This is what I said to the guy who's building rutu (cited earlier in this thread), and his response, and my counter-response:
question has nibbled its way back into my consciousness...
My electrical contractor is very confident that we should be very
comfortable with our electrical budget
, in general. On his boat, whenever
was piping, and the batteries charged, he'd run his watermaker. In
our case, there's a diversion to the hot water
heater's dual element. I'm
discovering that the water is *very* hot, and, more, I don't use but the
least amount, now, when I'm showering, as the outside temps start going
tropical. So, likely we'll not use much hot water, either, in our cruising,
particularly since our deck shower
is cold only.
Back to the story, however, I saw your website mentioned in a watermaker
discussion in cruisers and sailing forums' website, which got me thinking,
again. The discussion was mostly about build vs buy, and the cost in money
not amps, or 12 vs 110volt.
I think I recall
your system as being pretty massive, and running off your
pto unit at the front of your engine
or genset? Or was it PTO to be both
genset and water?
Anyway, whichever it is, I'm wondering about your thoughts for our boat.
PUR40E seems about the least expensive you can buy, but doesn't make very
much water, and at a very expensive amp/gallon cost. Spectra (various
models) seems to be the most efficient, but even so, ~1.5A/G seems like it
would be very expensive on the electrical budget, limiting water generation
to only high wind
days. And, of course, the cost of a Spectra is
breathtaking. My current
water usage, intentionally profligate in order to
fully flush the tanks
and lines and keep stuff fresh, is about 15G/D.
However, I have very little dishwashing, which might change the equation a
bit, and having Lydia and the occasional guest aboard might make that about
equal out, on average. OTOH, if we had essentially unlimited water, we
might change our salt
washdown to fresh, and some other accommodations to
With my salt water
washdown, or another of the ER TH currently blocked, I
intake, and could run the effluent out the same port as the bilges
scuppers). If I teed off the salt wash, I could even use the
prefilter that has (the one which used to filter the AC) lots of capacity.
Currently the ER shelves are unpopulated except for the shower
pump, and so
I'd have a variety of places to put stuff if I needed.
If I were to make a 12V unit, sized, I'd think, to make something on the
order of 4 or more G/H, as I can't expect we'd have a reliable generation
for more than 4 hours a day, what would you expect for the A/G costs, and
what would you suggest for the components?
> There is no easy answer. I went with an engine driven high pressure pump
> because I want a lot of water and even my 900 AH banks with 2 140W solar
are not enough to run a big watermaker and keep up the refrigerator
> by themselves.
> First of all, if you plan to hang around St. Thomas for a few years it may
> not be worth the investment. Water is available at all the marinas
> Charlotte Amalie and Red Hook for about 5 cents/gallon. However, leaving
> your mooring
every few days to fill the tanks
can be a hassle. The parts
> for a simple system will cost you about $2K. Not as fancy as mine but
> What is the average output of your solar panels
and/or wind generator
> 10-12 knots? You will need to plan on about 80 AH minimum to make 14-16
> The PUR system is intended to run for hours on end to produce any usable
> amount but I think it is more efficient to make a lot of water quickly.
> of the R/O makers give figures for brand new membranes which is
> They push the membranes to beyond maximum and the output quickly drops to
> around 85% or less of what they claim. Think more about running the R/O
> system for maybe 2 hours a day and let the solar/wind generator
the rest of the time.
> Here is a rough idea based on a few rules of thumb: The pump pressure
> vary greatly as you tune the system so this is the worst case running at a
> full 800PSI.
> The maximum recovery rate of one SW30-2540 40" membrane with average
> is 13% and the minimum supply rate is 1.3 GPM so you will get a minimum of
> about 10GPH. (That could be a higher in warm water.)
> The pump should be Ni/Al Bronze or stainless. Regular brass heads will
> hold up. Small bronze or stainless pumps are hard to find but CAT makes
> bronze 237 that puts out 2.3 GPM @1725RPM.
That will give you a little
> extra to grow into.
> The required HP is (GPM*PSI)/1460 so (2.3*800)/1460 = 1.26HP which is hard
> to do on 12VDC. The best you can do in a stock motor
is a Leeson 1HP
> (108322) and it eats 80 amps at full load. With 1HP you can get about
> 1.8GPM at 1350RPM so you would need a pulley of say 6" on the motor
> on the pump. (or 3" & 4")
> So the pump will output 108GPH and with a 13% recovery rate you will get
> about 14 GPH of fresh water. The motor will eat about 75 amps at that
> so we are getting about 5.3AH/gallon. It will be closer to 4AH/gal for
> first 1,000 gallons or so but I feel safer using long term numbers. You
> also increase output by adding another SW30-2540 (another $600). That
> give you about 25 GPH for the same AH. My program doesn't cover the 21"
> membranes but at these low flows they will probably be a little more
> efficient than the 40" size. In reality it will probably be considerably
> better than this but, as I said, this is a worst case scenario. As you
> for the 13% product you will find that the pressure required will be
> considerably less than 800PSI so the HP and therefore the AH will go down.
> If you decide to do it I will dig our my sources and get into more detail.
> Glenn Ashmore
> I'm building a 45' cutter
in strip/composite. Watch my progress (or lack
> there of) at: http://www.rutuonline.com
> Shameless Commercial
and my followup:
Hi, Glenn, and thanks for the detailed response.
Phew! I don't think I can handle 80 amps, for starters. 5-6A/gallon isn't
My KISS makes 15-25A at 15-25k, and higher as the wind goes up (not linear -
better as wind increases), but still would not keep up with that.
Until I get off the grid I don't know what to expect from my 360W of solar
Every real-life example report I've seen puts typical output at 1/4 minimum
in other than rain to up to 1/2 AH/W replacement, with ~1/3 being typical.
So, I'm hopeful of averaging ~120AH/day there. Given the proclivity of
in protected areas, I expect non-sailing-day wind power to be more
supplemental than big overage.
As expensive as it is, the commercial
guys (spectra, e.g.) are in the
1.5-2A/gallon range, which, if we're making, say, 15-20g/day, is something
we could likely keep up with, making more on windy days and less on not. I
don't know what their secret is, but perhaps they use smaller pumps, or
special gearings, or something else to achieve that.
Obviously, more research
is needed - and it's not high on the list,
particularly, as you say, if we're in an area where water is cheap
However, unless we get a budget hit which we've not yet identified, we don't
expect to hang around STT any more than any other island. Generally
speaking, we were expecting to capture our own water in most cases, whether
just by deck
, or by some awning, also not yet designed, drain attachment,
feed to the inlets - but there are the dry periods, and our initial time -
probably more than a year - will be in the Bahamas
where I'm told water is a
real challenge; Paradise Island and Nassau
, for example, having to barge it
in each day, e.g.