which prop size?

[rcbirk21]

I just bought a used Dumas 3.5 Hot Shot, but it did come with a prop. It doesnt seem to be running as fast as it should. What size prop would you recomend for this kind of boat? How do you tell what size prop you have, if you cant find any writing?



Thanks,
Erik

[Ron Olson]

Octura props from X437 to X440/3 or a Prather 215. Anything larger won't fit under the cavitation plate. Usually you will find a number on the hub of the prop, sometimes hard to see. Otherwise you'll have to measure the diameter which will be in millimeters. The last 2 numbers on an Octura prop is the diameter in millimeters and the first one after the "X" which is probably a 4 means that the pitch is 1.4 times the diameter of the prop. sounds confusing but not really.
An example; Octura X437/3 (the "/3" means that it's a 3-blade prop) has a diameter of 37mm or 2.07" and the pitch is 52.28mm or 1.48".
Other things to look at if you're new to OB tunnels is the prop depth and motor angle, both can greatly contribute to the speed of the boat. Minor changes as little as one degree can make a huge difference in speed and handling. Dialing in the carb for changing weather conditions makes a big difference also.

[martin richards]

Ron, for the X437 I think your converison to inches is reversed. Its 1.48" diameter x 2.07" pitch.

[Ron Olson]

Oops! Yeah, you got me! I was reading the chart backwards. :o

[rcbirk21]

Are all the props you are recommending 3 bladed. The boat has an Octura X440 on it now. Just by sound it seems to not allow the engine to rev high enough.

[Ron Olson]

There are some 2-bladed props in what I recommended. Try the X438 or the Prather 215, both 2-blade props if that's what you want. The X440 also comes in a 2-blade version but that seems to be too much for your boat.

[martin richards]

I'd guess Octura would go out of business pretty fast if they only counted on the turnover their three bladed props give them ;) No the two bladed are by far the most commonly used. I've no experience with K&B outboards, so can't add much more.