Low RPMs at WOT - 1964 Mercury 85hp

[jm999]

My old prop was in dire need of replacement so I ordered a new 13 1/4 x 19 prop. Put it on and the boat ran like crap. Wouldn't get up to 4000 rpm or over about 10mph. I obviously ordered the wrong size :facepalm:

I took the old one into the shop and the guy told me I need a 13 1/4 x 17 prop.

Would 2 inches in pitch really make that much of a difference? Granted I did have myself, 2 other adults, and a child on board, so I'm sure that accounted for some of the poor performance. My boat is a ~18ft, 2200lbs dry weight. Motor is a 1964 Merc850.

Thanks for any advice.

 

[chum1]

Re: Prop problems - 1964 Mercury 85hp

Re: Prop problems - 1964 Mercury 85hp

thats what i have, runs good, you may also want to check prop section forum

 

[Gomer50]

Re: Prop problems - 1964 Mercury 85hp

Re: Prop problems - 1964 Mercury 85hp

Dropping 1 pitch size equals out to about 200 RPM gain.So dropping from a 19 pitch prop to a 17 would give you 400 rpm.So in theory a 15 pitch prop with the load and rpm you stated would bring you up to around 4800 rpm.

 

[jm999]

Re: Prop problems - 1964 Mercury 85hp

Re: Prop problems - 1964 Mercury 85hp

Thanks for the replies. I was wrong in my initial post about the RPMs. It was actually running at 3000 rpm WOT. Should be between 4800-5200. I moved down to a 17p prop and it won't even get up to max RPM with just me on the boat. Only gets up to around 4000-4200. The manual calls for a 17p prop, for a boat/weight like the one I have. Something is starting to tell me that it might not be a prop issue at this point. The motor runs ok, but not like a top. Doesn't sound like its missing or struggling except when idling in gear it tends to cut out every once in a while.

Could it be something out of adjustment? Maybe an air leak in the fuel line somewhere? Maybe its not firing on all cylinders? I figured that with 85hp on an 18' open bow I shouldn't have any problems lugging a few people around. Thanks again!