Hot water on Boats - best solution ?

Discussion in 'Boat Design' started by rwatson, Jun 17, 2013.

  1. rwatson
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    rwatson Senior Member

    I did a search on this, but the only thread was very old

    I am checking out caravan solutions, but maybe they are not robust enough for boats ?

    What success has anyone else had.

    I have checked out


    any other solutions would be interesting, thanks

    PS - also

    A bit of background - the planned environment varies from fully serviced sites, to out of the way places. Having a mains power option would be great, as well as bottled gas. There is no potential for engine based hear exchangers or diesel for me, as he boat will be outboard powered.
    Last edited: Jun 17, 2013
  2. Richard Woods
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    Richard Woods Woods Designs

    My boats have also always been outboard powered. I have fitted an on demand LPG heater in the past, but had to do it myself as the gas installer refused to fit one on a boat because of the naked flame and CO risk. In fact I think they are illegal in Europe now. However they work great on multihulls (no heeling) and I always had a hatch open when the heater was lit.

    Also not sure how your insurance might be affected.

    When tropical cruising we use solar showers, and of course we then tend to swim more and wear fewer clothes. People also use the pump up style garden sprayer with a kettle full of hot water. But I suspect you want something better than that

    The 12v unit seems the best option, a 25amp draw isn't too bad with an outboard setup (less than an anchor winch) providing you only have short showers and run the engine while showering.

    I will be following this thread to see what others suggest

    Richard Woods of Woods Designs
  3. Willallison
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    Willallison Senior Member

    As I said in the thread to which you refer, I suggest you take a look at this thread (You probably already have). The science hasn't changed and I'm afraid I still haven't seen anything that can be legally installed on a boat that is up to the job, other than a small diesel-fired heater like the excellent Webasto units, which would also provide you with cabin heating, but which would obviously need a 2nd fuel supply...
  4. rwatson
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    rwatson Senior Member

    ah, thanks for that - that is probably what I should have been searching for. I will go through those posts in fine detail.
  5. philSweet
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    philSweet Senior Member

  6. Safe2breathe
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    Safe2breathe Junior Member

    what kind of boat are we talking about?
  7. rwatson
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    rwatson Senior Member

    This is planned for a 28ft , trailer sailer, with 60 hp outboard, built out of ply/epoxy.

    I have ~300 litres of fresh water ( starting out as ballast ) to heat as required.

    Seems to indicate caravan size/performance gear would be in order.

    I might have to do some special LPG lockers at the stern.
  8. Safe2breathe
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    Safe2breathe Junior Member

    My initial thought was utilizing your engine/generator cooling system ... Something like a coil around or in a segregated tank - imagine a radiator- but it is a good deal of work - for what it is worth the contact water heaters do work very well - they also have smaller water heaters for Rvs and such - just some thoughts.
  9. John Perry
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    John Perry Senior Member

    The duoetto 12v 10 litre electric water heater sounds quite interesting to me, but the distributor is an Australian company and a quick internet search did not reveal any other sources for this unit.

    My first thought on seeing the Duoetto was that no way could the typical electrical system on a small yacht cope with heating domestic hot water, other than when shore power is available, in which case you probably have a marina shower block not far away so don't really need on board hot water. I would add that the type of 'small yacht' I have in mind is a performance sailing yacht with a small petrol outboard for an auxiliary. The argument could be a bit different for more of a motorsailor type with a diesel inboard that probably runs for long periods whenever the boat is not on shore power. Could also be a bit different for RWatson's boat with its monster 60HP outboard.

    Then I remembered the 'Supercharger'

    I quite like the idea of the Supercharger. Most small petrol generators seem to be designed for the camper van market where house battery charging is primarily by the alternator on the vehicle engine and a generator is mainly needed to power various mains AC appliances, with battery charging as a bit of an afterthought, hence most of the electrical output is at 240V or 110V AC with only a few amps at low voltage DC. If you want to use the full electrical output of a typical small petrol generator for charging batteries then you have to accept the weight, cost and inevitable power loss (admittedly not a large power loss) of another box of complex modern electronics to fit between the generator and the battery. Besides which, 12VDC is surely safer on a small sailing boat than mains voltage.

    The Supercharger gets around this by providing its entire output at a DC voltage suitable for battery charging so it would seem to be more suited to small yacht applications where nearly all electrical equipment on board naturally requires 12V DC, or can be specified to require 12V.

