Definition of Planing

Why should it be allowed at all, do you mean? Sounds like adding a paddle stroke to me.

Surely the idea here is that the boat can break displacement wave forms and go faster than waterline length would normally allow? Hull shapes, weight and rig design will all affect a boat's ability to successfully go faster than displacement speed. Long thin hills don't create the same wave forms, so yes, I'd say that cats shouldn't be allowed to pump. Bet they do anyway.

So cat sailors can't pump to get planing 

Rule 42.3c states Except on a beat to windward, when surfing (rapidly accelerating down the front of a wave) or planing is possible, the boat’s crew may pull in any sail in order to initiate surfing or planing, but each sail may be pulled in only once for each wave or gust of wind.

“Surfing” is defined in the rule, but “planing” is not, nor is it defined in the definitions section of the rules. For that matter neither is “gust”.

I read the interpretations of Rule 42 on the WS website, but can’t see anything that defines planing (or gust), and I don’t believe either is as straightforward to define as may first appear.

The WS interpretations do say that planing need not be successfully achieved, but it does have to be possible (and if you fail to get planing after more than two attempts are likely to have broken the rules).