The clue, of course, was it was raining when the fire started. That means
hatches were probably dogged. Sealing up the companionway may limit oxygen
levels enough to snuff the fire. The Tartan 37 cabin volume is quite moderate.
Binary Bill's answer is essentially correct. Of course, there is no guarantee
that this will work. The fire may continue long enough to sink the boat by
melting seacock hoses open underway.

RB