    A friend of mine has just brought a Supercharger and the first thing he found was that with his installation at least, the Supercharger will only output a maximum of 30A whereas the rated output is 50A. I see from the internet that there seems to have been a formal complaint to the Advertising Standards Authority regarding the Supercharger, not sure if this deficiency is something to do with that. Despite this shortfall in output, my friend is well pleased with his Supercharger and he has found that so far it meets the charging requirements on his small sailing boat, including a small electric outboard for occasional auxiliary propulsion. The real maximum output of the Supercharger is about the same as the requirement of the electric outboard at full power, so it can be run as a petrol/electric transmission system for long periods if you can put up with the racket it makes. I have heard the unit run and it is quite noisy so if this type of generator catches on perhaps someone will come up with a quieter model soon. It is light and compact and does seem to start reliably with just one tug on the string.

    30amps may fall short of the rated output of the Supercharger but it is still several times more low voltage DC than you will get from a typical small generator of more conventional design and it is enough to provide a rapid bulk charge into a typical lead acid house battery on a small yacht.

    However, a problem with using a high output charger such as the 'Supercharger' with lead acid batteries is that following the bulk charge you should then apply a topping charge to take the battery from around 75% charge up to fully charged. I would add at this point that it looks like lithium batteries of one kind or another may quite soon become the first choice for marine house batteries, in which case units like the Supercharger would be more appropriate since Lithium batteries don't need a topping charge. However, if you omit the topping charge with lead acid, which is still most peoples choice for a small yacht house battery, then not only do you not get the full capacity from your battery but you also ruin the battery through sulphation. The current during the topping charge varies but for a typical size house battery on a small sailing yacht the average current during this period is likely to be a fraction of the 30amp maximum output of the Supercharger. That means that the duration of the topping charge will probably be longer than the bulk charge and during this extended charging period the engine of the Supercharger will be running at low load and so probably rather poor efficiency. One way of looking at this is that for more than half of your likely charging period you will have 'spare' 12V capacity available and it might be good to have a use for this spare capacity since it is available at relatively low cost in terms of fuel use and wear on the generator engine and mechanics. That is where I thought there could be a place for a unit such as the Duoetto water heater (or an alternative if there is one, even a home made alternative). It would seem quite nice to use your 'spare' generating capacity to heat a small insulated tank of hot water that could then be used for a few quick showers. How long you can leave it from the end of battery charging to taking a shower probably depends on how well your hot water tank is insulated. This would seem to be a simpler (no flue), more reliable, cheaper and arguably safer (no gas hazard and only low voltage electrics) then the gas fired 'geysers' and chimney stacks found on some yachts.

    I am not an electrical engineer so perhaps someone can point out if this argument is flawed.
  10. michael pierzga
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    michael pierzga Senior Member

    remember the also develop strategies to save this hot water in a well insulated tank. for practical reasons you will probably need an on demand gas heater for showers and whatnot..

    gas is a pain in the as s . you always run out of gas
  11. P Flados
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    P Flados Senior Member

    It is a shame that it is so hard to do something that should be so easy.

    If boat motors could be obtained with a "secondary cooling cooling coil", the combination of engine heat plus electric (from the alternator) to boost the temp just a little could easily do "on demand" water heating very very efficiently.

    Alternately, if a manufacturer was willing to make a marine rated LP "on demand" heater, it would probably be a really good solution for the RV industry.
  12. rwatson
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    rwatson Senior Member

    That surprises me. In a caravan, gas for hot water lasts months for a couple.

    That would be the same on a boat or dry land, less in a boat if you weren't hooked up to shore water and you were conscious of water levels.
  13. michael pierzga
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    michael pierzga Senior Member

    Well...I seem to run out of just about everything when Im on a cruise !!!!!!!!!!!

    Precoius little water left in my tanks

    Im 23 days into my present cruise , 2 to 5 persons onboard, and just about every consumable is running low.

    Gas can be difficult to get on coast. I would have to sail 50 miles off my present route to find a port with availble gas bottles...or I could anchor off the next village and take a 50 mile taxi ride.

    Perhaps in your part of the world gas and consumables are easier to get

    The less you use...the easier life is.
  14. Mark Cat
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    Mark Cat Senior Member

    Is the on-demand system attractive because of weight?

    Its hard to say the best way to heat water on-board unless the vessel requirements are understood.

    For larger yachts I am still perfecting hot water custom Aluminum insulated welded tank approaches, while minimizing corrosion; poultice, crevice, and galvanic.

    Once the tank is designed, which is a large part of the task, there are several sources and ways available to heat it.

    For example, for a single tank you can have multiple electric heating elements and controllers. One from the storage batteries and the other solar PVs.

    With a tank you can heat the water over time, which reduces peak power demands.

    As has already been said, when cruising in remote areas it can be difficult to find LPG of the correct type. And at times a challenge to find good diesel.

    All for now,

    Mark Cat

  15. michael pierzga
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    michael pierzga Senior Member

    Put a big inspection hatch in your hot water storage tank for cleaning. The inside on a hot water tank looks like a science project very fast.
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