Re: Force Prop Question
I don't think the late 1980s Force 85s ever had a dual exhaust lower unit on them, although they did use a prop that looked like it had thru-hub exhaust. I believe in the 3 cylinder engines, only the 90 HP engines had the dual exhaust with thru-hub exhaust.
The easiest way to tell, is to look at your lower unit. If it has a vertical line of water pick-ups above the gearcase on each side, then it is dual exhaust and you should not use a solid or closed hubbed prop. If it has water pick-ups on each side of the gearcase just behind the prop, then you can use either closed hub or open hub thru-hub exhaust.
Stock props are 13 3/8 X 15, 17, 19, and 21 inch pitch. Which one you choose depends on the size of the boat and the expected use. Aftermarket props will differ in diameter slightly but I believe only those four pitch sizes are available.
As a rough guesstimate, If the engine is on a boat with 90 HP max capacity, then a 19 pitch prop would be a good starting point, unless you wanted to use the boat primarily for water skiing. In that case, reduce prop by 2 pitches. If on a boat with a higher horsepower capacity, then a 17 pitch prop would be a good start. Of course, the ultimate test is tachometer reading. You want to prop the engine so that at wide open throttle it will run between 4500 and 5500 RPM with it being more desirable to be at the upper end of the range.
As far as markings, your original (if it is stock) prop was made by Michigan. It will have a number inside the hub where the prop nut and washer are. It may also have a number on the outside of the hub between two blades.
If your original prop looks like a Merc or OMC prop, that is, you can see through the hub, that would be a thru-hub exhaust type. If the hub is tapered and solid at the prop nut end, then it is a closed hub, non thru-exhaust type.
You can check here at iboats for a new prop, or you can buy a used one at the auctions.
