Help with selecting the right prop before I blow up my motor.

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Spikeo

Cadet
Joined
Aug 18, 2010
Messages
10
I have a 1997 Merc 40HP 4 cylinder 2 stroke that is on a 14ft RIB inflatable. Total weight with me in the boat is approx 700lbs The current prop is 10 1/2" dia. 13 pitch. WOT the tach is red lining at over 6000 rpms :eek: I am trying to figure what prop I need to get me in the 5000 to 5500 rpm range. I was thinking an 11" dia with a 15 pitch. I don't want to purchase and then find out that its not the right one for my boat. Any formula to calculate what would be the right prop? Or is it a hit or miss, trail and error approach in selecting the right one.
 
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steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Help with selecting the right prop before I blow up my motor.

Without a gps speed to go along with the rpm its difficult the evaluate your setup.
I'm assuming the motor is a Merc.?
A rule of thumb "about" 200 rpm change with each inch of pitch. Increase pitch lowers rpm.
Don't worry about the diameter; if the prop is intended for your motor it will fit. It does appear you need a 16 or 17" prop.
 
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Spikeo

Cadet
Joined
Aug 18, 2010
Messages
10
Re: Help with selecting the right prop before I blow up my motor.

SteelSpike The next time I go out I will clock the boat, with my GPS. It feel like I am doing 60 mph. Not used to sitting so close to the water. The Speedometer on the boat reads 35 mph.
 
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