1990-92 90HP Force

[zaufe]

I have a early 90's HP Force outboard on a Maxum 1800XR. I have the Clymer manual, but only see a range for max RPM range of like 4500-5500. Which is a huge gap! Trying to make sure I get things setup right with Prop selection/etc and what I have seen is there should be a listing of the RPM for the HP rating of the engine, but I can't seem to find one.

Right now WOT I was about 4500-4800 depending on other conditions (wind, wake, trim, etc). What should I target for on this setup?

 

[zaufe]

I should mention the SN is missing, so I can't 100% identify. My boat is a 92. I know the engine is a dual exhaust, and as far as I can tell its not the mercury LU.

 

[Nordin]

Looks like you are in a good range.
Maybe if you want to try something to pull up the RPM, go down 2" of the pitch of the prop.
It will give you about 2-300 PRM up.

 

[zaufe]

Might give it a whirl - going to try a while before making that change.

Looking at the manual, it says Check 91 and older engines with dual port exhaust (exhaust exit propeller hub and trim tab) with a 17 in pitch aluminum propeller (part No 48-FP685-3) turned down by a propeller shop to a diameter of 9 in for the 90 hp.

What does "turned down" mean?

Currently it has a 13 3/4"x17" pitch stainless steel prop.

 

[Nordin]

What manual are you talking about? Factory service,owners or an aftermarket.
Never heard about "turned down" BUT I think they mean "machined down" to 9 in diameter.

You can gind the blades if they are a little harmed BUT not turn them.

 

[jerryjerry05]

Your boat with 1 person 1/2 tank of fuel and coastguard equipment should run 52-5500 rpm's
​If your loading the boat down 2-3 heavy's and a cooler full of drinks and a dog and the fun equipment.
That can hold the rpm's down.

Now if the loads ok? and the motors running the way it should?
Then the prop needs to be changed.
Say yours is a 19p then a 17p would bring up the rpm's into the right range.

Most Force props are 13 1/2in to 13 3/4 by 15/17/19/21p or somewhere close.
The 9in would make the rpm's blast off the chart.


They can make a 17p prop into a 16 or an 18p but usually it's not needed.

The people at Michigan Propeller can lead you to the right prop.
They also have a prop forum here and the guys there know way more than I about props.

 

[zaufe]

What manual are you talking about? Factory service,owners or an aftermarket.
Never heard about "turned down" BUT I think they mean "machined down" to 9 in diameter.

You can gind the blades if they are a little harmed BUT not turn them.

Aftermarket Clymer manual

 

[Nordin]

Must be some kind of printers error. 9 in diameter prop is for a OB in 25-30Hp range not a 90Hp.

Do as jerryjerry suggest if you want to increase the RPMs. If you have a 17 in pitch prop try with a 15 in, it will raise with about 2-400 RPMs.