Hot Water after converting to solar charging

[Alec H]

I'm looking forward to not running the engine to charge batteries, but wonder what is the recommendation for heating water? This is a 40' mono with 3 showers. Do I remove the 110v/engine heat tank, and replace with a propane unit? In this forum I have seen diagrams for solar panels that feed to a heat exchanger, but after I install 3 - 100 watt panels on an arch, I don't think there will be space for more water heating (glycol) panels. Replacing the existing tank would be the most straight forward way to go, but there must be downsides. This must be something others have dealt with. Any suggestions will be appreciated.

[CaptsWife]

I talked to the captain about this as I know we have solar heated water on our boat. We have a dual element, ac/dc, that heats our water either with shore power or with solar power. We've got an on/off switch that we use for solar when not in a marina using shore power. This is separate from the water that is circulated around the tank by the engine under power.

We bought our dual element from e-marine. Talk to Harry.

Hope this helps.

[ReMetau]

We use a propane instant heater.

[Alec H]

Thanks for the tip; seems to be just what I need. Your web site is terrific; inspirational. I will become a liveaboard this spring, but will confine my cruising to the eastern Caribbean. I had a chance to sail a boat like yours a few summers ago - good, stiff boat and they look great.

Safe voyaging,
Alec H.

[Rapanui]

The simplest way would be to use an inverter to heat water assuming you have an electric element in your tank. It may not be the most efficient way, but it really is the most simple and convenient. On the hook I give our tank one or two 5 minute blasts and that is good for 2 showers. We have 300Ahr batteries with 480W of solar and we get back to 100% by lunch time most days.
To me, the complexity of solar heated water is not justified for a small possible gain in efficiency.

[Alec H]

This would certainly be the easiest approach. All I would need to add is an inverter, which I intend to do anyway. Thinking 1800watts. What size, in gallons, is your heater? How many amps? I will have 300 watts of solar, and 220 amp hr batteries. I think the heater takes 7 amps. This seems under powered compared to what you have. The other plan for me would be to plumb in a propane heater. Thanks for your input.

[Rapanui]

We have a Quick Nautic boiler about 30 litres with a 1200W element. Our 1600VA true sine inverter heats the boiler from our 300AHr sealed wet lead acid batteries. We use it in bursts of 5 minutes to let the batteries recover as it pulls the battery voltage down on load. It takes about 1% of the battery capacity per minute on load as measured by SmartGauge. You have a lower battery capacity but also if the heater draws 7A @110V approximately 800W element which cancels that out so it should work ok, but extra battery capacity would be better. Typically, for us, if the batteries were at 100% in the afternoon, the next morning would see the batteries at about 90% capacity from the fridge draw etc overnight and I might take another 10% heating water in the morning before the solar kicks in and takes the batteries back to 100% by early afternoon. That's in Croatia, so you need to read across to your location.
If you intend fitting an inverter anyway, give it a try for free. The worst that can happen is you will have to fit more battery capacity.

[Pizzazz]

This is optimistic. If you you run a 1,200W heater for 5 min in a 30L tank, you can expect to increase the temp by max 5F. It is noticeable but definitely not hot enough for a shower. In a more typical setting you need to raise the water temp by at least 30F, from 60 to 90F which means 30 min with no losses.

[Rapanui]

We have been doing this for 2 years now. Of course, if the tank water is stone cold, then you need a longer input but in practice you normally start with lukewarm or warm water and make it warmer. We spend periods of 6 weeks onboard, and do not have shorepower except occasionally. Most days we will move and utilise engine heating for however long enough until we can sail, so we are talking about bringing the temperature up in the mornings enough to have a hot shower for a couple of people.

[Rapanui]

There are some useful calculators for heating hot water on the web, e.g.
https://bloglocation.com/art/water-h...e-energy-power
The other thing to remember is that you do not have to heat the whole body of water in the calorifier as the hot water rises to the top where the take off should be.
Using the calculator for our example, to raise 5 litres of water from 40 - 60C with a 1200W element will take about 6 minutes (~10Ahr @12V ignoring Peukert). Thats a useful amount of hot water, enough for a shower or two. Not a massive hit on battery capacity but the limiting factor will be the voltage droop from the batteries delivering a high current. Hence we tend to heat water for 5 minutes, let the batteries recover for while and then if required give it another 5 minute hit.