Increase Pitch for 85HP Force?

[Mark9740]

First I want to thank everyone again that has helped me breathe life back into my 1986 85 HP Force. It took some time but once I slowed down to listen and follow the advice on here and in my Clymers, she's back running strong. Now that I've got her running I have a question about the prop pitch. The boat is a 16' bowrider and she's carrying extra weight from waterlogged foam and stringers under the deck. I know Ii need to re-do the deck but I'd like to get a couple of good fishing seasons under my belt first to be sure the engine is in it for the long haul before I invest in a major restoration project.

She runs at 30MPH at WOT which is 4400 RPM's on the tach. It's not too bad but I'd really like to see the RPM's firmly in the acceptable WOT range of 4500 - 5500 RPM's. The prop is a 13 1/2 x 17. I'm thinking about dropping to a 15 pitch to increase the WOT RPM's. Realistically, how much will the RPM's increase if I drop to a 15? Is doing this advisable? Any feedback would be appreciated. Thanks.

 

[roscoe]

Re: Increase Pitch for 85HP Force?

Dropping to a 13p will likely get you to 4800-4900 rpm.
I wouldn't bother with a 15p.

 

[hwsiii]

Re: Increase Pitch for 85HP Force?

Mark, Roscoe is very right about you need to buy a 13" pitch prop in the same make and model you have now, but I think he was preoccupied, because your RPM ought to go up to about 5,200 RPM and your hole shot will improve tremendously, and you should not lose more than about 1 MPH at WOT.

You might try reading this for a better understanding of motor height, changing props for better performance, and for the major differences between aluminum props and stainless steel.

http://forums.iboats.com/showthread.php?t=369057

And get back with me after reading the above white paper.



H

 

[Mark9740]

Re: Increase Pitch for 85HP Force?

HWSIII,
Lots of great info. I read the link you sent me and can tell you that I have been experimenting with my engine height as well. It was plowing a lot of water with the mid leg so I raised it an inch or so to the bottom hole in the motor mount. It runs fine but when I turn sharply I get lot of sucking sounds from the prop getting air. I think once I get the deck replaced and lose the extra weight from the waterlogged foam & stringers, the original hole will be fine.

I couldn't find the gear ratio (model #856F6A) but I think it's 2:1 so I calculated my prop slip based on that and came up with 15.2%. Do you still agree that a 13 pitch prop is the way to go?

Thanks again for the info and help with this.

 

[Frank Acampora]

Re: Increase Pitch for 85HP Force?

I think you will be disappointed with a 13 pitch prop.--but I could be wrong, after all I did get married LOL.

That 85 is a small bore engine (3.3125 bore, 2.80 stroke, 72 cubic inches displacement) and does not have gobs of torque. I have a similar one on a 15 footer. 21 pitch prop gives 44 MPH at 44-4500 RPM. 19 pitch prop gives 44 MPH at about 5000 rpm. 17 pitch prop gives 36-38 MPH at I forget what RPM. RPM wasn't important to remember, speed was too low.

The faster you turn the prop, especially thick bladed aluminum props, the greater the parasitic drag (the force necessary just to turn the prop through the water) and the less horsepower available to move the boat.

Base on this , I would recommend that you drop down no more that 2 pitches, that is, a 15. Anything more and you will be disappointed with top speed

 

[hwsiii]

Re: Increase Pitch for 85HP Force?

Frank, I just want to let you know that i am NEVER wrong, UNLESS I am breathing. LOL

Mark, I will tell you I have great respect for Frank's opinions, especially when it concerns Force outboards, as I also have a Force myself on my flats boat.

With that being said, I believe it is better to have the motor turn as many RPM as we can to try and reach the manufacturers recommended maximum RPM. And the 13" pitch will still not reach that goal, but it will be very close. Just remember to buy the same make and model of prop that is on it now.



H

 

[Mark9740]

Re: Increase Pitch for 85HP Force?

I'm undecided about which way to go...13 or 15 pitch. You both have made very good points. I guess I'll need to sleep on this one for awhile. Thanks.