Cabin heat from engine

[Pizzazz]

I am considering installing a heating system on my 31' sailboat. Since I sail in California, the primary use of the system will be to raise the temperature by 10-15 degrees F during the night while at anchor or a mooring (mostly for the kids). I ruled out non-diesel heaters (carry separate fuel, exhaust piping, too much effort for little gain). Diesel heaters make sense but they are expensive, noisy and there is always the question why carry a separate engine when there is the main propulsion engine that sits mostly unused.

So I came up with the following system. I have a 13 hp Yanmar diesel with a heat exchanger and a 6 gallon hot water heater. I would install an additional 40 gallon water tank under the starboard settee in series with the water heater, a couple of radiators and a small water circulation pump. Running the engine under way will heat 46 gallons of water to, say 140F. Assuming I target 70F cabin temperature, I would have about 27,000 BTUs stored in the system which is equivalent to running a typical household 1,500W space heater for about 6 hours (much more than I typically need). Then, while at anchor I could just circulate the hot water with the small inline pump.

Do you think this would work? On the plus side, I see a simple and cheap installation (just hoses and the water tank), increased tankage (I have always wanted more fresh water), convection heating (better since you do not lose the heat if you open the hatch for a few minutes), the option to use the water heater when docked with AC to warm the boat without running the 1500W heater all night long.

On the minus side, the engine will heat the water under load but likely not at idle. One way to fix this is to install a bypass hose from the water pump to the engine exhaust (either manual or thermostat driven) that will bypass the raw water cooling circuit until the 46 gallons of water is heated, then will open the raw cooling water loop - kind of dangerous but doable. I have not done the calculations but I believe the 13 hp engine will heat 46 gallons of water rather quickly. I do not think the additional tank needs to be insulated but this should also be a consideration.

Any feedback is much appreciated.

Pizzazz