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Diesels for boats

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Old 04-06-2008, 06:47 PM
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No, it's not all in the torque.
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Old 04-06-2008, 06:48 PM
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Fight fight fight!
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Old 04-06-2008, 07:01 PM
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There's no fight. There's physics. Steady state speed (e.g., top speed) is determined by the amount of power transfered to the water. Horsepower measures power and is the relevant figure, not torque. If you are interested in acceleration, not steady state speed, then torque matters; but no amount of torque will allow you to reach a speed faster than the available power will allow.

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Old 04-06-2008, 07:04 PM
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No fight, I have respect and regard for everyone on here. I also have opinions and am proved wrong from time to time.

In order for diesels to progress in this market I think it is important that we all have an understanding of the dynamics and I want someone to prove me wrong.

With that said I'll fire the first real world example of gas vs diesel.

Everyone knows what a Bertram 31 is. You take the 31 Bertram with twin 350 hp, 454 cui gas engines and top end is almost 10 mph faster than with 300 hp Cats 3116's or Cummins B's. You can go onto Yachtworld and look at used boat speeds, you can go anyone of 50 forums about that subject and see the same thing.

I will give you that the cruise mph with the diesels is far better than the gas engine but the debate is does HP = HP?

Look at a repower that has 210 B's in it, it's close to 20mph slower than the gas boat with 350's!!!!
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Old 04-06-2008, 07:17 PM
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There is definitely some tricky math involved, and a myriad of factors that scew the results. I've swapped 240hp inline fours that crushed the 330hp 7.4l's that came out. A lot of the increase could be due to the more efficient (at cruise) duo props. NJAWB is right, unless there is a way to apply the same amount of prop shaft force at the same RPM, then ya aint gonna go as fast. And "theoretically", if you could somehow harness a big torque monster with overdive transmissions to acheive this, would it be any more efficient?

You guys are way over my head so I'm just askin..
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Old 04-06-2008, 07:28 PM
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here's your math

http://www.elec-toolbox.com/Formulas/Motor/mtrform.htm

and this is a great article:

http://www.vettenet.org/torquehp.html

it put into simple terms why Hp out does torque.
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Old 04-06-2008, 07:40 PM
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Cheers!
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Old 04-06-2008, 07:51 PM
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How come my old gas engine builder was all about torque ? he said torque is the key thing in boating not hp ? maybe he was hittin the crack pipe ?
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Old 04-06-2008, 08:25 PM
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No crack pipe. He just realized that there is more to boating than just top-end speed in steady state conditions, and that the typical single gear ratio and prop power curve don't work well with a high-revving, high-horsepower, low-torque engine.
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Old 04-08-2008, 12:29 AM
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Originally Posted by Njawb View Post
You mean these? Looks like a pretty cool answer to my lingering question about how to get rid of the gasoline requirement for a yacht tender.
Damn looks like they got a lot further than I knew about. Joe what do you think of their piston supercharging?
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