The OP doesn’t tell us much. No mention of the pitch of his props, which of course is what determines the potential speed of his boat. Doesn’t mention anything about the motor either, a converted weedwacker with a top rpm of maybe 10,000 or a race motor with closer to 17,000 rpm. I assume he runs them in a surfacing configuration rather than submerged, perhaps I’m wrong.

The classic formula for boat props is to size a 3-bladed prop to 80% the diameter of the 2-bladed prop, assuming the same blade shape and pitch. It is a rule of thumb, which means it is usually not the optimum. But it’s a place to start. Frankly,
I would not go under 55 mm on a boat that large regardless of blade count.


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