Pumping to catch a wave

The situation

It's breezy with rolling waves downwind. Dev pulls his sail in sharply again and again to get the boat surfing down the faces of the waves.

The question

Is pumping the sail to surf allowed under the propulsion rule?

The ruling

Only within strict limits. Rule 42.1 bans using your body or sail to propel the boat, and repeated pumping (rule 42.2(a)) is illegal. But rule 42.3(c) gives a narrow exception: to initiate surfing or planing, you may pull each sail in only once for each wave or gust. So one strong pull to catch a wave is fine; pumping repeatedly on the same wave, or pumping when there's no wave or gust to catch, breaks rule 42 and earns a penalty — often a Two-Turns Penalty taken on the spot.