Re: Propellers I can use
Noah--They are called splines. The prop shaft and prop hub are splined to take rotational loads and the pin in the nose of the hub takes thrust loads and locates the prop on the shaft.
The only props I ever saw for that engine were 10 3/8 by 11, 12, 13, three blade. Long ago, Michigan did make a 2 blade 10 by 14 bronze racing prop.
My 1969 55 was so potent that on a 15 foot fiberglass Glastron runabout, I was swinging the 10 3/8 by 13 and the bronze racing prop. That combo outran the (then new) 3 cylinder OMC 55 HP engines. The bronze prop tended to vibrate a bit and I needed to cup it to gain maximum performance.
Surprisingly enough, the same exact 3 blade props with a different hub and part number was sold for older OMC 40 and 50 horse engines. SO: it is possible to get a propeller from a different engine and re-hub it to fit your 55.
I think I have a brand new 10 3/8 by 12 which I re-hubbed. If you are interested, send me a PM.