The Liverpool Standard and General Commercial Advertiser.
Liverpool, Merseyside, England, 1844-11-19 00:00:00

CORRESPONDENCE.

SHIP-BUILDING.

TO THE EDITOR OF THE LIVERPOOL STANDARD.
SlR,—Having in my last shown the importance of the
centre of gravity, with the responsibility attached to ship-
builders, in the correct or incorrect placement of its cen-
tre; if correct, its influence for good is diffused throughout
every part of the vessel, arid the reverse if incorrectly
placed, the truth of which must be proved by its forming
a close compact with the centre of displacement. To this
union theorists of the present day have made a bold stand,
by declaring that rather than submit to its laws they will
overturn the course of nature, and from it produce a new
system of shipbuilding, that shall overturn every-objec-
tion. The theory on which they have attempted to esta-
blish their hypothesis is by affirming that at one-third of
the vessel's length from forward, is the best position for
dead flat. On this principle many ships have been built,
both in the government and merchant's yards, and with
each it has proved a failure. Disappointed in their hopes,
and baffled in their expectations, yet being wedded to their
false principles, recourse has been had to filling out the
lower after water lines, whereby the centre of gravity falls
the aft side of the vessel's half-length, and the displace.
ment again shows a failure ; and to add to their consterna-
tion, those ships not only load by the head, but labour
greatly in a sea, and steer with difficulty. When about to
give up their airy phantom, an expedient presented itself,
that of giving their ship's upright stems, thereby enabling
them to add from six to twelve feet length of keel forward
(keeping the head of the stem as before). Through this
artifice the centres of gravity and displacement are brought
into closer connexion ; from this extension of the keel, the
centre of the lower water lines (which proved their
greatest obstacle) is now brought nearer the half-length
of the vessel. Having by this subterfuge delivered them-
selves from a dilemma, those men, without giving the real
cause for such a novelty, not only take credit for the dis-
covery, but strangely enforce the expediency of upright
stems, without showing one benefit accruing therefrom :
all experienced shipbuilders well know that the form of
the stem of a vessel is regulated by the fair termination, of
the bow. Feeling the importance, and with it the neces-
sity of again stating, that on the commencement and ter-
mination of the bow, as a first principle, in the construc-
tion of all vessels, depends, not only their sailing qualities,
with the correct placement of their cargoes, but that which
is of greater importance, the safety of the ship, and with,
it the lives of thb crew, I proceed to give a mechanical
method for finding the centre of gravity of each water
line, likewise of the whole structure. Haring constructed
your vessel on paper, on, a quarter scale, run off on each
water line in the half-breadth plan the thickness of the
plank of the bottom, prepare yourself with a dry piece
of American fir board, which plane up to the thickness of
the space between the water lines, as shown in the sheer
plan; cut as many pieces from it as there are water lines,
of the length, from the fore part of the rabbet of the stem
to the after part of the rabbet of the stern post, their
breadth the half-breadth of the drawing. Observe, the
, lower piece must taper agreeably to the vessel's set by the
stern ; this piece must fill up,. in the sheer plan, all the
space from the lower edge of the rabbet of the keel, round
to the fore part of the rabbet of the stem, and after part of
the rabbet of the stern post, up to the height of the first
water line : in like manner prepare each following piece.
When coarplete, place them together on the sheer plan,
and see that they fill up the whole space, from the rabbet
of the keel to the load water line, taking care tliafthe edge
that forms the middle line is straight and square, that
when placed together they may all form one fair surface ;
next place each piece separately on their respective water
lines in the sheer plan, and mark on the upper surface all
the stations fore and aft, which square across ; race the
line of dead flat down the edge of each piece, as a guide to
keep them well in place, when applied' to each other.
Commence with the lower piece, by setting thereon the
half-breadth of the water line on each station, taking it
from the line of the plank of the bottom e through those
spots run in the curve of the lower water line, with a sharp
firmer; trim into this line very correct, and.rough away
the part below to the rabbet, on which edge guage in the
half-siding of the keel, stem, and stern-post. In like
manner set off the half-breadth of the second piece, apply-
ing the lower piece to the under part, marking-it round
with a pencil, whereby you have the upper and lower edge
of the second piece-. By nicely trimming round the upper
edge, you prepare it for the lower edge of the third piece,
which continue till each separate part is prepared ; then
with a few brads fasten them all together (so as to sepa-
rate them without injury to the edges); clean, them off
very exact and fair e when performed separate them, and
apply each piece to its respective half-breadth, from the
middle line of the half-breadth plan of the drawing, to
prove them ; if welLeagain secure them together,.and fas-
ten thereto the half-siding of the keel, stem and stern-post,
and again separate the block, with a pin applied at each
edge of the lower piece, at the line of dead flat, when sup-
ported thereby, and the plane of the surface shows hori-
zontal.; or if the after part should drop a little,occasioned
by the set by the stern, you may then considerthe lower
water line as well, and the centre of gravity at dead flat ;
but should the fore body preponderate, in that case the
fore body is too full, and the drawing on that line must
be taken in. Fix the pins in the second piece six inches
the aft side of dead fiat; if the plane rest horiaental that
water line is well, but if either end drop, the line on that
part must be taken in ;• from which proceed, by applying
the pins to each separate piece, increasing six inches aft
on each water line, (so that the centre of gravity of the
upper water line, for a slip of 260 tons measurement, with
one foot set by the stern, may be two feet the aft side of
dead flat,) and as either end preponderates, alter the line.
Observe, it may require to fill out the lean parts, instead
of reducing the full ;. to this great attention must be paid;
as such the alteration must first be made on the drawing,
and the lines fared, in the body and half-breadth plans.
Should any of the lines require filling out, and the model
prove slack, glue on a slip at the middle line, wad again set
off the half-breadth, till it correspond with the drawing.
To find the main centre of gravity, the drawing having five
water lines, place the two upper pines in one scale, and
the three lower ones in the other. It is probable their
weight may be equal ; if so, the main centre of gravity
will be on the third water line, and one foot.the aft side of.
dead flat. Should the two upper pieces be the greatest
weight, in that case the main centre of gravity would be a
little above and aft the third water line ; if the three lower
pieces are the heaviest, the main centre of gravity will fall
a little below the third line, and a little forward ; in either
case it is of little moment. This method-of finding the
centre of gravity, if performed correctly, wall prove equal
with the nicest calculations. Having thus produced the
exactmodel, a displacement myplac nextement snhfatihl
ebeshsihnonwtnanfryolminnitnfhoiniwlntoergseintnt.he
Be pleased in your next to correct the following errors,
which appear in your paper of the sth instant :—for the
" stern in the main direction," &c., read stem ; for " the
deepest part of the ship," read dryest ; for " being one-
third from the stern," read stem ; for " let them be equal,
move," &c., read, " let them be equal as to weight,
move, 8(..c.-1 am, sir, &c.,
JOHN COCK.
Pembs'oke Dock, 11th Nov. 1814,