You don't say how many will be onboard.. If you have several or more in your
crew, then body heat in a closed cabin will produce enough comfort to allow
the removal of jackets (or down to sweaters). However, the humidity will
raise significantly.

Without a supplimental heating unit, your not going to be able to dry
clothing and even the modern fabrics simply 'give up' their moisture to the
cabin air. Be prepared for some steamed up portholes and dripping metal
fitting in the cabin area. Even with a cabin heater, the air, albeit warm
will still carry all the moisture from wet gear. If the weather is dry
outside, I recommend air drying the jackets in the wind to get rid of as
much moisture as possible, then warm them in the cabin. Also a wet gear
locker or shower stall is a good place to put anything that is dripping wet.
This lets a lot of the water go down the drain or into the bilge.

I know of people who try to heat with the propane galley stove and this is
or any other unvented heat source is dangerous since it will burn up the
oxygen and produces carbinmonoxide.

If your engine is FW cooled you might consider installing a heater coil in
this circuit and heat the cabin up while the engine is running. SW cooled
engine don't get hot enough to do this (mine only gets to 140 F).

I won't recomment any of the ventless heater, but others may have favorable
comments FWIW.

Steve
s/v Good Intentions