It’s Baffling
CFR Yacht and ABYC do not set a fixed re-
quirement for baffles on pleasure-craft tanks,
and CFR Yacht doesn’t cover diesel tanks.
Nevertheless, baffles are essential in all tanks
larger than a few gallons for proper stability
and to reduce noise. CFR Commercial
requires that all nonintegral tanks must be
built with internal baffles spaced no more
than 30 inches (76 cm), both athwartships
and fore and aft. My strong recommendation
is that tanks for yachts follow this 30-inch
(76 cm) spacing in most instances. (Without
proper baffles, tanks would fail the slosh and
pressure-impulse tests, in any case.)
Still, 30 inches (76 cm) is a somewhat
smaller distance than required to prevent ex-
cessive sloshing on larger craft. Such vessels
can have their baffles spaced farther apart. A
reasonable rule is to space baffles no more
than 15 percent of beam, but never more than
44 inches (111 cm) apart. When using spacing
greater than 30 inches (76 cm), the tank walls
must be strengthened with stiffeners between
the baffles, as structural calculations require.
For metal tanks, these are standard angles or
tees welded on as done on structural bulk-
heads. Tank strength must be calculated to
meet the required head (pressure) over the
wider panels with the larger distances
between baffles, but including the addedstiffeners. Note that CFR Commercial does
not set requirements for baffles on integral
tanks, only on nonintegral tanks.
Baffles must be fitted with large limber
openings at all top and bottom corners to per-
mit adequate fuel and air flow (Figure 5-13).
The total opening area can’t be more than
30 percent of the baffle—roughly 18 percent
is a good average. On integral tanks, I try to
center any cleanout access ports over a baf-
fle with the baffle itself cut back in a half
circle directly underneath. This way a single
cleanout panel gives access to two or four
baffle spaces. You can see these cutouts in
the baffles in Figure 5-13. On nonintegral
metal tanks, the baffles should be welded
every 2^1 / 2 inches (63 mm) with the weld one
size smaller than the thinnest plate being
welded to. Rivets can be used on internal baf-
fle connections, but use only welds when
connecting to the tank’s walls.
You can’t see inside your tank (unless it’s
large and fitted with cleanout ports), but it pays
to be nosy and learn about the construction.
Ask the manufacturer how they build and test
their tanks. Though most tank fabricators are
reliable and conscientious, I’ve occasionally
seen bizarre things. Once, for instance, I was re-
tained to “fix” a 54-foot (16.4 m) production
diesel motor cruiser with a single 15-foot-long
(4.6 m), 500-gallon (1,892 L) tank equipped with
just two baffles! The result was some odd bang-
ing noises, some very peculiar trim problems,
and a strange difficulty getting up on plane—
but only some of the time! No one had thought
to question the tank construction previously.
DON’TSHAREWALLS Speaking of baffles, it is
tempting—to save space and simplify
construction—to have integral water tanks
and fuel tanks share a wall. This is not per-
missible. Any slight pinhole leak in the shared
wall will contaminate both. Black- and gray-
water tanks are no exception. It is acceptable
to have two fuel tanks (of the same fuel type)
share a wall—basically a baffle with no open-
ings in it. This is sometimes a convenient way
to create a day tank inside a larger tank.Round the Corner
Best practice for nonintegral sheet-metal
tanks with walls 0. 190 inch thick or less is not
to weld them together at the corners. CornerPART TWO: FUEL SYSTEMS
Figure 5-13.
Baffles in
aluminum tank