One of the last remaining tasks aboard Nine of Cups was to replace the hot water tank. The old one faithfully heated our water for 13 years – which makes it about 130 years old in marine water heater years. During his survey, Cups' soon-to -be-new-owner discovered that the old one had started leaking, and we agreed to replace it. We certainly got our money's worth out of the old one.
Our marine hot water heater is designed differently from the typical residential heater. If we are plugged into shore power, the water is heated via an electric coil, much like a household electric water heater. If we are away from shore and are running the engine, however, the water is heated by hot coolant from the engine which is piped through the heater. The heater is insulated, so once the water is heated, the water will remain warm for about a day – assuming we don't use all 6 gallons in the meantime.
If we are at anchor, or sailing and not running the engine, there is a third way of heating the water. We have solar panels, a generator connected to the prop shaft and a wind generator which, on a good day, provide more power than we use. If the batteries are charged, the excess power can be fed to the hot water heater. We use a diverter type charge controller which connects the alternative energy sources to the batteries until they are near full charge, and then, rather than overcharging the batteries, redirects the excess power to another load – our hot water heater.
One concern with this plan is how we switch from one electrical source to the other. Connecting 120 vac shore power to the hot water heater while it is still connected to the alternative energy sources would be a very costly, and likely a very exciting faux pas. When we first installed the system, I checked into switches, but these were expensive and not always foolproof. The cheapest, easiest, and most foolproof method I could find was to install a normal 120 vac three-prong plug on the end of the water heater power cord, and two outlets next to the water heater, one connected to the shore power breaker, and one connected to the alt energy charge controller. When we want to connect to shore power, we plug the heater into the shore power outlet. When we leave the dock, we move the plug to the other outlet. If I forget to move the plug, the worst that happens is the water doesn't get heated.
Seems like a good plan, but in actuality, even when we have a lot of surplus power from our alternative energy sources, the water temperature is rarely ever raised to slightly above lukewarm. This isn't bad for a cockpit shower on a hot afternoon, but if we need hot water for doing the dishes or washing our hair on a cold day, we resort to the tea kettle.