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19-02-2007, 01:42 PM
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#1
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Registered User
Country: United kingdom
Location: West Sussex
Occupation: Grease Monkey
Interests: Makin my boat faster!
Boat name: S.B Racing
Boat make: Ocke mannerfelt canopied B23 / Zapcat
Engines: Merc 200XS Gen 2 / Tohatsu 50
Cruising area: littlehampton/Southampton
Join Date: Jan 2005
Location: West Sussex
Posts: 2,656
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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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Class 3C Mono EPA National speed record holder 95.35 mph Avg!
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19-02-2007, 03:36 PM
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#2
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Registered User
Country: US
Location: Virginia
Interests: Quiet desperation
Boat name: A Little Noisy
Boat make: Excalibur 24
Engines: Express Racing 525
Cruising area: Mobjack Bay (where?)
Join Date: Mar 2004
Location: Virginia
Posts: 1,974
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I thought that having more patience was a result of my getting older. Turns out that I just don't give a sh*t.
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19-02-2007, 03:56 PM
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#3
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Registered User
Country: hamburg , germany
Occupation: electronics
Interests: boating, audio , PC
Boat make: Fletcher Arrowshaft 199
Engines: Yamaha 225
Cruising area: baltic sea
Join Date: Dec 2005
Posts: 115
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if you use a cupped prop , just download my propcalc-tool
click!
greetings!
Andi
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19-02-2007, 04:20 PM
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#4
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Registered User
Country: United kingdom
Location: West Sussex
Occupation: Grease Monkey
Interests: Makin my boat faster!
Boat name: S.B Racing
Boat make: Ocke mannerfelt canopied B23 / Zapcat
Engines: Merc 200XS Gen 2 / Tohatsu 50
Cruising area: littlehampton/Southampton
Join Date: Jan 2005
Location: West Sussex
Posts: 2,656
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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?
__________________
Class 3C Mono EPA National speed record holder 95.35 mph Avg!
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19-02-2007, 04:44 PM
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#5
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Registered User
Country: hamburg , germany
Occupation: electronics
Interests: boating, audio , PC
Boat make: Fletcher Arrowshaft 199
Engines: Yamaha 225
Cruising area: baltic sea
Join Date: Dec 2005
Posts: 115
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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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19-02-2007, 05:10 PM
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#6
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Senior Member
Join Date: May 2006
Posts: 277
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Quote:
Originally Posted by scottyboy
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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Assuming you know your gear ratio and you know the prop pitch,
Quote:
Originally Posted by scottyboy
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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You can calculate exactly what the slip is:
Then you can get exactly the new speed...
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19-02-2007, 05:36 PM
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#8
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Senior Member
Join Date: May 2006
Posts: 277
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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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19-02-2007, 06:55 PM
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#9
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..
Join Date: Jun 2004
Posts: 1,567
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19-02-2007, 07:22 PM
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#10
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Registered User
Country: United kingdom
Location: West Sussex
Occupation: Grease Monkey
Interests: Makin my boat faster!
Boat name: S.B Racing
Boat make: Ocke mannerfelt canopied B23 / Zapcat
Engines: Merc 200XS Gen 2 / Tohatsu 50
Cruising area: littlehampton/Southampton
Join Date: Jan 2005
Location: West Sussex
Posts: 2,656
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Thankyou all! Matt yours was the simplest though!
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Class 3C Mono EPA National speed record holder 95.35 mph Avg!
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