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Old 03 November 2006, 13:07   #1
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Country: UK - Wales
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Boat name: Too Blue
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Props

Just tried my 24" stainless prop - despite what some people said it does get on the plane - quite easily. Acceleration not as brisk as before but quite acceptable. Top speed was poor - 39mph - but I was pulling 5200rpm. I would have thought the engine would have bogged down more? Worked out the slip[ and it was 0.382 - gulp!!!

With my 19" 4 blade I get about 46mph at 5500rpm - much better accelration.

I think the next step is a 22" or 21" prop - any ideas? Why could I still hit 5200rpm with a 24" prop???
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Old 03 November 2006, 16:17   #2
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Quote:
Originally Posted by codprawn View Post
Worked out the slip[ and it was 0.382 - gulp!!!
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Originally Posted by codprawn View Post
Why could I still hit 5200rpm with a 24" prop???
I think you answered your own question! I've done a quick calc, and if I had 0.382 slip I would only achieve 36.4mph, at 5500rpm. Sounds like the 24" prop is WAY too big! Imagine pulling away from a standing start in something equating to 8th gear in a car... you can rev all you like, but the clutch will still be slipping when you reach maximum speed that high gear will allow.

What's the gearbox ratio on the Suzi 225?
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Old 03 November 2006, 16:57   #3
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About 1.8:1

Now you can do your calculation properly.
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Old 06 November 2006, 13:39   #4
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As Richard said, you've answered your own question.

Your motor is driving the prop faster than its holding water, hence the misleading rpm figure; but the pitch of the prop is still too high.

Your motor, I believe, is spec'd for a max rpm of 5000 - 5600, so your 19, assuming your 5500 rpm figure was WOT, sounds just about spot on.

A taller prop will slow accelleration and hole shot, and bog down as you load the boat heavier.

jky
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