Re: Question about Michigan hub construction
Well, John---There is nothing wrong with your Michigan prop. When looking at the forward end, you should be able to see the prop aluminum (or stainless) hub--looks like a 1/4 inch thick ring--, the rubber hub itself, and the bronze drive bushing. In an effort to standardize and save money, the bushing is common to Merc and Force props (and some Evinrudes) so the splined portion is made only long enough to fit all three.
Your prop should ride on a thrust washer, otherwise it will contact the gearcase and wear it. Given that you say the original prop does not look like the Michigan, I suspect that the thrust washer is corroded into the original prop hub. Either buy a new one or using a long screwdriver or drift, hammer the old thrust washer out. You must reach in through the prop hole and tilt the screwdriver as much as possible because there is not much to drive against.
The next complaint that people usually have is that the prop hub is too short and rides back and forth on the shaft, even with the nut fully tightened. This is not so. The prop hub is the correct length.
Force closed hub props used what was called a "Flare Washer" or "Anti Cavitation Washer." This was a black plastic flared piece about 4 inches in diameter and one inch thick. It fit under the prop nut and washer. Its purpose was to help keep the closed hub props from cavitating.
(Backstory---When Chrysler --later to become Force--originally developed the one piece lower unit, the midleg/lower unit combo was one inch shorter than the old two piece lower. Effectively, the engine was raised one inch on the transom and they cavitated like crazy. Original equipment was a tapered metal spacer matching the taper of the prop hub and tailcone. Very elegant looking. But buyers were complaining about the cavitation problem. The flare washer was a quick engineering fix, made to replace the tapered metal spacer, to help reduce cavitation. Early Force engines still had the tailcone, later Force engines had the propshaft tailcone mounting portion removed and came with no tailcone.)
Open hub props like the Michigan stainless do not need this plastic flare washer because they already have a small flare, but still have enough room to let it be mounted if you want. Force black aluminum, open hub, semi-cleaver style props do not have sufficient room to use the flare washer. IF YOU DO NOT USE THE FLARE WASHER-- there is about 3/16 inch extra splines showing on the drive shaft and you MUST use a spacer. This part is still made and you can probably order that very expensive part through the Merc Dealer. OR: Lacking that, a couple of stainless or aluminum washers with 1 inch holes can be used to take up the extra and preclude prop movement with the nut fully tight.
BTW---Science content--beware-- Many people mistakenly call the hubs and old props brass. They are bronze. Brass is an alloy of copper and zinc and because of the zinc content corrodes quite nicely in water. Bronze is an alloy of copper and tin and is more resistant to corrosion. Naval bronze is an alloy with other metals added and is even more resistant.
