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Can I get hot water without the engine?

2020 November 1

Dear Boat Doctor,

I am setting up my boat for long-range cruising and trying to be as self-sufficient as possible. I can cover my electrical needs with solar panels. My watermaker produces all the water I need. I have sufficient fuel tankage for any passage and weeks at anchor, but I’m running into problems with hot water. Making hot water requires running the engine for a fairly long time, which I’d rather not have to do. Is there an alternative?

Charlie Barr

Portsmouth, Virginia

Dear Charlie, 

Hot water great to have, but it takes a lot of energy. The engine is an option, with some negatives, but there are no good electrical options.

Running your engine solely for charging or to make hot water is inefficient and hard on the engine. Diesel engines like to run with a load and at a standard operating temperature. By just running to heat water, you have near zero load on the engine, and this will make it run cool.

A diesel engine, the fuel and other consumables (oil, filters, belts and more) are expensive commodities. A diesel engine can easily cost $15,000, and the fuel and consumables run $5 to $8 an hour or more. With an average engine life of 7,000 hours, a diesel engine’s real operating cost is roughly $10 per hour. There are cheaper and quieter ways to make hot water.

I’d consider a diesel boiler. Webasto makes very nice, small marine boilers. These are designed for heating the boat but work well for hot water too. The systems are efficient also, burning about a quart of fuel per hour of running time. You essentially plumb the boiler to circulate through your water heater. The boiler makes the heat, and a small circulation pump cycles water through your water heater.

You will likely want the option to heat water with the engine or the boiler. This is done with a small heat exchanger on the heating loop. Your engine coolant loop will cycle through the heat exchanger, and the heat exchanger will heat the heating loop. You have the option of heating the loop with the boiler or with your engine when underway.

A boiler like this is very pleasant at anchor. They are thermostatically controlled and will run automatically to keep a steady supply of hot water. Properly installed, they don’t make much noise, a tiny bit of start up noise, but pretty quiet once up and running.