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The very popular general plumbing, toilets, sinks, faucets, pump problems, questions and answers discussion Forum
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Author:
SailorBob (Non-US)
Hello,
Yes, ... I'm a newbie here and I'm interested in knowing if you can help me with a project.
I wish to build an irregular shaped, stainless steel water tank, whereby one side will conform to the compound angled shape of the hull of a sailboat, in order to gain maximum usage of its allotted space.
Generally speaking, the tank will measure 18" x 8" x 6" & will have a capacity of approx 18 liters. It will be built by a tradesman skilled in S.S. tank manufacturing.
It will be installed horizontally in the location allotted to it, with the 6" dimension being the height.
There will be a fill pipe (garden hose size); An overflow pipe with an integral "one-way valve" (to prevent reverse flow contamination). The latter will be drained overboard via a sink outlet.
There will also be an outlet at the bottom, supplying hot water for two adjacent sink units separated only by a bulkhead.
The tank is to be heated by 120v shore-power, but only when tied up in a marina. The heated water will be used for personal toiletry and dishwashing purposes only.
My questions are:
1. What physical size element and Wattage shall I require for the tank dimensions and capacity described?
2. Will I require a relief valve in addition to the overflow system I've described?
3. What additional requirements might there be other than the above?
4. Where in the tank should I install the element(s) eg: Top cover plate or end plate?
5. Do you have any other recommendations for me?
Thank you for showing an interest in my project. ... SailorBob.
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Author:
SMSPlumbing (PA)
Here is some small water heaters that are ran by electric that the sponser sells. This will give you some idea, or you could just buy one of these.
[www.plumbingsupply.com]
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Author:
packy
watch this video and then decide if a home made water heater is safe. (especially without a relief valve)
[www.youtube.com]
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Author:
hj
If it is an "open tank" meaning it is not, and CANNOT be pressurized, then it does not need a relief valve. The element should be installed so that it heats the bottom of the water since heat rises and any water below the element will only be heated by convection currents, which also means it will ALWAYS be cooler than the water above the element.
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Author:
jimmy-o (CA)
1. What physical size element and Wattage shall I require for the tank dimensions and capacity described?
How many amps do you have available from shore power? If you have 120 volts and a 15 amp ciruit available, a small 1500 watt or even 1000 watt element will heat your 18 liters.
2. Will I require a relief valve in addition to the overflow system I've described?
Since the boat is not covered by any building codes, hard to say. If the overflow system could in any way fail closed, then you need a relief. Coast Guard rules govern boat construction?? As for the pressure rating of the relief, you would in theory need to know an ANSI Test Pressure Spec. for the tank, which of course you don't know. Since it is apparently a gravity system, I would get a 25 PSI relief valve, and keep your fingers crossed.
3. What additional requirements might there be other than the above?
4. Where in the tank should I install the element(s) eg: Top cover plate or end plate?
5. Do you have any other recommendations for me?
Motel 6
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Author:
bernabeu (SC)
purchase one of these (or similar): [www.plumbingsupply.com]
'gut it' and use the parts (element, thermostat, wiring, prv, etc.)
show / give the parts to your fabricator
good fortune 
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Author:
SailorBob (Non-US)
Thanks everyone for all your input. It could be that the suggested "on-demand' water heater is the way to go. I was unaware there is such a device. It looks neat, compact, & makes far more sense than the job I was about to embark upon, & it's certainly a whole lot cheaper than "reinventing the wheel"
I'll have to find the physical dimensions of the smallest unit available to see if it fits the allotted space on the boat. It would certainly save heating up 18 litres just to do the dishes ... but of course, the volume to be heated would be forever lessening as water is consumed (another consideration that had escaped my thinking) requiring some safety device to prevent burning up the elements in an empty tank.
... Yes, as me Irish wife would say: "There's a whole lot more to makin'-a-pot-o'-tae than just settin' a flame under the kittle". This forum has already proved its worth to me ... thanks to all who responded. 
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Author:
bernabeu (SC)
i think we all assumed that the tank would have an inlet pressurized water supply hooked up when in operation
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Author:
hj
If as you said, the water would be heated while tied up at the dock, and then used while at sea, there would be no power to "burn out the elements as the water was consumed".
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Author:
hj
You may have, but I DID NOT, which is why I said he probably did NOT need a relief valve.
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