Freshwater Tanks and Water Tank Capacity
We no longer store water in wooden casks;
rather, water is stored in purpose-made
tanks. This is cleaner and much less prone to
algae growth. It also permits a lot more water
per person. How much? Well, in the old days
the rule of thumb was 1 gallon (3.8 L) per per-
son per day. This still works, but only for
drinking and rinsing your hands and face. A
survey of long-range cruisers recently came
up with an average of 1.9 gallons (7.2 L) per
person per day for personal use only. But all
together, after adding in cooking, washing
dishes, and so on, the total was actually
3 .7 gallons (14 L) per person per day.
This is a pretty good guide to the way
water is used on modern boats with pressure-
water systems. Accordingly, a family of four on
a seven-day cruise would need 103.6 gallons
(494 L), plus at least a 30 percent reserve, say
135 gallons (510 L). This is still taking some
reasonable care—using salt water for pri-
mary dish-washing, keeping showers short,
and being sure not to leave the taps running.
(Yes, if you’re frugal, you could still get by on
less than a third of this—saltwater showers
with a quick fresh rinse, no pressure-water
system, all-manual pumps, careful attention
to conservation.)
The rule of thumb for tank capacity for
average cruising boats is a minimum of
15 gallons (57 L) for each usable berth
aboard. This is a bit low, as a typical 36-footer
(11 m), might have six berths and thus
90 gallons (342 L) by this rule. As we’ve seen,
this wouldn’t really be enough for a week’s
cruise for a family of four.
Cruisers equipped with watermakers
have topped 15 to 16 gallons (57 to 60 L) per
person per day or more. You could never
carry that much fresh water, but if you can
“make” it from the ocean, you’re OK. We
won’t go into watermakers here, but they’re
generally not as practical for coastwise cruis-
ers as you might think. This is because long-
shore waters are often muddy, brackish, pol-
luted, or some combination thereof. Without
clean, fresh ocean water, watermakers won’t
work well, or at all. In fact, a megayacht trick
is to build in big saltwater tanks. These can
be filled offshore and used to make fresh
water in harbor. Vessels this big, however,
can’t pull in just anywhere to fill up the tanks
as small to mid-size vessels can.Tastes in Tanks:
Tank Material
So what is the best material for freshwater
tanks? Stainless steel. Use only 316L for
corrosion resistance; everything else is sec-
ond best. Of course, second best is perfectly
acceptable. I’ve installed plastic, fiberglass,
wood-epoxy, and aluminum freshwater
tanks. They all work well enough, but each
leaves some taste in the water. Water from a
well-cleaned stainless tank—frequently
emptied and filled—can taste nearly as good
as fresh spring water. (Even better than
stainless is Monel. However, this is harder
and harder to find these days and quite
expensive.)Tank Details
Check to see that your tanks (regardless of
material) are well secured. You don’t want
them ever coming loose. (Fresh water weighs
8 .32 pounds per gallon, so a moderate-size
40-gallon tank would weigh about 360 pounds,
not a thing to have rolling around loose in a
storm.) Refer to Table 5-1 to make a quick,
close estimate of total tank weight. As with
the fuel tanks discussed in Chapter 5, water
tanks should have one or more clean-out
openings where possible (though this isn’t
practical on smaller tanks). It’s a nice plus to
have a tank drain at the bottom, ideally with a
sediment sump to collect any gunk. This, too,
isn’t practical on many boats because of
access limitations. If you do have a tank
drain, it should not only have a shutoff valve,
but the end should be threaded. This allows
you to install a screw cap so that an acciden-
tal opening of the valve won’t empty your
tank. It also means you can screw on a hose
to pump out the tank rather than just empty-
ing it into the bilge.
Baffles are required for both strength and
boat stability. In general, water tanks should
follow most of the requirements for fuel
tanks in Chapter 5. Making water tanks this
strong is not a legal requirement, but having aChapter 20:Freshwater Systems