GENERAL PRINCIPLES OF WORKING A SHIP.
Action of the water upon the rudder. Headway. Sternway. Action of the
wind upon the sails. Head-sails. After-sails. Centre of gravity or
rotation. Turning a ship to or from the wind.
A ship is acted upon principally by the rudder and sails. When the
rudder is fore-and-aft, that is, on a line with the keel, the water
runs by it, and it has no effect upon the ship's direction. When it is
changed from a right line to one side or the other, the water strikes
against it, and forces the stern in an opposite direction. For
instance, if the helm is put to the starboard, the rudder is put off
the line of the keel, to port. This sends the stern off to the
starboard, and, of course, the ship turning on her centre of gravity,
her head goes in an opposite direction, to port. If the helm is put to
port, the reverse will follow, and the ship's head will turn off her
course to starboard. Therefore the helm is always put in the opposite
direction from that in which the ship's head is to be moved.
Moving the rudder from a right line has the effect of deadening the
ship's way more or less, according as it is put at a greater or less
angle with the keel. A ship should therefore be so balanced by her
sails that a slight change of her helm may answer the purpose.
If a vessel is going astern, and the rudder is turned off from the line
of the keel, the water, striking against the back of the rudder, pushes
the stern off in the same direction in which the rudder is turned. For
instance, if sternway is on her, and the helm is put to the starboard,
the rudder turns to port, the water forces the stern in the same
direction, and the ship's head goes off to the starboard. Therefore,
when sternway is on a vessel, put the helm in the same direction in
which the head is to be turned.
A current or tide running astern, that is, when the ship's head is
toward it, will have the same effect on the rudder as if the ship were
going ahead; and when it runs forward, it will be the same as though
the ship were going astern.
It will now be well to show how the sails act upon a ship, with
reference to her centre of rotation. Suppose a vessel to be rigged with
three sails, one in the forward part, one at the centre, and the third
at the after part, and her left or larboard side to be presented to the
wind, which we will suppose to be abeam, or at right angles with the
keel. If the head sail only were set, the effect would be that the wind
would send the vessel a little ahead and off to the starboard on her
centre of rotation, so as to bring her stern slowly round to the wind.
If the after sail only were set, the vessel would shoot ahead a little,
her stern would go off to the starboard and her head come up into the
wind. If only the centre sail were set, the effect would be the same as
if all three of the sails were set, and she would go ahead in a
straight line. So far, we have supposed the sails to be set _full_;
that is, with their tacks forward and their sheets aft. If they were
all set _aback_, the vessel would go astern nearly, if the rudder were
kept steady, in a straight line. If the head sail only is set and
aback, she will go astern and round upon her axis, with her head from
the wind, much quicker than if full. So, if the after sail alone were
set and aback, she would go astern, and her head would come suddenly
into the wind.
These principles of the wind acting upon the sails, and the water upon
the rudder, are the foundation of the whole science of working a ship.
In large vessels the sails are numerous, but they may all be reduced to
three classes, viz., head sails, or those which are forward of the
centre of gravity or rotation, having a tendency to send the ship's
head off from the wind; after sails, or those abaft the centre of
rotation, and which send the stern off and the head toward the wind;
and lastly, centre sails, which act equally on each side the centre of
rotation, and do not turn the ship off her course one way or the other.
These classes of sails, if set aback, tend to stop the headway and send
the ship astern, and also to turn her off her course in the same
direction as when set full, but with more rapidity. The further a sail
is from the centre of rotation, the greater is its tendency to send the
ship off from the line of her keel. Accordingly, a jib is the strongest
head sail, and a spanker the strongest after sail.
The centre of rotation is not necessarily at the centre of the ship. On
the contrary, as vessels are now built, it may not be much abaft that
part of the deck to which the main tack is boarded. For the main
breadth, or dead-flat, being there, the greatest cavity will also be
there, and of course the principal weight of the cargo should centre
there, as being the strongest part. Therefore the centre of rotation
will greatly depend upon proper stowage. If the ship is much by the
stern, the centre of rotation will be carried aft, and if by the head,
it will be carried forward. The cause of this is, that when loaded down
by the stern, her after sails have but little effect to move her stern
against the water, and a very slight action upon the forward sails will
send her head off to leeward, as she is there light and high in the
air. Accordingly, to keep her in a straight line, the press of sail is
required to be further aft, or, in other words, the centre of rotation
is further aft. If a ship is loaded down by the head, the opposite
results follow, and more head and less after sail is necessary.
A ship should be so stowed, and have her sails so trimmed, that she may
be balanced as much as possible, and not be obliged to carry her helm
much off the line of her keel, which tends to deaden her way. If a ship
is stowed in her best sailing trim, and it is found, when on a wind,
that her head tends to windward, obliging her to carry a strong weather
helm, it may be remedied by taking in some after sail, or adding head
sail. So, if she carries a lee helm, that is, if her head tends to fly
off from the wind, it is remedied by taking in head or adding after
sail. Sometimes a ship is made to carry a weather helm by having too
much head sail set aloft. For, if she lies much over on a wind, the
square sails forward have a tendency to press her downwards and raise
her proportionally abaft, so that she meets great resistance from the
water to leeward under her bows, while her stern, being light, is
easily carried off; which, of course, requires her to carry a weather
helm.
The general rules, then, for turning a ship, are these: to bring her
head to the wind,--put the helm to leeward, and bring the wind to act
as much as possible on the after sails, and as little as possible on
the head sails. This may be done without taking in any sail, by letting
go the head sheets, so that those sails may lose their wind, and by
pointing the head yards to the wind, so as to keep the head sails
shaking. At the same time keep the after sails full, and flatten in the
spanker sheet; or, if this is not sufficient, the after sails may be
braced aback, which will send the stern off and the head to windward.
But as this makes back sails of them, and tends to send the vessel
astern, there should be either head or centre sails enough filled to
counteract this and keep headway upon her. On the other hand, to turn
the head off from the wind, put the helm to windward, shiver the after
sails, and flatten in the head sheets. Brace the head yards aback if
necessary, being careful not to let her lose headway if it can be
avoided.
The vessel may be assisted very much in going off or coming to, by
setting or taking in the jib and spanker; which, if the latter is
fitted with brails, are easily handled.