Looking for rough guestimate

[metriccrescentwrench]

I recently bought a 1981 15 ft. Bomber w/ a 1981 90 HP Johnson. (Hey - it was cheap, it's in reasonably good shape, it's all paid for, and I figure it's better to have a boat NOW that I can use rather than waiting for who knows how long for my dream boat ).

I don't have a tach yet (but it's on my shopping list). The boat currently has a 13-1/4 X 17 prop. By my estimation total weight with an average load is around 1300 pounds. Top speed (w/ gps) is about 40 mph. I'm thinking this may currently be somewhat under-propped, would appreciate some input.

At the moment I'm not so much worried about eeking out that last bit of speed as I am over-revving the engine. We do go WOT for several minutes at a time getting from one fishing spot to the next, and running back to the boat ramp at the end of the day.

Also, for when I do get around to adding a tach, does anybody know what red line is on this engine?

 

[Dhadley]

Re: Looking for rough guestimate

There really isn't any such thing as over-revving or a redline on a 2 stroke. There are no lifters to float or springs to collapse etc. The limiting factors on that motor will be a rev limiter in the pack, if it has one, the stock steel reeds and the porting.

The motor is nothing more than an air pump so to speak. As long as it can get air in and out it'll keep gaining rpm. The stock steel reeds in a crossflow will flutter and quit breathing at about 6200 or so. The 90 porting will give up in the mid 6000 range, about 6600 or so.

What you're looking for is efficiency. Once you get the tach installed you'll have your baseline. My guess is that if the motor is mounted all the way down and has no set back, a 17 won't be that far off. With that motor on a recreational boat you'll be looking for 5800 with an average load.

Guessing further, you might be looking at a 19" prop with a good set up. Again depending on your load and requirements. If that works out the 17" would be a good ski / water toy prop.

Keep us informed once you get the tach and know the true rpms. Then we can figure the slip % (efficiency) and go from there.