Some time ago Bill Roberts answered many questions on this same issue. In summary, he said the following:

1-Cats sail in displacement mode, but the famous "1.34" factor does not apply. As Wouter said, this is old stuff.

2-A cat can plan.

3-In order to keep the balance of the forces, a cat would necessarily plan on the forward part of the hull, like a hidroplane.

4-A cat with a planing fore section is perfectly feasible, but the ride upwind is very uncomfortable. He tried it once, replacing only one hull. This way he could compare hull perfomance just tacking or jibing.

I insisted on the subject in message directly to Bill, proposing the concept of a variable geometry hull. The idea is copied from planes, that change the geometry of the wings according to the need: they extend flaps.

The hulls would display small retractable lateral flaps in the forward sections, so that upwind the bottom would be conventional but downwind the flaps would be extended and the bottom would be proper for planing.

Bill said that it would work and suggested to try with a powerful cat, like the ARC 22. But he also said that in his trials the speed improvement downwind was less then 5% so there is a possibility that the heavier weight and complications are not worth the effort.

I called the device "flapcat" but never tried it because I decided to jump directly into hydrofoils. The draft drawing is available for anyone interested in spending time and money developing a planing cat.

Cheers,
Luiz