A sailing prop or a motoring prop? The best of both worlds? Variable /
folding prop? You need a low speed prop, but not too fine. One sheet of
luan and 3.5 hp will go 30 knots, but develops best thrust going fast,
coarse pitched. You want high rpm, going slow. Fat blades are best, but
drag most when sailing. 3 blades sounds like a tugboat, with torque but
no speed. Could it be too fine?

My two bladed prop is 13R12

10.5 Hp two cylinder sounds like a universal medallion atomic 2. 12 HP?

I've got one. It has a WICO dog tooth magneto on a gear train out the
left looking fwd side, and that also drives the water pump.

A head gasket was an early problem. My head seldom gets hot, barely
warm, no thermostat. So long as the impeller is OK, that is.

Floods out when hot and power reduced. Work it all day, turn it off
without a cool down and pop goes your head gasket, first delivery.

I have no knotmeter, no tach. My timing is accomplished by rotating the
body of the magneto, having loosened off it's nuts while under power.
The one below is loosened before starting, the one above, under power,
or idling.

There is no spark advance to accommodate high rpms, but a manual lever
seems possible. This is more basic than Ford's "T", though it does have
a front mounted electric starter and flywheel.

I have about 100 pounds dry compression, each cylinder. An old Zenith
carb with leaky top and bottom shaft seals makes getting the mixture
right a pain. I have wired rubber grommets around the shaft bearings,
things are better, now. I think the idle jet is plugged.

Normally, at idle, I leave 1" of wake, at wot, about 6". Pleny of
power. I could look up the reduction ratio.

I have a 2 bladed prop, with a small blade set, 13R12. I have a low
drag asymmetrical bilge keeler, Tyler 29 6800 lbs. I don't bother with
a shaft lock, but could turn the flywheel, and mark the shaft for a
vertical propellor, half hidden behind the skeg. A mirror or webcam
might be helpful, there. The single exposed blade would make the boat
turn right under sail.

I sail in a river. I have a larger prop, steeper, but never tried it.
Once under sail, I don't care how fast I'm going, really.

Westerbeke carries spares and sells xeroxes of parts lists, etc.
Ridiculously expensive. Half shell bearings 65 bucks per half. I
ground down some 3 dollar shells to fit it. I'll Make rings out of
fence wire or a slinky, some day. I'll have to buy an anvil.

I've never seen the cam shaft or valves, except for when I replaced the
head gasket, but the adjustment is easy, so long as the engine is
upside down, or carb, water pump, magneto, timing gears and covers and
flywheel are removed. The valve access panel leaks oil something awful.
Perhaps the new gasket there has solved that problem. I think an
exhaust valve stuck open for a minute, once, about 5 years ago. The
exhaust header got white hot until a different idle speed was
commanded. It sounded funny.

The oil pump looses a ball bearing, and it's function, when removed
with the bolts extracted from the pump body. This ball bearing can be
retrieved from within the front main bearing cage by shaking the block
while right side up. Do it on a clean sheet of something, it's hard to
pick out a small ball bearing from sawdust and spider web dust bunnies.
The engine will run with this extra ball loose in the crank bearing,
but will develop no oil pressure. Don't ask how I know.

I no longer suffer from fuel starvation caused by a too warm fuel line
and vapor lock, but my pancreas has settled down, and now I need to do
crunches, lots of crunches. After belly muscle repair surgurey,
likely.

If more info might help, let me know. I'll dig out the book. Got a
picture of your engine? If so, send it to me, not the group, or post it
and send a link.

I do have a nice self tacking jib.

Terry K