The Seaman's Friend / Containing a treatise on practical seamanship, with plates, a dictionary of sea terms, customs and usages of the merchant service

by Richard Henry Dana

CHAPTER X.

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.