Another Prop Question

fishermanaddict · Mar 26, 2009

I am new to the forum. I need some help with my prop. I have Yamaha 150hp 2 stroke Saltwater SS. My manual call out for 4500-5500rpm WOT. The most I get is 5000rpm trim up. 70gallon fuel, T top , 2 people , no bottom paint. Is 5000 rpm would be ok or have to be 5300-5500 rpm at WOT? Would 5000 rpm will short the engine life?I am running with Saltwater stainless prop 13.75 X 17pitch , need some advice.

Megabyte · Mar 27, 2009

Some folks would say that it should be closer to the 5500 rpm max, but my OX66 runs about the same as yours - 5300 in the spring and 5000 with a 'dirty bottom' by fall. I'm happy with that.

Simplest thing to do would be to take the prop to a local prop shop and ask them to tweek it to give you a bit more rpms. That could be a pretty low-cost solution.

Or... ask the prop shop to make a recommendation based on your circumstances. Most good shops will loan a prop to test if they know you are a serious (buying) customer.

Good luck!

DaleH · Mar 27, 2009

Megabyte":1uibfzkz said:
Some folks would say that it should be closer to the 5500 rpm max, but my OX66 runs about the same as yours - 5300 in the spring and 5000 with a 'dirty bottom' by fall. I'm happy with that.

Based on my read, your setup is quite different from Capt Kevs. I'd be OK with his setup, but if you're only running 5K now, with a clean bottom in the Spring, you'll lose another few hundred RPMs as the bottom gets dirty.

IMHO you should NEVER ever allow the max RPMS to be below 5K on a 5.5K max RPM motor, while @ WOT. Another thing, all normally aspirated combustion engines lose up to 200 RPMs during the heat of the Summer. All Yummie dealers up here recommend being off by 200 preferably, but no more than 300 RPMs off the max RPMs.

Less not adding oil or having an oil transfer issue, over-propping motors is the surest way to kill an engine.

Megabyte · Mar 27, 2009

Dale is absolutely right.

5000 rpms in the spring when the air is cool and the bottom is clean is a bit too low. Talk to your local prop guy and get his recommendation for bumping that closer to 5500.

fishermanaddict · Mar 27, 2009

I didn't know the spring time would make the engine perform different. Do you guy think I should drop down in pitch. like 15.75 X 15 pitch? If I go with bigger dia. and smaller pitch how that make any different?

DaleH · Mar 27, 2009

fishermanaddict":3asbh0f8 said:
I didn't know the spring time would make the engine perform different. Do you guy think I should drop down in pitch. like 15.75 X 15 pitch? If I go with bigger dia. and smaller pitch how that make any different?

Definitely drop your pitch. You might even get a good shop to dial in the prop you have, though they can usually only move a SS one about an 1". Props can be more testing and voodoo magic than science. I have found that going too high on the diameter, when it exceeds the pitch, didn't work for me. I would think a premium 15.25" x 15" wheel would be slick on that rig. Many here like the Merc Enertia.