142 
SHIP-BUILDING. 
an angle of 45 degrees with the keel, it may be readily seen, 
by the water-lines abaft in the half-breadth plan, that the 
immediate shock it receives from the water increases as it 
approaches to the load-water-line, where they become nearly 
at right angles with the side of the rudder in that position, 
and this holds good, whatever angle the rudder makes with 
the keel; hence some are of opinion, that the rudder should 
be made broader near the line of floatation, and narrower 
towards the keel; but the present mefhod of making the 
rudder with increasing breadth downwards is only in pro- 
• portion to the obliquity of impulse the water acts*against it 
near the keel. It must be observed, that the above force 
strikes the rudder obliquely, and only strikes it with that 
part of its motion which, according to the sine of incidence, 
forces it in a contrary direction, with a momentum which 
not only depends on the velocity of the ship’s course, by 
which this current of water is produced, but also upon the 
extent of the sine of incidence. This force is by conse- 
• quence composed of the square of the velocity with which 
the ship advances, and that square of the sine of incidence, 
which will necessarily be greater or smaller according to 
circumstances; so that if the vessel increases her velocity 
three or four times faster, the absolute shock of the water 
upon the rudder will be nine or sixteen times stronger, under 
the same incidence; and if the incidence is increased, it will 
yet be augmented in a greater proportion, because the square 
of the sine of incidence is more enlarged. 
Many useful discoveries may be made by blocks or models 
of ships, and with as great certainty as by the nicest calcu¬ 
lations. For it must be allowed, as before observed, that in 
calculating from a draught drawn from a quarter of an inch 
scale, it will be liable to some inaccuracies, which cannot 
be obviated in practice, by reason of various little altera¬ 
tions which may be made in laying off the ship in the 
mould-loft; consequently the draught and the ship will, in 
those points, disagree. And likewise, upon strict exami¬ 
nation, we shall be enabled to find, that there are not many 
ships that have both their sides exactly equal in every re¬ 
spect. 
Let the block, or model, be constructed to a scale of one- 
quarter of an inch to a foot of the corresponding parts on 
the ship; and care should be taken to provide the wood as 
light and dry as possible. 
The model being accurately constructed, it may be also 
proved by suspending it by a line, fastened to a hook in any 
part of a straight line, drawn from the middle line of the 
stem to that of the stern-post. This hook may be moved 
forward and aft to different places in the middle line, and a 
weight may be suspended from the upper part of the middle 
line, on the post. If the two sides be exactly of equal di¬ 
mensions, and homegene, they will then be of equal weight. 
A plane passing through these three lines, whatever part of 
the middle line the hook be in, will likewise pass through 
the middle line of the keel, stem, and post: therefore, if 
the model stands this proof, it will be as true to work from as 
the nicest calculations. 
The model, having stood this test, may be suspended by 
the same line, or silk, in different positions, until it points 
out the centre of gravity; which will be found, when the 
block hangs in a state of equilibrium. This practice is, 
doubtless, very simple; but it will be found very convenient. 
Further, the model being suspended by the hook, the lines 
hanging at the stem and post corresponding to their middle 
lines, and to that which suspends the block, we may hold a 
batten out of winding with the line that suspends it, and, 
with a pencil, draw a line upon it. A plane passing through 
this pencil line, at right angles to the keel, and passing like¬ 
wise through the line that suspends the block, will likewise 
pass through the centre of gravity, which, therefore, must 
be somewhere in this plane. Again, move the hook to some 
other part of the middle line, and let the block be suspended 
from that point; draw also another pencil line, out of wind¬ 
ing with this last line of suspension, and the intersection of 
the two lines will give the height of the centre of gravity 
above the keel, and likewise its distance from the post and 
stem; and if the hook be moved to any other part of the 
middle line, and a pencil line be drawn as before, it will 
likewise intersect in the same point; or, let there be ever so 
many points assumed in the middle line, and the block sus¬ 
pended by each, and pencil lines drawn, they will all inter¬ 
sect in the same point; and as the centre of gravity will 
always be in the plane which passes through the middle line 
of the keel, stem, and post, it may with certainty be marked 
on the draught. 
Of the Timbers of a Ship .—Of all large machines des¬ 
tined to undergo severe shocks, a ship is perhaps the least 
skilfully and artificially contrived. Her several parts are put 
together on a principle so much opposed to that which 
constitutes strength, that if a ship, on the old construction, 
should be put upon wheels, and drawn over a rough pave¬ 
ment, the action of a day would shake her in pieces; but 
being destined to move in an element that closes upon her, 
and presses her equally on all sides, she is prevented from 
falling in pieces outwards, and her beams and decks pre¬ 
serve her from tumbling inwards. Whoever has observed a 
ship in frame, as it is called, on the stocks, that is, with 
only her timbers erected, must forcibly be reminded of the 
skeleton of some large quadruped, as of a horse or ox, 
laid on its back; the keel resembling the back-bone, and 
the curved timbers the ribs, which is, in fact, the name by 
which they sometimes go. These ribs, issuing at right 
angles from the keel, consist, in a 74-gun ship, of about 
800 different pieces, the space between each rib seldom ex¬ 
ceeding five inches. These ribs are covered with planks 
of different thickness within and without, also at right 
angles to the ribs, and fixed to them by means of wooden 
pins or tree-nails. In the inside three or four tier of beams 
cross the skeleton from side to side, at right angles to both 
planks and ribs. These beams support the decks. At right 
angles to the beams are pieces of wood called carlings, and 
at right angles to these other pieces called ledges, and upon 
these the planks of the deck are laid in a direction of right 
angles to the beams, and parallel to the planking of the 
sides. From this sketch it will be perceived, that all the 
parts of a ship are either parallel or at right angles to each 
other. The ribs form a right angle with the keel, the planks 
inside and out are at right angles to the ribs, the beams at 
right angles to these, the carlings to the beams, the ledges to 
the carlings, and the planks of the decks to the ledges, the 
beams, and the ribs. 
Now', it is well known to every common carpenler that 
this disposition of materials is the weakest that can be 
adopted—consequently, when a ship of the old construc¬ 
tion is .first launched into the water, it is invariably found 
that the two extremities, being less water-borne than the 
middle, drop, and give to the ship a convex curvature up¬ 
wards, an effect which, from its resemblance to the shape of 
a hog’s back, is usually called hogging. In very weak or 
old ships this effect may be discovered in all the port-holes 
of the upper-deck, by their having taken the shape of lo¬ 
zenges declining different w r ays from the centre of the ship 
to each extremity. 
We are indebted to Sir Robert Seppings for remedying 
this defect. He applied the principle so familiar to carpen¬ 
ters, that of placing a diagonal brace across all the right 
angles formed by the timbers, and thus rendered them im¬ 
movable. The expense is great, but it amply repays itself 
in the greater security and durability of the vessel. Sir 
Robert, in order to give a continuity of strength to the whole 
machine, and leave no possible room for play, filled the 
spaces between the frames with old seasoned timber cut into 
the shape of wedges; but recently with a prepared cement, 
thus rendering the lower part of the ship or floor one solid 
complete mass, possessing the strength and firmness of a 
rock. 
The same principle of trussing is carried from the gun- 
deck upwards, from whence, between every port, is intro¬ 
duced a diagonal brace, which completely prevents the ten¬ 
dency 
