wise, had to be making some big power to match the torque of a diesel, which typically works out to about 2:1 over the HP curve...

That said, the limiting angle of the diesels will be the low RPM range. You could try to change props and gearing, but you'll end up setup for low speed or top end, that is, unless you run a 2 speed transmission between the engines and drives.... Having owned a 21' Schiada v-drive with a trans in it, I gotta say, it's pretty cool watching the people who are with you as your boat shifts gears.

What kind of engines are you looking at? Dollar for dollar it may seem cheaper on the surface, however, once you get down to the brass tacks, probably not so much... And I've been kicking around buying a Top Gun with no power and going either the Yanmar or D-max route... I'd almost be better off with a pair of 700's and NXT's fo close to the same money.
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Old 03-24-2008, 01:17 PM
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If more than 300, you're going to take a hit in top end at the least, maybe degrading performance everywhere.
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Not exactly... once you figure in the torque curve of a diesel engine, whatever was in there gas wise, had to be making some big power to match the torque of a diesel, which typically works out to about 2:1 over the HP curve...
Of course the diesel will make more torque, maybe even 2:1. But what does that have to do with top speed? Horsepower is what determines the terminal velocity, where the power available to propel the boat (engine power converted to thrust by the prop) exactly equals the drag caused by water and air.

My suggestion of regearing is the correction factor. Change the ratio to alter the effect at the prop (where it matters). He'd need less reduction ratio, reducing torque multiplication of the gearbox, to get the prop speed back where it was. The exact math here depends on what he has for gas motors.

I stand by my original statement. If he was making more than 300 hp before, he'll take a hit in top speed.

I'm intentionally ignoring the fact that he might be able to improve propeller efficiency in the process of the switch which may skew things, but it's just as likely to get worse, so its easier to ignore it for now.

I like the Dmax idea...
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Old 03-24-2008, 02:27 PM
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Is Harley still building boats?
I went for a test ride on one back in the early 80s.
It was a diesel boat... thought it had a pair of cats.
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Old 03-24-2008, 02:35 PM
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yeah he says he is
most of his boats were cat 3208s in the 375 hp range
couple guys are running gs engines pretty successfully...one is using stock 415 PCMs and hits 50. One have 350HP stock motors and hits 40.

I keep blowing motors and I think it is a combo of gear overload (not enough ratio) and REALLY BAD motor builds. So out of digust I plan to start over....hence the diesel questions.

Some low hour 300 HP Volvo units around...about the same cost as buying a pair of factory new GM 502 longblocks....?
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Old 03-24-2008, 02:50 PM
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I've stayed out of the diesel threads for awhile now because people like mthill and 29firefox are just too technologically past where I am. Where my knowledge ends there's is just getting started fuel system wise!!!

That said, mthill is right, read all my old threads - HP is HP and top speed is all about prop speed. You need to swing those props the same speed to go the same MPH. Slower bigger props will help mid-range but will hurt top end.

All this talk about diesels and torque curves and torque, non of it means a thing, when you gear the diesel up and gas engine down to the same output shaft rpm and they are swinging the same prop then the amount of work is equal. It doesn't matter what is on the driving end of the prop.

How fast the boat goes with 300 hp gas enegines is dam near what it will do with 300 hp diesels. Through gearing(maybe a 2 speed) and maybe some better props you might squeak out a couple mph on either engine but a 300hp gas can never run with a 500 hp gas engine.

Just my humble opinion.
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Old 03-24-2008, 03:32 PM
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yes, but you can run the diesels near WOT much of the time, where a gas bbc, you should be pulling back 25-30%.
IMO, having a 15000lb harley means you want a good comfortable ride, not looking to be so fast.

and the diesels should be able to spin a much bigger wheel.
contact Hurley and see what they say.
i'm sure you may have a slower top end, but there is something to be said about reliability.
Magnum isn't running 1500hp blower motors in their big boats. There is a reason.
A big hp BBC w/o the torque ain't gonna get your boat moving.
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Old 03-24-2008, 04:11 PM
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You dont really need to get "tank clean outs" if there is less then 2-4 gals of gas in your 100 gal tank you will be fine. Truckers used to add gas to their tanks to keep it from gelling in the old days, called it "hot diesel'
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