RPMs at WOT?

[Nitro2014]

I have a 1985 2 stroke Johnson 115 hp outboard. It runs 4500 rpms at WOT trimmed all the way up. With an added weight of around 300 lbs it will run 5000rpms with the trim in the same position. Do you think I am over-propped and working my motor too hard?

 

[octofoil]

I would definitely check out the info on this website at: http://boatpropellers.iboats.com/propeller-tutorials/how_to_choose_prop/?cart_id=848645851

Also, if you are adding weight and increasing RPM I'd be curious to know if weight distribution is an issue. If I ever have a heavier load than normal it helps to get some weight on the bow.

Not sure what the spec is for your motor, but my1977 55hp is 5000-6000rpm WOT. It came factory with a 15 pitch and I had to back it off to an 11 pitch to get 5500 rpm target. I was really surprised that I only lost about 3-4 mph top speed.

 

[emdsapmgr]

That engine's normal/original operating range is from 4500 to 5500 per the factory. Current thinking is that it should be propped to run in the upper part of that range, or 5500 rpm's. So, technically, you are lugging the engine when you run it at only 4500 rpm's at WOT. It will be dog out of the hole, run warmer than it should and will burn a lot of fuel for less speed. Re-prop it.

 

[Nitro2014]

Thanks for the information! Right now I have a a 13 1/4 x 20 turbo-o 4 blade stainless prop. I would like to keep the next prop under $200. I prefer another stainless 4 blade but am not opposed to trying out and aluminum or 3 blade. (Just need to choose the right one) Any suggestions on a lower pitch change and/or different prop styles in general?

 

[Nitro2014]

Sorry, I should have posted in the prop category. Regardless, I appreciate the great info!

 

[emdsapmgr]

Borrow a 17 pitch aluminum 3 blade and test that for WOT rpm's.