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Old 13 March 2006, 18:09   #1
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Country: UK - England
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prop choice 2 smaller and faster

hi i have a avon 400 with yam 30 running a 9&7/8 x 12 prop
i am getting max around 30mph on gps at a wot of 5000
and i am considering a pitch change up to a 13
what increase do you recon that this will give and will it efect plane time
as at the momement its almost instant, although speed drops 4 up to 24-26 mph

thanks andy

ps yes a 50 hp would be nice but missus wants new fire place
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Old 13 March 2006, 18:23   #2
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You'll go slower.
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Old 13 March 2006, 18:35   #3
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so what would i need to go faster as i seem to get conflicting reports on which way to pitch up or down
should i increase prop size and decrease pitch ?
or decrease prop size and increase pitch


Ta Andy
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Old 13 March 2006, 18:41   #4
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I don't think you can go faster. At least, not much. I can't remember what the max revs are for the 30 Yam but I'd guess at 5500. If that's a correct assumption, pitching down will gain you a couple of horsepower but it will also need some of the extra revs to make you go as fast as you are now. Then the few extra revs on top of that will be extra speed but it will be a small gain. I'll go and calculate for you...hold on.

Edit: I'll need your gear ratio cos It looks as though there is a bit of inaccuracy in your figures when I calculate backwards.
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Old 13 March 2006, 18:48   #5
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were would i find that in the manual or on the engine

Andy

found it 1.85

according to book wot is 4500-5500 with max hp @ 5000
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Old 13 March 2006, 18:52   #6
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The manual.

That's it, then. You're at max power as it is. End of story.
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Old 13 March 2006, 18:55   #7
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oh well thanks for help

guess itll be another cold winter then !

Andy
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Old 13 March 2006, 18:57   #8
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Still a problem with your figures cos at 1.85 you've got only about 3.4% prop slip.

I'd rough guess at about 20%.
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Old 13 March 2006, 19:00   #9
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Using your figures and 20%, it gives a boat speed of 24.5mph. If your GPS is accurate at 30mph, you must be pulling higher revs.
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Old 13 March 2006, 19:04   #10
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HI

No definently got 30+mph 1 up usually drops between 1&2 into wind but starts t o lift out of water more when fully trimmed up
main slowing is when laidened with wify,kids and toys

then as it says drops down to 24-26 on gps

Andy

but revs could and probably are out as only fitted tacho recently (bought of ebay)
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