SkipperBob,
Lucky you....a Shannon 43 and in Wickford RI! Great place for breakfasts and nice strolls ashore.
I have a 42' sloop with an Espar D5L. Don't live aboard anymore, but visit the boat each day and use the Espar as backup to an electric heating source (in case of REALLY cold spells and/or shorepower failure).
For past several years, I've left the five 4-inch vents in the overhead open all winter. The Espar easily provides enough heat to overcome the cold air from these vents. In fact, just yesterday I plugged three of these because I believe 2 vents are more than enough.
My air intake for the Espar is also in the aft cabin. Have been shrinkwrapped for two of the past three winters. I think there's plenty of fresh air under the shrinkwrap, unless you have a really, really tight covering. If you do, you can always leave the zipper door open a bit.
So, my advice would be to provide some sort of opening to allow fresh air to get belowdecks. Hawsepipe? Vent? Port? Louvers on the
hatch boards?
Bill