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

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Old 08-24-2008, 04:11 PM
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It was a single in a direct drive inboard Parasail boat.Never had a pyrometer on it .
The heat damage was the exhaust manifold melting through by the Turbo on 2 occasions and the turbos housings cracking after every thousand hours.

Engine never ran hot on the gauge And was always left to idle and cool down after running on plane.

Only answer I ever got from Yanmar was the motor was a light duty motor and if I wanted reliability to step up to the 350 engine whick I think is now a 420
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Old 08-24-2008, 04:11 PM
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Quote:
Originally Posted by articfriends View Post
Assuming the motors will pull the prop a 30 pitch at 3800 at 10% slip will put you at 64 mph,at 32 pitch at 15% slip will put you at 65 mph,you can play with the numbers on the prop calculator website,Smitty
http://www.rbbi.com/folders/prop/propcalc.htm
That calculator is bubu.... It doesn't take into account Diameter & Area Ratio.
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Old 08-24-2008, 04:16 PM
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I heard they where discontinuing the 315.
Buddy of mine that does all the warranty work on Hinkleys has had a field day working on the Yanmars lately.A lot of problems with the after market coolers and such on them creating major problems.
He told me Hinkley is swicthing to Volvos from now on.

Don't shoot me I am just the messenger.
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Old 08-24-2008, 04:23 PM
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Quote:
Originally Posted by tommymonza View Post
It was a single in a direct drive inboard Parasail boat.Never had a pyrometer on it .
The heat damage was the exhaust manifold melting through by the Turbo on 2 occasions and the turbos housings cracking after every thousand hours.

Engine never ran hot on the gauge And was always left to idle and cool down after running on plane.

Only answer I ever got from Yanmar was the motor was a light duty motor and if I wanted reliability to step up to the 350 engine whick I think is now a 420
How long was the average parasail ride and what RPM did you run at???
Did you have one of the earlier 315's?
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Old 08-24-2008, 04:35 PM
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Quote:
Originally Posted by METAL BROS RACING View Post
Right @ 60-mph would be my guess. I have already put in an email to to Max Machine Worx guys, I've never met them but their work is well spoken of in our area. I will have them do a 1.36 swap and if they have any goodies they can add while they are in there we will do that too. None of this happens til Feb anyhow so there is time to get a solid plan together.

Probably something in the 26" range then at 3800 will put you into the low 60's.
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Old 08-24-2008, 04:39 PM
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Quote:
Originally Posted by 29Firefox View Post
That calculator is bubu.... It doesn't take into account Diameter & Area Ratio.

Its not perfect, but it is a good reference to plug in some numbers and get a starting point.
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Old 08-24-2008, 05:21 PM
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Somebody please give me some real speeds with real prop pitch of 30' - 35' boats running their gas engines at 3,800 RPMs with bravo drives?
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Old 08-24-2008, 06:20 PM
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At 3800rpms, in my 32 Active Thunder with 1.36 drives and 24 pitch props, I'm right around 55mph.

My propslip remains pretty constant at about 13-14% from 3800rpms through WOT.

Last edited by Griff; 08-24-2008 at 06:23 PM.
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Old 08-24-2008, 07:57 PM
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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 pitch 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

Last edited by HabanaJoe; 08-24-2008 at 08:10 PM.
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Old 08-24-2008, 08:05 PM
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Quote:
Originally Posted by daredevil View Post
Acording to this your boat would have 0.7 % slip !??????

3800 X 24 / 1.36 X 0.01 / 12 = 55,882 MPH

And if you do it that way you get the most acc. speed and slip calculation.

RPM X PITCH / RATIO X 0.01 / 12 = SPEED with 00.00slip !
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.

Last edited by Griff; 08-24-2008 at 08:10 PM.
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