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Old 28 January 2007, 10:46   #1
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Yet another prop question!

Finally managed to get new boat in the water and I think it's a bit over-propped. With Ribcraft 5.3 and Tohatsu 50 (yes I know it's a small engine for a 5.3), max Rev's I get are 5100 with engine trimmed as far out as I can. If I change the prop to one with pitch 1 inch less than current I am estimating I will get close to max RPM of 5800 (Spec is something like 5100 to 5850 at WOT).

My quesiton is - if I change the prop, will this actually give me any more speed? I'm getting approx 25Knts at moment.

Thanks.
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Old 28 January 2007, 18:07   #2
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Quote:
Originally Posted by al40 View Post
Finally managed to get new boat in the water and I think it's a bit over-propped. With Ribcraft 5.3 and Tohatsu 50 (yes I know it's a small engine for a 5.3), max Rev's I get are 5100 with engine trimmed as far out as I can. If I change the prop to one with pitch 1 inch less than current I am estimating I will get close to max RPM of 5800 (Spec is something like 5100 to 5850 at WOT).

My quesiton is - if I change the prop, will this actually give me any more speed? I'm getting approx 25Knts at moment.

Thanks.
Al40,

I am by no means an expert in this area but,

(i) I don't think you will get 700 extra RPM by going down just 1 inch. (Rule of thumb people here normally quote is 100-200 RPM/Inch)
(ii) even if you did I doubt you would get any significant extra speed. Maybe 1 knot extra with a 1" smaller prop and the full 700 extra RPM.

Actually to me 25 knts seems not bad for that size of boat and engine.
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