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Old 07 December 2009, 14:05   #1
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Yamaha prop Nuts!!!

i want to achieve a higher speed, at the moment i can get 40 knots at 5400, is that the limit??
currently running a 13x 19 pitch ally Yam K series Prop.

Handbook says 4500-5500 rpm, But its a lighter boat than the searider 54 by 80 kgs, (approx worked out).

what size stainless prop would achieve a higher top end,
I tried a 13x19 stainless and got 34 knots and 4900 max on the searider, so no go there.

if i tried a 13 or 13.3/4 by 17 pitch, would that do the trick??

does the larger prop help or hinder??
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Old 07 December 2009, 14:15   #2
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Did I read that you've just dropped the motor down on the transom by 1 hole?
That isn't going to help you achieve higher top speed.
If you're getting 5400rpm now, then it sounds like you're just about there, with your current set-up.
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Old 07 December 2009, 14:38   #3
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brad, yes just dropped the motor one hole and handles so much better, i will refuse to break out under heavy cornering and trim down really pushes the nose down now, so i can get more speed in a heavier sea,

the speed has increased, weirdly?????

also it now revs to 5400 which is 100 short of its max stated rpm,

i just wonder if its to do with the engine running in, getting slacker.. now done 49 hours
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Old 08 December 2009, 04:06   #4
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If it's pure top speed you're after, and don't mind cr@p accel and the prop letting go at the slightest provocation, Yam used to do a series of props that were higher pitched but smaller diameter - the increase in torqure needed to turn the higher pitch was offset by the drag reduction of the smaller diameter.

Works better on a lighter boat, as it's not needing to push so much load. My old Yam 55 had one when I got it - it flew in a straight line on flat water, but was about as much use as a chocolate teapot for cornering & in big waves.
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Old 08 December 2009, 05:14   #5
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I'm no means an expert, but I'd have rough weather handling and speed over top end any day.

You're always going to compromise one set-up with another to a degree. On my BWM I used to run two different sized props. One for calm conditions and light load and one for a heavier load or where I needed the handling and acceleration but didn't need the top end.

As far as I can see there are two easy options for you.

A Jackplate or lighten the boat as weight is a big killer of speed. I lost 4 knotts from the first run to the time I had Venom kitted out.
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Old 08 December 2009, 12:57   #6
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as much use as a chocolate teapot for cornering & in big waves.
No use at all to Matt then !
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Old 08 December 2009, 13:31   #7
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No use at all to Matt then !
From the photos he seems to spend most of his time ot the water, in which case one of these might be better: http://en.wikipedia.org/wiki/File:Pr..._1500x2100.jpg

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)?
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Old 08 December 2009, 13:54   #8
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now thats a question!!

if i increased the dia a quarter of a inch to a 13 1/4 x 19 ally, would that give me more beans???

bottom line is that if she is revving to near maximum RPM and 3-4 knots short of desired speed (love to acheive 44 knots on the flat) then i guess im looking at upping the motor hp, which has been discussed many times i know with limited results.

hmmmm , what to do!!
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Old 08 December 2009, 14:01   #9
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now thats a question!!

if i increased the dia a quarter of a inch to a 13 1/4 x 19 ally, would that give me more beans???

bottom line is that if she is revving to near maximum RPM and 3-4 knots short of desired speed (love to acheive 44 knots on the flat) then i guess im looking at upping the motor hp, which has been discussed many times i know with limited results.

hmmmm , what to do!!
Twin 90's!
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Old 08 December 2009, 16:25   #10
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hmmmm , what to do!!
Give up numb nuts maybe if you filled the boiler with some more coal you might get a few more revs outta that old stove.
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