Best Prop Pitch for my Engine/Boat

[djt283]

I've been learning from this forum all winter while I've been working on our boat and finally joined to ask a question. Last summer we bought our 1990 16' Chaparral (162 XL) from my girlfriend's sister. We were eager to get it in the water so didn't bother with any of the fixes we wanted to do until this winter. Among all the new seats, gauges, upholstering, etc. we want to replace the badly chipped prop.

I'm looking for the best WOT speed possible. I don't know anything about the existing prop other than it's aluminum and I think 13 1/4" diameter. The writing on it is not really legible any more. I don't know how to measure the pitch, but the engine manual says 13 1/4 x 17 was supplied when new. The engine is an early 80's (I think) Evinrude 70hp model E70ELCN that at WOT will reach 6000 rpm with the current prop. Since the redline in the manual is 5500rpm, I'm just curious about changing the pitch.

If anyone has any experience with this engine or a similar one who could tell me the best prop pitch for high end speed, I would really appreciate it.

Thanks.

 

[Silvertip]

Re: Best Prop Pitch for my Engine/Boat

You cannot measure the pitch of a prop unless you are geometry whiz and you have the actual procedure in front of you. Like this one posted a bit ago. The quick way to determine pitch is to go to a prop shop and have them put it on a pitch block.

http://www.propellerpages.com/?c=articles&f=2006-03-27_manual_pitch_measurement

Since you are well over the 5500 rpm recommendation you could raise the pitch. If the pitch really is 17, go to a 19. That should bring the revs down to about 5600 which is perfect. THe question however is when you saw 6000 rpm, were you alone or did you have what you consider an "average load" in the boat. If you were alone, don't change pitch. You can always throttle back. However if you change pitch based on a light load and you then try and run heavy, you will be lugging the engine.

 

[djt283]

Re: Best Prop Pitch for my Engine/Boat

Thanks for the reply. It runs at 6000 rpm with two of us on it, and maxed out at 5400-5500 with 4 adults (one in bow, two center, one aft). I figure since it currently capable of on average 500 rpm above the max, increasing the pitch so it maxes out at 5500 with two people would increase the speed. Is that generally a correct assumption?

 

[hwsiii]

Re: Best Prop Pitch for my Engine/Boat

Djt, I could be wrong but I show that model to be a 1982 Evinrude with a maximum manufacturers RPM of 5,000 to 6,000 RPM.


H

 

[djt283]

Re: Best Prop Pitch for my Engine/Boat

The closest model # I can locate on Evinrude's website is a 1982 E70ELCNB, which is why I'm figuring it's an '82. Mine does not have the B at the end of the model # though, but I have no idea if that means anything or not. I do have the original owner's manual, and it is for the 70HP long shaft (mine) and the 75HP.

Under the specifications it says "Full Throttle Operating Range 70 .... 4500 to 5500 RPM." That's why I've been assuming redline is 5500.

 

[steelespike]

Re: Best Prop Pitch for my Engine/Boat

Heres another twist;My aftermarket ser. manual indicates the motors prior to 86 are rated at 5,000.Most suggest running "motor"about 500 rpm over that at hp rating.