I recall filling in the outside 2 slots, as shown in the picture, worked. It reduced the water pressure and increased speed 1/2 to 1. I left the holes open in case of weed stuffing. Shorter is always faster, maybe 2 mph (or more) per inch depending on the boat. The problem is planning off, low speed slippage, higher overall prop slip, usually resulting in a prop change. More blades and a higher pitch can generally correct these problems.

It seems, from all I have heard, the IMCO is more streamlined and faster, at the same drive height.

The later model? sportmaster may be more efficient, the originals were real dogs. Their extra beef did prevent cracking of the housing just above the carrier, on very high X dimension cats.