"Neil" wrote in :

Heater is a 12v 100watt ceramic heater

Make a little test before going farther down this slippery slope.....

Put a 100W lamp in the boat at dusk. Measure the temperature when you
start. Come back in the morning and see how hot it is inside the boat from
this 100 watt heat source. I doubt it will raise the boat 1 degree. That
will make it 11F inside when it's 10F outside....unless the wind is blowing
through any hatches not hermetically sealed.

If you can't try this on the boat, try it in your garage WITHOUT parking
the warm car in there. Set a 100W lamp on the center of the floor at dark
and check it in the morning. The boat isn't insulated like the garage,
either.

--
Larry