I got one of these a couple of years ago. They come in a variety of BTUs, but mine also has the dual 1lb propane tank holders. I wanted it for the dead of winter, to do some projects in the cabin. With the hull of the boat at 30 degrees, it simply would not dent the interior temperature. I had it running so hard, the tanks were icing up from the rapid gas expansion. Nothing. The boat itself, was essentially a huge heat sink.
That's because your boat is huge and has the internal volume of a 3br/2ba home on land.
Most of us don't have that problem.
I used my Coleman Black Cat propane heater last weekend while I was out cruising. The temperature dropped down to 41F (which I know is no big deal), but I ran the heater on "high" to take the initial edge off, and then on "low" the rest of the time, and I had absolutely NO problem with condensation or excess moisture in the cabin, and the cabin was sooooo cozy. (or was it all the rum I drank?) If the ambient humidity is already high when you use one of these, then
you might experience some condensation.
Also, Coleman offers a regulator accessory that allows you to plumb the heater to a large propane tank. Huck could keep the tank out in the cockpit and cut a notch in a spare companionway panel to run the hose through:
Fuel outside of cabin + flameless, catalyzed heat inside of cabin= safer.
Do be careful with all of the open flame on the boat, will you? All it takes is a mis-step, and a plate full of tea candles will be scattered all over the cabin lighting the wood bits on fire.
The fellows are right about insulating the hull. You can buy sheets of 1 inch thick, foil-backed insulation panels for about $7.00/sheet from Home Despot. Just ONE of those sheets, properly cut, will insulate all of your window glass, the inside of your forepeak hatch, and probably the hull area along the sides of your V-berth.
I'm building a cockpit cover/boom tent using plastic conduit at $2.00 per 10 foot piece and a $12.00 canvas tarp. This will keep the snow out of the cockpit. Snow is heavy and it sinks boats.
If you can afford a generator, you can scrape a few pennies together for this stuff.