6 gl or 12 gl water heater

[Boby_Hunter33]

My Kumma 8 Gl leaks. It will be replaced by a Raritan 6 or 12 gl.
It is a 45 DS, usually 3 people on board. coastal cruising.
The 6 will heat faster but will have less water.
Usually we motor very little, just to get in and out of marina/anchorage/mooring. So the motor heating the whole 12 Gl will not happen too often.

Thanks

 

[jssailem]

Heating water in a standard water heater is either electrical through AC power which from a practical issue is attachment to a marina outlet or conductive through the heat generation from running the engine.
If limited time then a 6gal tank means short showers but warmer water.
Alternatively you can hoist a 5gal black plastic outdoor camp shower and finish cleansing the soap out of your eyes in the cockpit.
No easy answers. Note on subs, before nuclear power, ship board showers were limited to 2 minutes. One quickly learned to de soap early.

 

[plenny7]

You don't have to run the engine very long to heat up the water

 

[MetricMountie]

Experience tells me that the time to heat 12 gallons as opposed to 6 gallons is very little while running the diesel under load and the extra hot stuff is especially appreciated by the female crew members, just sayin.

 

[JamesG161]

The only thing we [2-4] use our Hot water for is comfortable showers. If you want to hot wash dishes, you can make hot water on the stove.

Our water heater is too hot [180°] from engine production. It normally lasts in the heater tank for 16 hours, no use.

If you heater has 180° [scalding temp ≈150°F] use with cautions and..

6 gallon

Jim...

PS: Remember that water heat will enter your Cabin slowly as it cools down.

 

[Stu Jackson]

You don't have to run the engine very long to heat up the water

The laws of physics have not changed.
Without doing the calcs which anyone can find online [temp rise * input heating source * quantity of water being heated; it's a simple heat transfer calc], I can share real world experiences. In SF it took less than a half an hour to heat my 6 gallon tank with electricity at the dock. Here in BC it takes well over a half an hour. Why? The surrounding water temperature is 10 degsF lower here, that's why, so the water tank is much cooler and takes longer to heat. That's the "temp rise" part of the equation.

Experience tells me that the time to heat 12 gallons as opposed to 6 gallons is very little

Simply not true. And heating is linear: twice the volume, twice the time. In metric, SAE, F or C!!!
Or else someone's got a perpetual motion machine that we all wanna see.

And yes, engine heating can make water very hot. Do a search here on "thermostatic mixing valves" to find a solution. I have chosen to use my faucets to do that with a water saver button at the faucet, so I can set the temp and not get scalded. If you have kids, get one or both.

 

[plenny7]

Simply not true. And heating is linear: twice the volume, twice the time. In metric, SAE, F or C!!!
Or else someone's got a perpetual motion machine that we all wanna see.

While the heating of the water is linear, the time it takes for the engine to heat up is the same in either case.

For 6 gallons you have (Time to heat engine) + (Time to heat 6 gal.)
For 12 gallons you have (Time to heat engine) + 2*(Time to heat 6 gal.)

Heating 12 does not equal 2 x heating 6

 

[Gunni]

I have always found a 6 gallon water heater adequate. No reason to give 12 gallons of hot water a ride.

 

[Stu Jackson]

For 6 gallons you have (Time to heat engine) + (Time to heat 6 gal.)
For 12 gallons you have (Time to heat engine) + 2*(Time to heat 6 gal.)

Heating 12 does not equal 2 x heating 6

That's extremely math "challenged" and incorrect. The ONLY part that mattes is: 2*(Time to heat 6 gal.).
Why? 'Cuz "(Time to heat engine)" is a CONSTANT in both of your equations.
Please don't offer to balance my checkbook.

 

[plenny7]

That's extremely math "challenged" and incorrect. The ONLY part that mattes is: 2*(Time to heat 6 gal.).
Why? 'Cuz "(Time to heat engine)" is a CONSTANT in both of your equations.
Please don't offer to balance my checkbook.

Exactly my point.

 

[Stu Jackson]

Heating 12 does not equal 2 x heating 6

Exactly my point.

Huh?

 

[JamesG161]

While the heating of the water is linear

Is not linear, but not so important idea as to sizing.

It is consumption and use that is the key. Which is "batch" use.

Gunni has it right 6 gallons.
Jim...

 

[plenny7]

Huh?

Perhaps we're making different assumptions. I'm assuming we start the timer as soon as the engine starts. I'm also assuming the engine is cold when it starts. Since it's cold, it is not heating water immediately. It takes time for the engine to heat up. That amount of time applies to heating 6 gallons as much as 12 gallons, hence it's applies to both equations.

I don't think @Boby_Hunter33 cares about any of this. The point is, it doesn't take long to heat water up using the engine (however you want to define the equation), so even though he doesn't run his engine much, he should base his decision on how much hot water he wants on hand, rather than the time it takes to heat it up.

 

[Stu Jackson]

...he should base his decision on how much hot water he wants on hand, rather than the time it takes to heat it up.

I agree wholeheartedly. Assuming he runs his engine long enough to heat up whatever size he buys.

 

[kloudie1]

Here is another fly in the ointment.. consider that on Yanmar engines (I assume others as well but don't know for sure) the heat to the water tank (engine coolant flow) bypasses the thermostat so the water in the tank starts heating very quickly and the engine warm up time is not really a constant (12 gal tank will not let the engine warm up as quickly while the thermostat is closed).. Your car heater works like that as well.. coolant flows to the heater before the thermostat opens.. Not wanting to begin a peeing match here.. the 12 will take longer to heat than the 6, however.. how much longer to a usable temperature is the question.. With the engine under moderate load, the difference is certainly measurable but to a person on board, that time won't be very different feeling.. I am talking a system with good flow and well installed..

 

[Boby_Hunter33]

Thank you all for the input.
I guess i need to clarify a few issues:
1. With the current 8 gl tank, only in rare occasions the engine ran enough time to heat the water in the water heater.
2. The main source of energy to heat the water is a small honda 2000 generator.
3. Working on increasing my energy production to be able to have 30 to 45 minutes of power for the water heater (1500 w) every second day.
Thanks

 

[JamesG161]

bypasses the thermostat so the water in the tank starts heating very quickly and the engine warm up time is not really a constant

Non linear.

2. The main source of energy to heat the water is a small honda 2000 generator.

Electrical heating elements work the same.

Normally slower since there is very little mixing in the water heater tank.
6-8 gallon tank then.
Jim...

 

[Gunni]

You should see the look of joy on the faces of the crew when I announce that a one-hour engine run will commence, the beer refrigerator will be chilled, and there will be hot showers for all!

 

[JamesG161]

one-hour engine run will commence

I was thinking the same thing. Water heater gets "free" heat while your boat moves forward under engine power.
I have done that in light winds, sails up, and an engine boost to make 5 knots.
Jim...

 

[Boby_Hunter33]

Personally, I think that running the engine and generating GHG contradicts the essence of sailing. My diesel bill for last season was $73. The engine was used only once for a long period of time (3 hours), this was when we ran for shelter before hurricane Michael arrived to Rhode Island.