We don''t directly put heat into the bilge but have not seen a condensation problem. We have a waterstop in our keel stepped mast and the bilges are dusty. Which is a good thing as our shallow bodied boat has minimal bilges under the floorboards. We have ducts delivering heat to key places like the head and wet locker. One duct runs to the forecabin under the settee. A locker or area where a duct runs through stays warm and dry. You can poke a small hole in a duct to dribble a little more heat in that area if needed. If you run a duct forward in the bilge it will throw off enough spill heat to keep things dry.
We drain our tanks, hot water heater and
lines each fall as we don''t trust everything to stay warm when off the boat. We haul water in a jug that has a spigot and basically treat it like camping. We''ll heat dishwater on the
stove. The harbor drains the water system but keep one
line running continuously up at the ramp. Some liveaboards will move their boat by the ramp and run a hose to their tanks in good weather.
A comment on solid
fuel or wick type diesel heaters. They have their charms but we got tired of cleaning tiny soot marks off the deck. We also got fumigated periodically when a gust would backdraft and fill the boat with smoke. We tried several different charlie nobles. A 3'' extension helps but the best strategy was put it out when it starts blowing hard.