Flotation
FEDERAL LAW
183.516 - Cellular plastic used to
encase fuel tanks
(a) Cellular plastic used to encase metallic
fuel tanks must:
(1) Not change volume by more than five
percent or dissolve after being immersed in any of
the following liquids for 24 hours at 29 deg.C:
(i) Reference fuel B ASTM D 471 (incorporated
by reference, see Sec. 183.5).
(ii) No. 2 reference oil of ASTM D
471 (incorporated by reference, see Sec. 183.5).
(iii) Five percent solution of trisodium
phosphate in water; and
(2) Not absorb more than 0.12 pound
of water per square foot of cut surface, measured
under Military Specification MIL P-21929B.
(b) Non-polyurethane cellular plastic
used to encase metallic fuel tanks must have a compressive
strength of at least 60 pounds per square inch at ten
percent deflection measured under ASTM D 1621 (incorporated
by reference, see Sec. 183.5), “Compressive Strength
of Rigid Cellular Plastics”.
(c) Polyurethane cellular plastic used
to encase metallic fuel tanks must have a density of
at least 2.0 pounds per cubic foot, measured under ASTM
D 1622 (incorporated by reference, see Sec. 183.5),
“Apparent Density of Rigid Cellular Plastics.” |
NOTE:
If cellular plastic was used to encase a fuel tank and is
counted as flotation material, it must meet the above stated
requirements.
Of the many foams available, foamed polystyrene
and polyurethane are the most common. They are noted here
because of their broad use, but not to the exclusion of other
types of flotation material.
Foamed polystyrene is a low-cost material produced
from expandable beads (“pop corn”) or extruded
in the form of billets or boards. The common name is “Styrofoam.”
In its common forms, it is readily dissolved in gasoline and
is highly flammable. There are special compounds of polystyrene
foam that are solvent-resistant and self-extinguishing. One
objection to foamed polystyrene is that it cannot be foamed
in place, and it usually cannot be produced in a boat builder's
plant. Caution is necessary because some varieties have “wormholes”
which reduce the flotation value.
Polyurethane foam is another widely used buoyancy
material. It is usually foamed in place or molded into specific
shapes before installation. It is also available in slabs
and billets. Polyurethane foam is normally highly resistant
to gasoline and oil, particularly in densities of 4.0 pounds
per cubic foot and higher. Even in densities of 1.5 to 2 pounds
per cubic foot, it is considered very resistant to gasoline
and oil. Polyurethane foam is flammable, but it can be made
self-extinguishing. In densities of less than 2 pounds per
cubic foot, it may absorb significant amounts of water.
Air chambers used to achieve flotation are usually
made of plastic materials. They must be capable of withstanding
the same solvent-resistance tests as other flotation materials,
and they must pass the required 18-hour submergence preconditioning
before undergoing the flotation tests.
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