prop questions... HELP!

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henry7o7

Cadet
Joined
Dec 23, 2010
Messages
23
I have 15ft klamath with full windshield with 2007 merc 40hp big foot.. when I bought the boat it didnt have a prop on it so I went to the outboard shop and they sold me a prop saying that this is the prop I need for my set up.. well long story short the boat does 30mph with 17p prop at 4000 rpm...

Should I buy another prop? ( i know if i lower Pitch it will increase RPM) spec is 5500-6000 RPM @wot
What size prop?
I was wondering if i go to lower pitch will it increase the speed?
What will I get out of buying the right prop?
 
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steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: prop questions... HELP!

Doesn't add up. Boat can't go 30 at 4000 rpm. Is speed by gps? Is the tach set right?
It appears you need to make up 2000 rpm you can't do it with a prop.
30 mph at 10% slip would be about 4800. Slip is a guess.
You might be able to make up 1200.
 
Silvertip

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,764
Re: prop questions... HELP!

That prop is about right for that motor/boat. You need to verify the tach as suggested, or get a more accurate speed reading. And NO! Decreasing pitch does not INCREASE speed. The engine runs faster with less pitch because the prop is taking less bite on the water resulting in less forward movement. Increase in speed with less pitch occurs ONLY if the existing prop is extremely over pitched for the engine. This is not the case with your setup. You might also ensure the engine is making full power -- as in is it running properly.
 
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henry7o7

Cadet
Joined
Dec 23, 2010
Messages
23
Re: prop questions... HELP!

thanksyou for all the input I will take the boat out this weekend to double check everything and respond then..
 
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