1975 Johnson 85 HP - Prop Size Question

[sramos]

What style and size prop do I need. The boat is a 15 ft Tri Hull. The motor is a 1975 85 HP Johnson. I really don't anything about props. The prop on there right now is really dented and dinged.

[emdsapmgr]

It's really hard to say what prop will work best on that engine/boat combination. Back in those years, the factory used to ship engines with props. That engine should have shipped with a 17 pitch prop. That's a good place to start, but don't be suprised if you have to go down to a 15 pitch on that heavy boat. You want that engine to run right a 5500-5600 at full throttle, with one person in the boat and a light load of gas.

[sramos]

Thanks for your reply. It has a 13 - 1/4 x 17. The tachometer is trashed so I really don't know what the RPM's are at WOT. I really never run the motor WOT as it doesn't handle well in rough water on WOT. I just listen to the wine of the motor and keep it reasonable for conditions. Doesn't really seem to be a problem. The prop is really chewed up and I will replace it at some point. If I drop down to a 15 it will probably help a little as far as getting up on plane?

[eavega]

I have a 15' tri-hull I run a '76 70 HP Johnson on. I have run it wit the 17P prop with a full load (three guys plus fishing gear) and it runs @ 5500 RPM. Boat is heavy for a small craft, so you should be okay running a 17P prop to start with an 85 HP. I don't think you should step down to a 15P, especially without a tach. See if you can find someone to loan you the right prop for testing, or try to pick one up online.

-Rgds

[sramos]

For now - I think I will stick with what I have. I will be fixing the tach this summer and maybe I will take a look at the prop at that point.

Thanks for the reply.