Encyclopaedia Britannica, or, a Dictionary of arts, sciences, and miscellaneous literature : enlarged and improved. Illustrated with nearly six hundred engravings > Volume 19, Scripture-SUG

S H I P - B U I L D I N C.
«4
[iecaufe
a (hip’s
Hsgtog.
|d fag.
:®jrts of AB (fig. 50.), which being a£ted upon by the forces
ie Water A C c, D r/, B b, may be maintained in equilibrio,
VehVl11 ProV^e^ ^ ^as a bitficient degree of ftiifnefs : but as
—_j foon as it begins to givo way, it is evident it mufl bend
in a convex manner, fince its middle would obey the
forces C c and D d, while its extremities would be ac¬
tually drawn downwards by the forces A a and B b.
The veflel is generally found in fuch a fituation j and
fince fimilar efforts continually aft whilft the veffel is
immerfed in the water, it happens but too often that
the keel experiences the bad effeft of a ftrain. It is
therefore very important to inquire into the true caufe
of this accident.
For this purpofe, let us conceive the velfel to be di¬
vided into two parts by a tranfverfe feftion through
the vertical axis of the veffel, in which both the centre
Plate of gravity G (fig. 51.) of the whole veflel and that of
•cccxcv. the immerfed part are fituated: fo that one of them
HS' 51* will reprefent the head part, and the other that of the
ftern, each of which will be confidered feparately. Let
g be the centre of gravity of the entire weight of the
firfi:, and 0 that of the immerfed part correfponding.
In like manner, let y be the centre of gravity of the
whole after part, and w that of its immediate portion.
Now it is plain, that the head will be afted upon by
the two forces g m and 0 //, of which the firft will prefs
it down, and the latter pufh it up. In the fame man¬
ner, the Hern will be prefled down by the force y y.. and
pufhed by the force a v. But thefe four forces will
maintain themfelves in equilibrium, as well as the total
forces reunited in the points G and O, which are equi¬
valent to them 5 but whilft neither the forces before
nor thofe behind fall in the fame direftion, the veflel
will evidently fuftain efforts tending to bend the keel
upwards, if the two points 0 u are nearer the middle
than the two other forces g m and y y. A contrary
effeft would happen if the points 0 and u were more dis¬
tant from the middle than the points g and y.
But the firft of thefe two caufes ufually takes place
almoft in all veflels, fince they have a greater breadth
towards the middle, and become more and more narrow
towards the extremities j whilft the weight of the veffcl
is in proportion much more confiderable towards the
extremities than at the middle. From whence we fee,
that the greater this difference becomes, the more alfo
will the veffel be fubjeft to the forces which tend to
bend its keel upwards. It is therefore from thence
that we muft judge how much ftrength it isneceffary to
give to this part of the veffel, in order to avoid fuch a
confequence.
If other circumftances would permit either to load the
veflel more in the middle, or to give to the part immer¬
fed a greater capacity towards the head and ftern, fuch
an effeft would no longer be apprehended. But the
deftination of moft veffels is entirely oppofite to fuch an
arrangement: by which means we are obliged to {Length¬
en the kneel as much as may be neceffary, in order to
avoid fuch a difafter.
We fhall conclude this chapter with the following
praftical obfervations on the hogging and fagging of
{hips by Mr Hutchinfon of Liverpool :
“ When fhips with long floors happen to be laid a-
dry upon mud or fand, which makes a folid refiftance
againft the long ftraight floors amidfhips, in comparifon
with the two {harp ends, the entrance and run meet with
■ \iiual
' man-
J t, P-13
little fupport, but are prefled down lower than the flat
of the floor, and in proportion hogs the ftiip amid¬
fhips j which is too well known from experience to oc-
cafion many total Ioffes, or do fo much damage by
hogging them, as to require a vaft deal of trouble and
expence to fave and repair them, fo as to get the hog
taken out and brought to their proper fheer again : and
to do this the more effeftually, the owners have often
been induced to go to the expence of lengthening them j
and by the common method, in proportion as they add
to the burden of thefe fhips, by lengthening their too
long ftraight floors in their main bodies amidftiips, fo
much do they add to their general weaknefs to bear
hardfhips either on the ground or afloat j for the fcant-
ling of their old timber and plank is not proportionable
to bear the additional burden that is added to them.
“ But defefts of this kind are belt proved from real
and inconteftable fafts in common praftice. At the
very time I was writing upon this fubjeft, I wras called
upon for my advice by the commander of one of thofe
ftrong, long, ftraight floored Ihips, who was in much
trouble and diftraftion of mind for the damage his ftiip
had taken by the pilot laying her on a hard, gentle Hop¬
ing fand, at the outfide of our docks at Liverpool where
it is common for Ihips that will take the ground to lie
for a tide, when It proves too late to get into our wret
docks. After Recommending a proper {hip carpenter, I
went to the ftiip, which lay with only a fmall keel, yet
was greatly hogged, and the butts of her upper works
{trained greatly on the lee fide ; and the feams of her
bottom, at the lower futtock heads, vaftly opened on the
weather fide : all which {trained parts were agreed upon
not to be caulked, but filled with tallow, putty, or clay,
&c. with raw bullocks hides, or canvas, nailed with bat-
tons on her bottom, which prevented her finking ivith
the flow of the tide, without hindering the prelfure of
water from righting and doling the feams again as ftie
floated, fo as to enable them to keep her free with
pumping. This veflel, like many other inftances of
fliips of this conftruftion that I have known, was faved
and repaired at a very great expence in our dry repair¬
ing docks. And that their bottoms not only hog up¬
wards, but fag (or curve) downwards, to dangerous and
fatal degrees, according to the ftrain or preflure that
prevails upon them, will be proved from the following
fafts:
“ It has been long known from experience, that when
fhips load deep with very heavy cargoes or materials
that are flowed too low, it makes them fo very labour-
fome at fea, when the waves run high, as to roll away
their mails ; and after that misfortune caufes them to
labour and roll the more, fo as to endanger their work¬
ing and ftraining themfdves to pieces: to prevent
which, it has been long a common praftice to leave
a great part of their fore and after holds empty, and to
flow them as high as poflible in the main body at mid-
fhips, which caufes the bottoms of thefe long ftraight-
floored ftiips to fag downwards, in proportion as the
weight of the cargo flowed there exceeds the preffure of *
the water upwards, fo much as to make them danger^
oufly and fatally leaky.
“ I have known many inftances of thofe ftrong ftiips
of 500 or 600 tons burdens built with long ftraight
floors, on the eaft coaft of England, for the coal and
timber trade, come loaded with timber from the Baltic
to
295
Efforts of
the Water
to bend a
Veffel.