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I can't figure out this math, please help.

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Old 08-24-2008, 08:08 PM
  #31
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Originally Posted by HabanaJoe View Post
Griff,

That's what I was looking for. Metal's original question is based on a 1.5 reduction gear not the taller 1.36's you have. You validate my point that it's prop speed and not pitch for going faster, of course plays into it but you need sheer rpm's to go fast!

Also how much Hp are those engines and at what max rpm? I'm driving at proving my point here a little more so help me out please?

Joe Gere

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502mpi's with CMI's/ 425HP. WOT 5000rpms at 72mph.
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Old 08-24-2008, 08:43 PM
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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!
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Old 08-24-2008, 09:12 PM
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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.
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Old 08-24-2008, 09:28 PM
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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 .
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Old 08-25-2008, 12:57 AM
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Originally Posted by 29Firefox View Post
That calculator is bubu.... It doesn't take into account Diameter & Area Ratio.
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
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Old 08-25-2008, 05:33 AM
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Originally Posted by HabanaJoe View Post
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.

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.
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Old 08-25-2008, 08:29 AM
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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....
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Old 08-25-2008, 08:34 AM
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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.
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Old 08-26-2008, 10:07 AM
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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?
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Old 08-26-2008, 11:00 AM
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Your asking a loaded question.

Here's your answer with regards to small light fast boats (as compared to a 50' Hatteras) and my personal feeling below that: You need more gas power than that and you need more cubic inches. You can't run a gas engine as close to peak as the diesel for as long. The 365 Merc's or something similar with fresh water cooling run slow to never exceed the same 50-55 mph would I think last as long as the Yanmars. Maintenance costs will be slighty higher but minimal.

My thoughts - I feel with today's advancements in gas engines that for smaller boats they are as good as any diesel engine. These small diesels don't have long life like the old 6-71's, the cost of diesel fuel is insane, and gas engine economy is sooo much better than years ago. The more electronics they put on a gas and diesel engine especially going forward make both engines susceptible to non-iron (iron would be piston, cam, valve, etc) related failure.

The longevity question is what people hang on when talking about switching to diesels, but you'll never wear them out in a go fast boat - period. People get bored of the go fast boats in under 3 years generally so they're sold /traded off or sit somewhere to be used when they want a thrill.

I can buy a gas boat new for much less than diesel and run the piss out of it. When I wear it out I'll come close to being able to put in 2 sets of replacement gas engines (stock power) with the money I saved on the purchase up front and the fact that diesel is almost $1 a gallon more in some areas.

Safety wise diesel can't be beat, diesel is hard to explode, gas is easy.

My Mercury outboards 225 Opti's are part of the class action suite, people call them junk etc, etc, etc. I put over 1,000 trouble free hours on them, no repairs nothing except a blown powerhead caused by a Merc Tech checking something on it for a Mercury recall and Merc ate it for me - pretty good for outboards!!!

After all that I still like the diesels better, the sound, the power, the fact I can run the boat harder longer for the same Hp. Cost wise though, they don't prove out.

Joe Gere
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