Simon,

Bill answered in previous posts that he did some experimenting using two different hulls in a cat: a normal one and a planing one. He tacked to compare the relative performance in identical conditions.

Apparently there was about 5% gain downwind but the slaming and pounding upwind was terrible.

One solution seems to be a variable geometry hull, with flaps or something alike extending from a normal hull to provide a greater planing area.

Foils are another approach. The inclined foils in open 60s provide only limited lift, but the figures seem to be increasing lately. The Catri 27 foils lift up to 90% of its displacement to assist it in planing. The downside of foils is the poor light wind performance.

Cheers,