For a catamaran the common forced-air diesel heaters don’t make much sense — there is too much distance between all the cabins. We think hydronic heat is the efficient solution — routing of the 20mm heater hose takes up little of your valuable storage space. And if thoughtfully routed, keeps your storage dry as a side effect of the radiated heat.
OTOH, if you are not concerned about warming the storage areas, consider insulating the heater hose. I once calculated the thermal losses through our 120-ft of hose — it is significant, and it really does not contribute to the space heating goal.
Adagio has a 40,000 BTU Webasto diesel boiler — just about right for our volume, but possibly a bit undersized considering our huge glass area.
One design tip: ensure that your hydronic ballast tank is BIG, and as high as possible above the otherwise highest point in the loop. The bigger/higher the less likely you are to get air bubbles in the liquid flow around the boiler’s heat exchanger. The Webasto does not like air bubbles, as we discovered after replacing 3 motors. Then in 2003 I designed a new stainless tank — never a fault since.