through hull drain tubes in the floor

[McGillicuddy]

Yo, Avian, Common problem. Couple of solutions include adding 2 1" drain plugs to the mix or considering the time tested "vortex" concept.

Here's a link to FELLOW-SHIP's "vortex" creation also known as a flapper valve or a duck bill valve.
http://www.classicseacraft.com/forum...rev=#Post40102

There you'll see also CSickNick's more modern creation (I think he used bronze and a check valve) and a really good discussion. Note the dates of the posts to give you an idea of how to use the search engine should you need to refer to them again...