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Old 14 April 2010, 19:25   #1
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Propeller diameter help

Hi

I have twin 60hp mariner 2 strokes on a 5.5m BWM, originally i think the props were 12.25 x 15" pitch. I bought a 10 1/8 x 15 p by mistake as i didnt realise there were different diameters for the same size pitch. Probably a school boy error, but my decision now is whether i purchase another 12.25 x 15 pitch or a 10 1/8 x 15 pitch.

As they are both 15" pitch what difference will the diameters make in regards to performance?

Thanks
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Old 15 April 2010, 02:55   #2
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Quote:
Originally Posted by ed_dash View Post
Hi

I have twin 60hp mariner 2 strokes on a 5.5m BWM, originally i think the props were 12.25 x 15" pitch. I bought a 10 1/8 x 15 p by mistake as i didnt realise there were different diameters for the same size pitch. Probably a school boy error, but my decision now is whether i purchase another 12.25 x 15 pitch or a 10 1/8 x 15 pitch.

As they are both 15" pitch what difference will the diameters make in regards to performance?

Thanks
There is more to it than diameter, the best thing to do is try them both back to back and determine the difference on your particular set up, then come back with WOT RPM's and speed although you will probably already know the answer. With a race boat smaller diameter would give you more RPM, so if you're near the limit with the larger diameter props they may hit the limiter - let us know how you get on!
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Old 15 April 2010, 06:18   #3
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diff diameters on a twin setup?

i only have 1 of the smaller diameter props, as it was bought by mistake. is it ok on a twin setup to run 1 engine on a 12.25 diameter and 1 on a 10 1/8 diameter providing i keep an eye on the rpm.
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Old 15 April 2010, 07:28   #4
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In really simple terms, the smaller diameter has less drag, so will tend to spin faster. The race boat trick cookee is tlking about is basically to get more speed, by using a smaller diameter, you load the engine back up by upping the pitch (Pitch is like the gears on your car, higher gear = high load). Race boat speed is slightly important, so the higher pitc, the faster you go!

So, if youi drop the diameter, at the same pitch the small dia prop will spin faster.

Other thing is that a smaller prop has less"grip" o nthe water, so will tend to "let go" more easily (e.g. corners, rapisdly opened throttles, waves) which with race boats isn't such a big problem as they are generally lighter than a cruising rib where "all round grip" is more important......


I certainly wouldn't run different props on twins. If the prop was new, can you not P/X it back in for the original size?
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Old 15 April 2010, 11:35   #5
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Ballpark rule I've heard is 100rpm per 1/4", so the 10-1/8 will spin about 400 to 500 rpm faster WOT.

A bigger problem will be the different thrust acheived for a given rpm, but that's not really ballparkable, as far as I know.

jky
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Old 18 April 2010, 04:34   #6
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Quote:
Originally Posted by ed_dash View Post
i only have 1 of the smaller diameter props, as it was bought by mistake. is it ok on a twin setup to run 1 engine on a 12.25 diameter and 1 on a 10 1/8 diameter providing i keep an eye on the rpm.
No! You will need different throttle settings and it will make the handling of the boat a nightmare - probably!

If it's brand new do as 9D280 says and part exchange it with something the right size!
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