We should remember that most of the propellers for I/C engine boats are OK for a few hundred horsepower, and the shaft size is OK for that service too. A 1/2 inch (12mm) shaft size would be fine for a 5A Stuart, and the only reason to go bigger is because most available propellers have a far larger bore, or to be less likely to bend the shaft when grounding.
From Ron, "….. that is the reason an endmill should never be used in a drill chuck…"
Thanks for that bit of knowledge. I have used a drill chick for holding an end mill, and although it was a Morse taper with draw bolt, I experienced a ruined part as the mill dug too deep into the work.
From Ron, "For our very low horsepower engines, it would probably be fine to omit the taper altogether….."
For my next steamboat project I am planning on 15 horsepower, far more than 95% of the existing steam launches in this world. In order to test things, I installed a 14 horsepower IC engine, geared 3:1 to the prop, a 14 x 17 aluminum prop from an I/O drive which had a splined shaft.
I bored out the splines, and installed a one inch (25.4Mmm) straight steel shaft, fastened only with a typical roll pin and loctite. Bearing centers are at 83 inches. It is plenty strong enough for that service, so would be good for just about any steam launch.
This setup is on video on "Boat design.net" website, thread is "So much for the "Nay Sayers" (Powered O'Day Javelin 14.9 mph)"
http://www.boatdesign.net/forums/