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Old 23-11-2014, 10:31   #1
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Macblaze's Avatar

Join Date: Nov 2013
Location: PNW
Boat: Hunter 386
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Hot Water Heat Retention

I was rereading my notes from last season and was reminded of a question that came up. We have done a lot motoring along French canals in hire boats. They all had diesel-fired water heaters. Universally the water would stay hot enough that we could have a warm to hot shower the next morning without firing up the engine again. But none of the sailboats we chartered this season (in the PNW) provided more than tepid water after any significant time had past.

How do the hot water tanks on a typical sailboat work? I do know the after berth above the water heater benefited from the heat produced, but the insulation didn't seem to be there to keep the water hot. Is it a matter of size? The canal boats tended to be 40-50 ft. I will say the water was generally way hotter on the sailboat after the motor was shut off: hot enough to melt a plastic pop bottle. Never had that problem in France. So probably completely different systems...

So what does one need to know about hot water on board?
Gaudeamus igitur iuvenes dum sumus...
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Old 23-11-2014, 10:38   #2
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Join Date: Jan 2011
Location: Cowichan Bay, BC (Maple Bay Marina)
Posts: 5,439
Re: Hot Water Heat Retention

Three variables: size (capacity), temperature and insulation.

Water to water heat exchangers are used in sailboats most often, using engine heat from the fresh water coolant system. With 160 to 180F thermostats in engines, the water can get quite hot. Dangerously so. Isotherm sells their heaters with thermostatic mixing valves to limit the output to a "legal" level.

Seaward/Raritan heaters are least expensive, least insulation. Isotherm or Isotemp (?) has the best.

We have a Seaward, six gallon. Usually not even tepid the next morning in 57F seawater surround. We rearrange our showering schedule to when there is hot water after motoring.
Stu Jackson
Catalina 34 #224 (1986) C34IA Secretary
Cowichan Bay, BC, (Maple Bay Marina) SR/FK, M25, Rocna 10 (22#) (NZ model)
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Old 23-11-2014, 12:09   #3
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Ann T. Cate's Avatar

Join Date: Nov 2011
Location: aboard, cruising in Australia
Boat: Sayer 46' Solent rig sloop
Posts: 7,120
Re: Hot Water Heat Retention

Just a note, here. The water in the PNW is cool enough for us to not refrigerate beer there, the bilge is cool enough. I bet the water in the canals is a lot warmer, first off, European summers can be quite warm, and that water is surrounded by warm land. So the difference in water temp around the boat will have some effect, too.

We had a water heater on a 1974 launched boat, and it had adequate insulation to retain warmth overnight, but not enough for a "hot" shower. Hot showers came after motoring, or by sun shower.

Ann & Jim, U.S. s/v Insatiable II, SE Qld, for a while
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