What prop should I be using?

[Bad Charlow]

Hey guys. I just put an old Mercury 90hp (thunderbolt/tower of power) on my 30ft pontoon. Seems as is the rpms are way high while not quite pushing the boat so well. I'm thinking that this may be a prop issue. Looking for advice on what size/pitch prop I should be using.
Thanks

 

[schematic]

in the neighborhood of 11-13p depending on motor health and boat weight. Try to achieve 5500rpm at WOT

 

[jimmbo]

An inline 6 90hp has a WOT range of 4500 - 5000. It makes its peak hp at about 4700 - 4800 rpm. At 5500 it will be making about 70hp. That engine has small intake and exhaust ports and is just unable to breathe above 5000 rpm. Not a defect, just engineered to produce 90 hp, below 5000 rpm. So prop it for 5000 tops

Edit: I am referring to the 1978 - 1986 90hp. If you are referring to the 1965 90hp, it had an RPM range of 4800 - 5200 rpm, but it was 10 cubic inches smaller

 

[Bad Charlow]

Great info. Thanks guys!

 

[jdaghir]

For another point of reference, I have a 30 foot pontoon with an enclosed cabin (dry weight of 2560 lbs). It's powered by a 1983 mercury v-150 (150 hp at the crank, probably closer to 135 hp at the prop) with a 2.00:1 gear ratio. I am currently running a 16" dia x 11" pitch, 3 blade aluminum prop. That gives me a WOT of 5400 rpm with a moderate cargo load and 22 mph.

 

[jimmbo]

A 1983 150 would be a prop rated engine. Merc started prop rating the 6s, both Vee and In-line, in 1982.