those damn rules

Ah yes, another rules question…

Scenario – Off the beach boats (dinghys), . Starboard Course, 0.5 knot tidal current going in the same direction as the wind. Wind 8 knots

I’m coming in on Port, just laying the windward mark. Call me boat B.

Boat A is 2 boat lengths clear ahead on Port tack also, but to leeward and pinching to make the mark, he’s slower than me and I can see he’s not going to make it, due to the tide.

Boat A reaches the zone still clear ahead. At the mark Boat A attempts a down speed tack to starboard. Boat B clears boat A’s bow and the mark and sails off. Boat A hails protest.

Boat A claims he had had completed his tack and had to luff to avoid hitting Boat B. Boat B doesn’t┬áthink Boat A completed his tack.

I’m generally OK with Rule 18 and mark room, but I’m not sure on this one. Is he entitled to room if he’s tacking in the zone? Does that even matter?

How do we apply Rule 10 and 18 here? Have at it.