On Wed, 13 Jun 2007 06:57:41 -0700, terry
wrote:

Thinking of using a 3" port that can also be blocked off by a screw in
plate. The cover plate to be stored nearby (lazarette?) possibly
secured by a cord or lanyard?.

A scoop type ventilator mounted near the stern is a commom
arrangement. If you get one that pivots you can turn the vent opening
forward in a following sea. In actual practice however getting some
water in them doesn't seem to be a big issue unless you are in extreme
conditions.