Selecting Propeller for Mercury

[Raider3:16]

I have a 84 Mercury inline 6 90 hp

It sets on a 24 foot pontoon boat

I am having trouble finding out what pitch prop I need

I have tried used the online guides but they all want to know RPMs and since my boat doesn't have a gauge I don't know that one.

Any help would be appreciated

Edited October 3, 2009 by Raider3:16

[Cracker Larry]

Without a tachometer it will be almost impossible to get the right prop, or to even know if you have the right prop. That engine should be running between 5500 and 6000 rpms wide open, that's where it's rated. If the prop has too little pitch the engine will over rev and could be damaged. If the pitch is too high, the engine can't turn the rpms it needs to make power and you'll be wasting fuel and going slow. Changing props is like changing gears in a car. You need to select a prop that will put the rpms in the power band.

Pontoon boats usually require less pitch than other hull types. If I were to take a wild guess, I'd say you'll probably need about a 13 pitch, maybe a 15. It would be worth the $100 to install a tach, it could easily pay for itself in fuel or engine damage. Then find a dealer who will let you try a couple of sizes, prop selection is trial and error. The right pitch will put your engine about 5800 rpms top end.