cruiser might carry 1,200 gallons (4,540 L) to
get adequate range—3.8 tons!
To convert lb./gal. to kg/L, multiply lb./gal.
by 0. 1198
To convert kg/L to lb./gal., divide kg/L by
0. 1198The optimum location for fuel tanks is
over the boat’s center of buoyancy. This way,
there is no change in trim with varying tank
levels, and the weight is kept out of the ends
to reduce pitching.
On sailboats it is also important to locate
tanks as low as possible to maximize sail-
carrying power with full tanks, but this is not
good practice for voyaging motor cruisers.
Such motor cruisers are sometimes designed
with huge fuel tanks built into double bot-
toms, but if the vessel is adequately stiff with
empty tanks, then adding 6 tons or more of
fuel this low down will make the vessel far
too stiff—dangerously and uncomfortably so.
Conversely, if the vessel relies on the weight
of the fuel in the double bottom for proper
stability, it will be dangerously tender when
empty. The proper vertical location for big
tanks on long-range cruisers is moderately
high, with the tank’s center of volume at or
even just above the design waterline (DWL).
Planing hulls actually can benefit from
tanks somewhat aft of the center of buoyancy.
The goal is to have the vessel trim level when
light and just a bit down by the stern when
heavy. Again, the tanks should not be too low.
Planing hull forms already have quick, snappy
rolls. The vertical center of the tanks are,
again, ideally at or just a bit above the water-
line. The drawing in Figure 5-4 shows a 66-foot
(20 m) express cruiser of my design that has
the tanks located right over the longitudinal
center of buoyancy (LCB) and at the optimum
height. In this case we were able to make the
tanks do double duty in blocking some engine
noise from the accommodations.
Though the ideal is to have large tanks
over the LCB, real-world considerations can
make this impractical. This creates trim
problems that must be addressed. One solu-
tion is multiple tanks and pumped distribu-
tion of fuel to maintain level trim. For long-
range voyagers, another option is seawater
trim-ballast tanks. Figure 5-5 shows an
82-footer (25 m) my office designed that hasChapter 5: Fuel Tanks and Fittings
Any nonpolyurethane foam used to encase fuel tanks must have a compressive strength of 60 psi (413 kPa) at
10 percent deflection. And polyurethane foam must have a density of at least 2.0 lb/cu.ft. (32 kg/m^3 ).
The foam must not be the sole support for a metallic tank. The metallic tank must be structurally supported inde-
pendent of the surrounding foam.
The summary of requirements for metallic gasoline tanks buried in foam (encapsulated in cellular plastic) is as follows:- The tank must be made from a nonferrous metal.
- The tank must be structurally supported independent of the foam.
- Water must drain freely from the tank’s top surface.
- The tank supports, chocks, and straps must be integral with the fuel tank orinsulated from the fuel tank surface with
non-moisture-absorbing material. - The tank must not support any other boat structure of any type.
- The tank must be restrained from moving more than^1 / 4 inch (6.3 mm) in any direction.
- All connections, fittings, and labels must be accessible for inspection.
- Failures in the foam or encasement material cannot occur at the joint to the surface of the fuel tank.
There are still more requirements for foam used to encase fuel tanks. If you intend to build a boat with foam-
encapsulated (cellular-plastic-encapsulated) foam—contrary to the recommendations of this book—you must care-
fully follow relevant portions of the CFR. You should use the ABYC Compliance Guidelinesfor fuel systems as a check-
list for compliance.
Again, though the practice is legally permitted if the preceding rules are followed, this book recommends against
burying or encapsulating any tank in foam or cellular plastic.