_Material_. In 1861, the British iron-plate committee fired with
68-pounders at many varieties of iron, cast-steel and puddled-steel
plates, and combinations of hard and soft metals. The steel was too
brittle, and crumbled, and the targets were injured in proportion to
their hardness. An obvious conclusion from all subsequent firing at
thick iron plates was, that, to avoid cracking on the one hand, and
punching on the other, wrought-iron armor should resemble copper more
than steel, except that it should be elastic, although not necessarily
of the highest tensile strength. Copper, however, proved much too soft.
The experiments of Mr. E.A. Stevens of Hoboken, with thick plates,
confirm this conclusion. But for laminated armor, (several thicknesses
of thin plates,) harder and stronger iron offers greater resistance to
shot, and steel crumbles less than when it is thicker. The value of hard
surfaces on inclined armor will be alluded to.
_Solid and Laminated Armor compared. Backing_. European experimenters
set out upon the principle that the resistance of plates is nearly as
the square of their thickness,--for example, that two 2-inch plates are
but half as strong as one 4-inch plate; and the English, at least, have
never subjected it to more than one valuable test. During the last year,
a 6-inch target, composed of 5/8-inch boiler-plates, with a 1-1/2-inch
plate in front, and held together by alternate rivets and screws 8
inches apart, was completely punched; and a 10-inch target, similarly
constructed, was greatly bulged and broken at the back by the 68-pounder
(8 inch) smooth-bore especially, and the 100-pounder rifle at 200
yards,--guns that do not greatly injure the best solid 4-1/2-inch plates
at the same range. On the contrary, a 124-pounder (10 inch) round-shot,
having about the same penetrating power, as calculated by the ordinary
rule, fired by Mr. Stevens in 1854, but slightly indented, and did not
break at the back, a 6-5/8-inch target similarly composed. All the
experiments of Mr. Stevens go to show the superiority of laminated
armor. Within a few months, official American experiments have confirmed
this theory, although the practice in the construction of ships is
divided. The Roanoke's plates are solid; those of the Monitor class are
laminated. Solid plates, generally 4-1/2 inches thick and backed by 18
inches of teak, are exclusively used in Europe. Now the resistance of
plates to punching _in a machine_ is directly as the sheared area, that
is to say, as the depth and the diameter of the hole. But, the argument
is, in this case, and in the case of laminated armor, the hole
is cylindrical, while in the case of a thick armor-plate it is
conical,--about the size of the shot, in front, and very much larger in
the rear,--so that the sheared or fractured area is much greater. Again,
forged plates, although made with innumerable welds from scrap which
cannot be homogeneous, are, as compared with rolled plates made with
few welds from equally good material, notoriously stronger, because the
laminae composing the latter are not thoroughly welded to each other,
and they are therefore a series of thin plates. On the whole, the facts
are not complete enough to warrant a conclusion. It is probable that the
heavy English machinery produces better-worked thick plates than have
been tested in America, and that American iron, which is well worked in
the _thin_ plate used for laminated armor, is better than English iron;
while the comparatively high velocities of shot used in England are more
trying to thin plates, and the comparatively heavy shot in America prove
most destructive to solid plates. So that there is as yet no common
ground of comparison. The cost of laminated armor is less than half that
of solid plates. Thin plates, breaking joints, and bolted to or through
the backing, form a continuous girder and add vastly to the strength of
a vessel, while solid blocks add no such strength, but are a source of
strain and weakness. In the experiments mentioned, there was no wooden
backing behind the armor. It is hardly possible,--in fact, it is nowhere
urged,--that elastic wooden backing prevents injury to the _armor_ in
any considerable degree. Indeed, the English experiments of 1861 prove
that a rigid backing of masonry--in other words, more armor--increases
the endurance of the plates struck. Elastic backing, however, deadens
the blow upon the structure behind it, and catches the iron splinters;
it is, therefore, indispensable in ships.
_Vertical and Inclined Armor_. In England, in 1860, a target composed of
4-1/2-inch plates backed with wood and set at 38° from the horizon was
injured about one-half as much by round 68-pounder shot as vertical
plates of the same thickness would have been. In 1861, a 3-1/4 plate at
45° was more injured by elongated 100-pounder shot than a 4-1/2 vertical
plate, both plates having the same backing and the weights of iron being
equal for the same vertical height. When set at practicable angles,
inclined armor does not glance flat-fronted projectiles. Its greater
cost, and especially the waste of room it occasions in a ship, are
practically considered in England to be fatal objections. The result of
Mr. Stevens's experiments is, substantially, that a given thickness of
iron, measured on the line of fire, offers about equal resistance to
shot, whether it is vertical or inclined. Flat-fronted or punch shot
will be glanced by armor set at about 12° from the horizon. A hard
surface on the armor increases this effect; and to this end, experiments
with Franklinite are in progress. The inconvenience of inclined armor,
especially in sea-going vessels, although its weight is better situated
than that of vertical armor, is likely to limit its use generally.
_Fastening Armor_. A series of thin plates not only strengthen the whole
vessel, but fasten each other. All methods of giving continuity to thick
plates, such as tonguing and grooving, besides being very costly,
have proved too weak to stand shot, and are generally abandoned. The
_fastenings_ must therefore be stronger, as each plate depends solely on
its own; and the resistance of plates must be decreased, either by more
or larger bolt-holes. The working of the thick plates of the European
vessels Warrior and La Gloire, in a sea-way, is an acknowledged defect.
There are various practicable plans of fastening bolts to the backs of
plates, and of holding plates between angle-irons, to avoid boring
them through. It is believed that plates will ultimately be welded.
Boiler-joints have been welded rapidly and uniformly by means of light
furnaces moving along the joint, blowing a jet of flame upon it, and
closely followed by hammers to close it up. The surfaces do not oxidize
when enveloped in flame, and the weld is likely to be as strong as the
solid plate. Large plates prove stronger than small plates of equally
good material. English 4-1/2-inch armor-plates are generally 3-1/2 feet
wide and to 24 feet long. American 4-1/2-inch plates are from 2 to 3
feet wide and rarely exceed 12 feet in length. Armor composed of light
bars, like that of the Galena, is very defective, as each bar, deriving
little strength from adjacent, offers only the resistance of its own
small section. The cheapness of such armor, however, and the facility
with which it can be attached, may compensate for the greater amount
required, when weight is not objectionable. The 14-inch and 10-inch
targets, constructed, without backing, on this principle, and tested in
England in 1859 and 1860, were little damaged by 68-and 100-pounders.
The necessary thickness of armor is simply a question of powder, and
will be further referred to under the heads of Ordnance and Naval
Architecture.