w v
P = ------
d² 576
[TEX: P = \frac{w \ v}{d^2 \ 576}]
for plates less than two and one-half inches thick, and
w v
P = ------ - 1.5
d² 400
[TEX: P = \frac{w \ v}{d^2 \ 400} - 1.5]
for plates more than two and one-half inches thick.
If [theta] be the angle between the path of the projectile and the
face of the plate, then v in the above formulæ becomes v sin [theta].
When we come to back the plates, their power to resist penetration
becomes greater, and our formula changes. The Gavre formula, given
above, is used to determine the velocity necessary for a projectile to
pass entirely through an iron plate and its wood backing.
Compound and steel armor are said to give about 29 per cent. more
resisting power than wrought iron, but in one experiment at the
proving ground, at Annapolis, a compound plate gave over 50 per cent.
more resisting power than wrought iron.
The Italian government, after most expensive and elaborate comparative
tests, has decided in favor of the Creusot or Schneider all-steel
plates, and has established a plant for their manufacture at Terni,
near Rome.
The French use both steel and compound plates; the Russians, compound;
the Germans, compound; the Swedes and Danes use both. Spain has
adopted and accepted the Creusot plate for its new formidable armored
vessel, the Pelayo; and China too has recently become a purchaser of
Creusot plates.
Certain general rules may be laid down for attacking armor. If the
armor is iron, it is useless to attack with projectiles having less
than 1,000 feet striking velocity for each caliber in thickness of
plate. It is unadvisable to fire steel or chilled iron filled shells
at thick armor, unless a normal hit can be made. When perforation is
to be attempted, steel-forged armor-piercing shells, unfilled, should
be used. They may be filled if the guns are of great power as compared
to the armor. Steel and compound armor are not likely to be pierced by
a single blow, but continued hammering may break up the plate, and
that with comparatively low-powered guns.
Wrought iron must be perforated, and hard armor, compound or steel,
must be broken up. Against wrought iron plates the projectile may be
made of chilled cast iron, but hard armor exacts for its penetration
or destruction the use of steel, forged and tempered. Against
unarmored ships, and against unarmored portions of ironclads, the
value of rapid-firing guns, especially those of large caliber, can
hardly be overestimated.
The relative value of steel and compound armor is much debated, and at
present the rivalry is great, but the weight of evidence and opinion
seems to favor the all-steel plate. The hard face of a compound plate
is supposed to break up the projectile, that is, make the projectile
expend its energy on itself rather than upon the plate, and the
backing of wrought iron is, by its greater ductility, to prevent the
destruction of the plate. It seems probable that these two systems
will approach each other as the development goes on. An alloy of
nickel and steel is now attracting attention and bids fair to give
very good results.
The problem to be solved, as far as naval armor is concerned, is to
get the greatest amount of protection with the least possible weight
and volume, and this reduction of weight and volume must be
accomplished, in the main, by reducing the thickness of the plates by
increasing the resisting power of the material. In the compound plate
great surface hardness is readily and safely attained, but it has not
yet been definitely determined what the proper proportionate thickness
of iron and steel is.
A considerable thickness of steel is necessary to aid, by its
stiffness, in preventing the very ductile iron from giving back to
such an extent as to distort the steel face and thus tear or separate
the parts of the plate. The ductile iron gives a very low resisting
power, its duty being to hold the steel face up to its work. If now we
substitute a soft steel plate in the place of the ductile iron, we
will get greater resisting power, but our compound plate then becomes
virtually an all-steel one, only differing in process of manufacture.
The greatest faults of the compound plate are the imperfect welding of
the parts and the lack of solidity of the iron. When fired at, the
surface has a tendency to chip.
In the all-steel plate we have the greatest resisting power
throughout, but there are manufacturing difficulties, and surface
hardness equal to that of the compound plate has not been obtained.
The manufacturing difficulties are being gradually overcome, and
artillerists are in high hopes that the requisite surface hardness
will soon be obtained.
The following may be stated as well proved: