Transom adaptations

Has anyone ever come across instances where the transoms of fast moving power boats have a radius on the bottom edge?

I recently had to radius the transom of a 9m rib because of problems with the outboards overheating. The problem was because the Suzuki’s used didn’t have a low water pick up. We radiused the transom to try and get a cleaner flow of water higher up on the leg and as expected the engines fared better. The addition of an adapted low water pick up put the problem to bed.

The unexpected result was the additional speed gained when we tested the boat again. I was of the opinion that planning hull transoms were sharp to aid separation of the water from the hull so I expected a reduction of speed. I then though it was perhaps due to the engines being mounted higher so I looked further into it and one of my colleagues informed me of some Greek friends of his that regularly round off the transoms to gain speed. I am guessing it is down to cleaner water reaching the props but has anyone had any experience or seen other examples of this because I am intrigued.

 

Most likely the gain in speed was due to an increase in the hull trim angle. Total resistance is the sum of wave making and skin friction and you may have decreased the friction part by the higher trim with less hull in the water. Wave drag was increased but not as much as the decrease in friction. Not all this is good though. Higher trim makes for a rougher ride and forward vision is reduced. Many boats on plane will go faster just by having someone move to the stern for the same reason.

Of course you may be right abut the prop getting better water flow with the radius. In that case, the prop level or position was not optimum to begin with.

 

you should have lowered the outboard ..Suzuki dont design an outboard that will not pick up cooling water when correctly mounted ....cavitation plate level with bottom of hull and hull edge as sharp as possible