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Old 19-02-2007, 01:42 PM   #1
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Prop calculation

If you've tested a prop and have a max speed at a max rpm on the engine whats the easiest way to work out from there if the rev's were at the highest desired point what the speed would be?
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Old 19-02-2007, 03:36 PM   #2
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Does this help?

http://www.rbbi.com/folders/prop/propcalc.htm
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Old 19-02-2007, 03:56 PM   #3
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if you use a cupped prop , just download my propcalc-tool

click!

greetings!
Andi
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Old 19-02-2007, 04:20 PM   #4
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thanks, but this does'nt take into account for diameter of the prop your running and how much slip should i allow for?
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Old 19-02-2007, 04:44 PM   #5
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diameter is not really important, most props are 13 1/2 or 14 1/2, it will change to 50-100 RPMS

If you have a lightweight boat , use 10 % or less, but minimum is 6 %
Some props have more slip but are faster!
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Old 19-02-2007, 05:10 PM   #6
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Quote:
Originally Posted by scottyboy View Post
thanks, but this does'nt take into account for diameter of the prop your running and how much slip should i allow for?
Assuming you know your gear ratio and you know the prop pitch,
Quote:
Originally Posted by scottyboy View Post
If you've tested a prop and have a max speed at a max rpm on the engine whats the easiest way to work out from there if the rev's were at the highest desired point what the speed would be?
You can calculate exactly what the slip is:

Then you can get exactly the new speed...
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Old 19-02-2007, 05:28 PM   #7
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For Scotty's specific question, I would use this:
http://www.go-fast.com/Prop_Slip_Calculator.htm
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Old 19-02-2007, 05:36 PM   #8
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Calculations:

(1)
( pitch x RPM ) / (gear ratio x 1056 ) = Theoretical speed in MPH

(2)
( TSpeed - ASpeed) / TSpeed = Slip

(3)
Calculation constant = 100% - Slip


You know your pitch, RPM, Ratio and actual speed, so the rest is just numbers...


Say you were actually going 90mph, and you were at 6410RPM with a 28 inch prop with a gear ratio of 1.7

(28 x 6410 ) / (1.7 x 1056 ) = 179480 / 1795 = 100mph Theoretical speed


TSpeed = 100mph
ASpeed = 90mph
therefore Slip = 10%
Therefore Calculation constant = 90%

Now you have your slip for your exact situation, so insert it back into the equasion, and move your RPM to the point you wanted it to be...


(28 x 7250 ) / ( 1.7 x 1056 ) = 203000 / 1795 = 113MPH Theoretical speed

Reduce the TSpeed by the calculated slip and you get the new expected true speed..

113 x 90% = 101mph Actual Speed
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Old 19-02-2007, 06:55 PM   #9
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try this one...

http://www.mybayliner.co.uk/boatmad/propnn.zip
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Old 19-02-2007, 07:22 PM   #10
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Thankyou all! Matt yours was the simplest though!
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