Question on a Force prop

[rpatton]

Ok here it goes. My brother inlaw bought a 17' bow rider with a 85 hp force on the back. They took it out and with 2 people on board it will only turn 4000 rpm. I'm thinking it should run more like 5500 rpm , right? I'm not there so I'm unsure of the prop size . My question is will any other makes interchange with the force motors? I have a spare Johnson 70 prop I'd like to try to see if the rpm's will come up. Or can someone tell me roughly what size prop should be on the boat? Thanks

 

[jimmbo]

You need to know what the WOT rpm range is, I think the 85 force was 4500 - 5500. Your Johnson prop will not fit the Force engine. Some later engines could use mercury props but since You have not posted the year of the engine... Older Force engine used Chrysler props.
Before you go running and buying props, make sure the engine is in good condition, as in, good compression, good spark, and fuel system. Also the tach could be wrong

 

[rpatton]

It's a 1982 with a piranha prop 13.7515B on it. I have no idea if it's right or not. I've been reading that it should be a 13x19 but at roughly 200 rpm per inch of pitch that doesn't seem to be enough. I think I need to personally run this boat and see , maybe it's just the tach