1989 force 125 prop size?

[maxum247]

I have a 20ft maxum cuddy cabin, weight about 2,150 lbs with a 1989 125 Force outboard, 307 lbs, purchased new in 1989 as boat motor and trailer package. Prop is the original that came on the engine when purchased new. Tryed to figure prop size, all I can find for a number is on the outside of the prop hub (15) no other numbers inside or out, front or back. How could I find out what the prop diameter/pitch would be?
Speed in 89 (not sure about rpm) with this prop was 37mph, 30 gallons of gas give or take.
Today tack reading is about 5000 rpm at full throttle with a full tank of gas and a speed of 35-37mph depending on weather conditions.
Changing boat over to center console, looking for a starting point to work with on prop size. Should the rpm be a little higher in general? I beleive the book calls for 4500-5500 rpm. Thanks!

 

[Frank Acampora]

Re: 1989 force 125 prop size?

I had a 21 foot Manatee cuddy with a 125 Force and it came with a 19 pitch prop. WOT RPM was way low--4000. So, I changed to a 17 which got me up to 4400-4500.

All the stock props are 13 3/8 inch diameter. Yours sounds like it is a 15 inch pitch. Usually there are three numbers and then the pitch stamped into the hub like 476-21. in this case the 476 indicates a stainless steel cleaver prop and the 21 indicates 21 inch pitch. Most likely the first three numbers did not properly stamp into your hub and only the pitch (15) is readable.

That stock prop in my opinion is a piece of crap and does not let the engine develop its best performance. If you decide to buy a new (or used) prop, Michigan is pretty much your only choice. They do make a couple of options but even there, the pitch is still only 4 choices: 15, 17, 19, and 21.

Now, even though the original prop is a Michigan manufactured for Force, The after-market Michigan branded props will perform better and you MAY be able to swing a 17 pitch. In reality, there is no way to be sure unless you can test before buying. Generally, 2 inches of pitch higher will drop RPM by 400, and Stainless steel props give 200-400 rpm more than an equivalent aluminum prop. However, the michigan stainless are all cupped which tends to reduce RPM by 200-400. So, if you could, say, test through iboats, then the first prop I would test would be a 17 pitch stainless prop. If money is a consideration, then 15 pitch aluminum followed by 17 pitch aluminum for test.

Of course, with props, nothing is cut and dried and if at all possible testing before buying is highly recommended.