Devon damp?? You should try the West of Scotland this autumn and winter!! I’m sure there is extra skin growing between my toes.
We use a 150W tube heater on Electra 2. I chose this becuse it is safe, economical (we have to use meter cards ashore at Sandpoint), no hassle with overflowing etc, and it seems to work well. The boat is always dry when I visit in the winter. I open all the lockers, under-seat and under-bunk storage, and engine access covers to make sure the heat gets everywhere.
I know someone who uses a low power fan heater on a timer which runs it for regular short periods each day. He claims that the fan has the advantage of circulating the air inside the boat better. I didn’t go for this because of the safety aspects, and because I was unsure whether the constant temperature cycling might actually cause more damp.
Here’s hoping for a drier Spring.