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Old 08 December 2009, 16:34   #11
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if i increased the dia a quarter of a inch to a 13 1/4 x 19 ally, would that give me more beans???
I'm no expert but that seems counter intuitive - if its a bigger diameter it will be harder to turn, and so the revs will drop? lower revs at same pitch = lower speed. So unless your current slip is quite bad then I'd guess no.
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Old 08 December 2009, 16:40   #12
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hmmmm , what to do!!
What's the max hp rating of the boat?
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Old 08 December 2009, 17:52   #13
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Old 08 December 2009, 18:42   #14
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Well, in terms of power to weight, at 90hp, it doesn't get any better than a 2-stroke Yam.
You could perhaps get in touch with the guys who race these motors in 3b. They may be able to offer you some tips on set up.
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Old 09 December 2009, 04:12   #15
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if i increased the dia a quarter of a inch to a 13 1/4 x 19 ally, would that give me more beans???
Increase in diameter will give you more grip on the water, but also will increase the drag.


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(love to acheive 44 knots on the flat)
Drop the diameter, up the pitch. Unfortunately there are no easy guestimators for diameter / revs like the "1" pitch = approx 200rpm" rule of thumb for pitch.
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Old 09 December 2009, 11:35   #16
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Seriously though if top speed is the priority is it better to get air borne (loosing drive but having no resistance from the water) or to keep the prop in the water (maintaining drive but with some drag)?
Both, I think. As I recall, the big offshore racing powerboats try to get just the prop in the water, with the hull riding on a cushion of air. Probably easier to do with >1000hp on tap, though.


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Drop the diameter, up the pitch. Unfortunately there are no easy guestimators for diameter / revs like the "1" pitch = approx 200rpm" rule of thumb for pitch.
I've heard 100rpm per 1/4" change in diameter. Don't know how accurate that is, as I haven't found diameter adjustments that fine in props I can actually afford.

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Old 10 December 2009, 04:04   #17
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I've heard 100rpm per 1/4" change in diameter. Don't know how accurate that is,
I think it depends on your starting diameter - as your prop gets bigger, 1/4" increase will give you a proportioanally higher area increase.......
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Old 10 December 2009, 11:35   #18
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No doubt; same as pitch - it doesn't always follow the 200 - 250 per inch rule, especially when making large changes.

But, as a starting point, it's probably close. Closer than "no idea", anyway.

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