Plug in the numbers. Its 14%.

http://go-fast.com/Prop_Slip_Calculator.htm

Actually probably closer to 12% since the pitch on stock Bravo's is on the heavy side.

502mpi's with CMI's/ 425HP. WOT 5000rpms at 72mph.

Griff,

I'm guessing here but your engines at 3,800 rpm are probalby not making any more than the 315hp the Yanmars Metal is talking about, your guess???

If that is true than his Yanmars with 1.36's should be right around 55 mph as well, give or take with smaller pitch wheels like yours.

Metal Bros - start there 1.36 and if the Yanmars can turn them easy bump up a couple inches and see what happens?

Remember this is all practical knowledge and assimilating with a gas boat. The logic is sound and proven but like most things it will take some T&E to get it right good luck!

Most of the rpms were below 1500 the majority of the time.The motor is a great motor but I strongly recommend underpropping it and letting it breath.
I had the motor in a 31 Rybo Runner hull with a direct drive inboard with 1.5 to 1 tranny swinging a 21x21 originaly. That got me 22 knots at 38oo empty with no Parasail behind me.
i believe I went down to a 19 x21 that got me p into the 4000 range but dropped my speed by a full knot.

I just don't see the reliability factor nor the fuel savings to justify all the other problems of dealing with these small high RPM diesels .
The long term benifits of the larger ones like the 420s are justified if you are running a larger boat that needs big power but if you are thinking you are going to run the chit out of these 315s and be hassle free you are dreaming.

If I was back in the business again I would go back to a stock Carb 454 for what I needed . The relaibility factor and repairbility is unmatched.I have gotten 4000 troublefree hours out of a 330 -454 with nearly the same fuel burn as the 315 with far far less problems. With todays differience in fuel prices I would not touch a diesel with a ten foot pole unless I was in some big sportrfish that absolutly had to have them.
This whole diesel miniscule fuel burn numbers is a complete Farce.
I will eat my words if any one can prove different.

The only place a Diesel is noteworthy is any large heavy boat or vehicle that a Gas engine can not provide the needed displacement or torque .

I see yours and others points,the prop is out of its "ideal" rpm for effieciency so the slip will be have to be factored much higher from whats being said. If thats truely the case then the outdrive should be regeared to achieve the same prop rpm equivilant to a gas motor turning at least 5000 rpm's.If a gas motor turns prop 3333 rpm's at 5000 w/a 1.50 drive and it is felt that is closer to ideal then play with the gearing numbers to achieve the equiv rpm's. A 1.25 lower gear set in a xr drive would give you 3040 prop rpm's at 3800,you can still use the prop calculator-punching in the numbers a 1.25 drive w/3800 input rpm's would need at 26.5 pitch prop to turn 65 mph at 15% slip,Smitty

Mine are probably making close to the same 315 HP at 3800rpms, but the Yanmars will have a lot more torque and should be able to spin a bigger prop. Probably a 26-28 pitch.

Sounds like I may be on the wrong path here....
Fact is I love diesel anything- cars,trucks etc, I like the efficiency and the reliability of it. I'll look at the CMD and the Volvo D6 stuff and see if there is anything that may work out better. 3800 rpm is max on these so55ish would be top speed and you are right that doesn't justify the re-power. So thanks for the pointers, it has been very informative guys. I'm open to any suggestions you may have in the meantime, Feb will be here before I know it....

Griff,

I know that is what people assume because they hear the word "diesel'. The fact is no matter what kind of engine it is, if it makes 315 hp at 3,800 rpm the torque is exactly the same - no difference.

T = 5252 X Hp / RPM

The equation does not matter where the power comes from.

Now I agree that the diesel will be in it's power band at a much lower rpm than the gas engine and therefore could in theory turn a larger prop because on the lowside of 3,800 rpm it has more Hp than a gas engine.

I think if Metal Bros is not looking for a rocketship it's a great project. The boat will cruise in the high 40's for at least 2,000 hours and only sip fuel.

Joe

If the 315s could push the boat at 50 turning 3600 the fuel burn would be 26 to 28 gallons an hour

A pair of 300hp small block mercuisers would burn less than that at that speed .And I believe you would get the same hours out of them.

What do you think?