Running engine at anchor for hot water - load

[s/v Jedi]

Originally Posted by pbdayger

Did not see any comments ref: propane on demand water heater. Have been thinking about rigging in my head, remove the propane tank when not in use… on Ontario I could utilize t lake water adding a small pump… on our 31 Dufour I think it would work well. Why wouldn’t it Wouk well on any size boat….???? Pros/cons????
PeterB

Propane? Isn’t that a fossil fuel that was used in the past?

[pbdayger]

Ya gotta do better than that��

[noelex 77]

A traditional boat calorifier is not very practical for those who are full time cruising and spend several days or more at an anchorage.

[GILow]

Originally Posted by s/v Jedi

Propane? Isn’t that a fossil fuel that was used in the past?

Yep, practically like banging rocks together to make fire.

[Letterkenny]

Reviving this thread a bit. Was just planning out my electrical usage and ways to replenish it. Worst case scenario, how hard is it in an on engine (Yanmar 3YM20 in my case with a 125a alternator) to run for an hour, 30 times a year (maybe 5 consecutive days in 6 separate trips without a load) to heat water and replenish batteries? On my last shorter trip I just used the Honda but thinking in the summer time that even with solar, I’d have to run it a lot to top off the batteries whereas here I could potentially gain 1 kw or so from charging off the alternator and saving 30 minutes or so of a 1200w load from running the water heater.

[donradcliffe]

Originally Posted by s/v Jedi

Well it isn’t exactly free. Having the water heater plumbed in the engine coolant loop, means that the engine takes much longer to reach it’s operating temperature which reduces lifespan significantly.

Wrong again. Engines use thermostats to quickly bring them up to operating temperature in about 5 minutes.

[donradcliffe]

Our main source of hot water was a black solar shower in the sun.

[JPA Cate]

Originally Posted by donradcliffe

Our main source of hot water was a black solar shower in the sun.

For showers, so was ours. It was "normal." Didn't feel "like camping." Those times are so very gone.

Ann

[donradcliffe]

Originally Posted by jackrabbit59

You can't change the laws of physics.
Anyway physics says running an engine for hot water production is very inefficient. Invest in another solar panel and lithium battery and save the fuel.

The physics says that using waste heat from an inefficient combustion engine to heat water while you are running it for propulsion, charging batteries, and making water is a better plan than throwing more money at solar panels and batteries.

[Cheechako]

Originally Posted by Letterkenny

Reviving this thread a bit. Was just planning out my electrical usage and ways to replenish it. Worst case scenario, how hard is it in an on engine (Yanmar 3YM20 in my case with a 125a alternator) to run for an hour, 30 times a year (maybe 5 consecutive days in 6 separate trips without a load) to heat water and replenish batteries? On my last shorter trip I just used the Honda but thinking in the summer time that even with solar, I’d have to run it a lot to top off the batteries whereas here I could potentially gain 1 kw or so from charging off the alternator and saving 30 minutes or so of a 1200w load from running the water heater.

It's easy. I have done this as my main procedure since the 1980's and several boats. A hot water heater plumbed to the engine using the heat exchanger will make good hot water in 20 minutes. You will also be charging your batteries and the most efficient charge time is the first 15-20 minutes.

Aside from that though, if running the engine is your main battery charger, THAT takes some running time if you have a refrigerator. With a 100 amp alternator at least an hour twice a day. And a bit more often also.

[Reeve21]

Originally Posted by Letterkenny

Reviving this thread a bit. Was just planning out my electrical usage and ways to replenish it. Worst case scenario, how hard is it in an on engine (Yanmar 3YM20 in my case with a 125a alternator) to run for an hour, 30 times a year (maybe 5 consecutive days in 6 separate trips without a load) to heat water and replenish batteries? On my last shorter trip I just used the Honda but thinking in the summer time that even with solar, I’d have to run it a lot to top off the batteries whereas here I could potentially gain 1 kw or so from charging off the alternator and saving 30 minutes or so of a 1200w load from running the water heater.

I've been doing pretty much what you describe on 3 different Yanmars for 25 years now. It didn't seem to hurt any of them, never any major issues or a rebuild necessary. But keep an eye on the exhaust elbow.

[mako]

Just throwing this out there... for those who have a large array of solar, you can fit your water heater with 12V, 24V or 48V heater elements and pump excess solar energy in. Just wire it in carefully so that you're not accidentally running off your batteries.

[sailingharry]

Originally Posted by donradcliffe

The physics says that using waste heat from an inefficient combustion engine to heat water while you are running it for propulsion, charging batteries, and making water is a better plan than throwing more money at solar panels and batteries.

And the math doesn't look good for solar panels and LFP batteries (actually LFP batteries make no power at all, so they aren't part of the solution).
Water has a specific heat of 1.16 * 10-3 kWh/(kg*K). A 10 gallon water tank is ~40kg. Heating from 60 to 105F is 45F, or 25K. So, to heat a 10 gallon heater from 60-105 takes 1.16 * 10-3kWh*45*25=1.3kW. Using the rule of thumb of output=3X rating, you'd need 400W of solar to do this once a day (and once a day is probably sufficient).


Big solar arrays (Jedi) might have 400W of excess capacity. I think most of us are closer to break even (or even wishing for more).

[FlyingScot]

Originally Posted by sailingharry

And the math doesn't look good for solar panels and LFP batteries (actually LFP batteries make no power at all, so they aren't part of the solution).
Water has a specific heat of 1.16 * 10-3 kWh/(kg*K). A 10 gallon water tank is ~40kg. Heating from 60 to 105F is 45F, or 25K. So, to heat a 10 gallon heater from 60-105 takes 1.16 * 10-3kWh*45*25=1.3kW. Using the rule of thumb of output=3X rating, you'd need 400W of solar to do this once a day (and once a day is probably sufficient).


Big solar arrays (Jedi) might have 400W of excess capacity. I think most of us are closer to break even (or even wishing for more).

I had done a quick observational estimate about 100 Ah to heat 6 gallons of water from our battery bank from out old solar/battery setup. That was from ambient to around 120F. When we installed new engines and replaced the water heaters I wasn't willing to give up the engine heating ability. Especially since engine heat is much higher than the standard water heater cutoff. I know some boats are switching to instant on heaters. This makes sense as a supplement, especially if your regular water heater is far away. I see many catamarans with one large water heater in the aft end of the boat and the owner's cabin shower on the oppsite side way forward. Some owners end up running the water into a bucket until it gets hot. Bad design.

As far as excess solar, that seems like a fairy tale to me. I don't think I've ever met a real life live aboard who wouldn't like just a little more solar. This presumes they live off the solar array, not using other methods of power generation. As a friend told me once, "you almost have enough". We literally doubled ours from 1440Wp to 2895Wp and some days it's still not enough.

[SV Confianza]

We use our hot water heater exclusively on the batteries. I believe it’s an 11 gallon isotemp spa 40. Uses 750 watts.

Will heat up water enough for a shower in 20 min in the tropics. Was about 40 min when we were in colder weather.

No problem with our current 1290 watts of solar (normally have 2,090 watts but are redoing the solar on the hard top at the moment. Lots of glue removal from a previous install. Pain in the…

But heating water never seems to be an issue even in our current state.