Forced air diesel heaters seem to work best on boats. First, the move air, this means you can intake fresh air which reduces moisture and helps with ventilation. Second, they are inexpensive (eBay units go for $200, brand names for $1,000, choose as you wish). Ductwork is expensive but you need it to distribute grate evenly around the boat. There is no other way, Dickinson sounds good until you realize the main saloon needs to be too hot for someone to sleep aft for example. Third, the ductwork typically lasts a long time. One can always replace the furnace with a newer unit. I recommend the eBay units for people who are casual and Wallis for people who are serious about winter sailing.