proportion of the longitudinal huff framework is parallel sided
with curved bow and stern portions, the radius of these curved portions
being, in the case of the bow, twice the diameter of the hull, and in
the case of the stern nine times the same diameter. Experiments proved
that the resistance of a ship of this shape was only two-fifths of the
resistance of a ship of the same dimensions, having the 1 1/2 calibre
bow and stern of the Zeppelin airships at that time constructed.

A considerable difference of opinion existed as to the material to be
chosen for the construction of the hull. Bamboo, wood, aluminium, or
one of its alloys, were all considered. The first was rejected as
unreliable. The second would have been much stronger than aluminium,
and was urged by Messrs. Vickers. The Admiralty, however, considered
that there was a certainty of better alloys being produced, and as the
ship was regarded as an experiment and its value would be largely
negatived if later ships were constructed of a totally different
material, aluminium or an alloy was selected. The various alloys then
in existence showed little advantage over the pure metal, so pure
aluminium was specified and ordered. This metal was expected to have a
strength of ten tons per square inch, but that which arrived was found
to be very unreliable, and many sections had, on test, only half the
strength required. The aluminium wire intended for the mesh wiring of
the framework was also found to be extremely brittle. A section of the
framework was, however, erected, and also one of wood, as a test for
providing comparisons. In the tests, the wooden sections proved,
beyond all comparison, the better, but the Admiralty persisted in their
decision to adopt the metal.

Towards the end of 1909 a new aluminium alloy was discovered, known as
duralumin. Tests were made which proved that this new metal possessed
a strength of twenty-five tons per square inch, which was over twice as
strong as the nominal strength of aluminium, and in practice was really
five times stronger. The specific gravity of the new metal varied from
2.75 to 2.86, as opposed to the 2.56 of aluminium. As the weights were
not much different it was possible to double the strength of the ship
and save one ton in weight. Duralumin was therefore at once adopted.

The hull structure was composed of twelve longitudinal duralumin
girders which ran fore and aft the length of the ship and follo