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Old 06 August 2009, 13:58   #1
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250 prop?

have replaced 2 x 200hp yammies with 2 x 250. the 200's had a prop of 13 3/4 x 21. the 250's have a prop of 14 1/2 x 19. can anyone explain what these dimentions mean..?
i feel the 250;s r over reving, or at least peeking out with very little effort. i put the 200 props on the 250..pretty much the same result.
thought being 21" they might be bigger or bite more...der!
its obviously down to pitch and surface area or somthing, i know not wot.
any way wot props should i use for the 250's to make them work a bit harder? and wot the hell do those dimentions mean..???
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Old 10 August 2009, 14:10   #2
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While we're waiting for some of the experts to respond, I can help you with the dimensions question.

The first figure is the diameter of the propeller itself (end to end) which is fairly straight forward to measure.

The second is the pitch (19", 21"). Measurement of the pitch will take a bit of doing of course not required in your case. Basically the pitch is the theoretical distance the propeller would travel in one complete revolution. Think of it as a screw and imagine the distance the screw would move in one complete turn - that's your pitch.

What RPMs are your engines running at WOT? is it within the recommended rev band of the manufacturer? If yes, then which side of the spectrum is it towards?

Basically, switching to a prop with a higher pitch would reduce the RPMs and conversely for a prop with a lower pitch.
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Old 14 August 2009, 02:04   #3
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Originally Posted by shahid View Post
While we're waiting for some of the experts to respond, I can help you with the dimensions question.

The first figure is the diameter of the propeller itself (end to end) which is fairly straight forward to measure.

The second is the pitch (19", 21"). Measurement of the pitch will take a bit of doing of course not required in your case. Basically the pitch is the theoretical distance the propeller would travel in one complete revolution. Think of it as a screw and imagine the distance the screw would move in one complete turn - that's your pitch.

What RPMs are your engines running at WOT? is it within the recommended rev band of the manufacturer? If yes, then which side of the spectrum is it towards?

Basically, switching to a prop with a higher pitch would reduce the RPMs and conversely for a prop with a lower pitch.
OK thanks, that makes sence, i guessed it was something on those lines. max rated rpm is 5600, on medium load the engines reach the max very easily. from wot u say on pitch and diameter then my different props r going to have the same affect which i have proven with trial, looks like i might need a 14 x 21 or simillar. obviously this will be an experts decision, i will contact ho yamaha here in egypt but they r a bit backward on tech stuff...cheers
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Old 14 August 2009, 16:07   #4
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I would say you are going to need 23" pitch props or maybe even 24". You should see quite an improvement in speed as well
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Old 15 August 2009, 01:05   #5
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Ever heard of:
http://bbcboards.zeroforum.com/zeroforum?id=14
They are kind and very helpful. Thanks to them I changed my prop from a SST 23" to a "Croxtonised" Tempest Plus 23". In one word: fantastic.
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Old 15 August 2009, 02:59   #6
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I would say you are going to need 23" pitch props or maybe even 24". You should see quite an improvement in speed as well
Ok, but wot diameter, i presume diameter is as important as pitch......???
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Old 15 August 2009, 03:02   #7
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Ever heard of:
http://bbcboards.zeroforum.com/zeroforum?id=14
They are kind and very helpful. Thanks to them I changed my prop from a SST 23" to a "Croxtonised" Tempest Plus 23". In one word: fantastic.
this was my next question..!! can anyone reccomend a reliable experienced company who can advise on and supply props...!! i will check them out thanks.

anyone any other suggestions...???????
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