HELM

The helm is usually composed of three parts, viz. the rudder, the tiller, and the wheel, except in small vessels, where the wheel is unnecessary.

The length and breadth of the rudder are represented in plate VIII. where it is evident that it becomes gradually broader in proportion to it's distance from the top, or to it's depth under the water. The back, or inner part of it, which joins to the stern-post, is diminished into the form of a wedge throughout it's whole length, so as that the rudder may be more easily turned from one side to the other, where it makes an obtuse angle with the keel. The hinges upon which it is supported are also expressed in this figure. Those which are bolted round the stern-post to the after extremity of the ship, are called googings, and are furnished with a large hole on the after-part of the stern-post. The other parts of the hinges, which are bolted to the back of the rudder, are called pintles, being strong cylindrical pins, which enter into the googings, and rest upon them. The length and thickness of the rudder is nearly equal to that of the stern-post, as represented in fig. I. plate X.

Plates VIII and X

The rudder is turned upon it's hinges by means of a long bar of timber, called the tiller, which is fixed horizontally in it's upper end within the vessel. The movements of the tiller to the right and lest, accordingly, direct the efforts of the rudder to the government of the ship's course as she advances, which, in the sea-language, is called steering. The operations of the tiller are guided and assisted by a sort of tackle, commnicating with the ship's side, called the tiller-rope, which is usually composed of untarred rope-yarns, for the purpose of traversing more readily through the blocks or pullies.

In order to facilitate the management of the helm, the tiller-rope, in all large vessels, is wound about a wheel, which acts upon it with the powers of a crane or windlass. The rope employed in this service being conveyed from the fore-end of the tiller k, to a single block i, on each side of the ship, (plate III. DECK) is further communicated to the wheel, by means of two blocks, suspended near the mizen-mast, and two holes immediately above, leading up to the wheel, which is fixed upon an axis, on the quarter-deck, almost perpendicularly over the fore end of the tiller. Five turns of the tiller-rope are usually wound about the barrel of the wheel, and, when the helm is amidship, the middle turn is nailed to the top of the barrel, with a mark by which the helmsman readily discovers the situation of the helm, as the wheel turns it from the starboard to the larboard side. The spokes of the wheel generally reach about eight inches beyond the rim or circumference, serving as handles to the person who steers the vessel. As the effect of a lever increases in proportion to the length of it's arm, it is evident that the power of the helmsman, to turn the wheel, will be increased according to the length of the spokes, beyond the circumference or the barrel.

Plate III

When the helm, instead of lying in a right line with the keel, is turned to one side or the other, as in B D, fig. I. plate V. it receives an immediate shock from the water, which glides along the ship's bottom in running aft from A to B: and this fluid pushes it towards the opposite side, whilst it is retained in this position: so that the stem, to which the rudder is confined, receives the same impression, and accordingly turns from B to b about some point c, whilst the head of the ship passes from A to a. It must be observed, that the current of water falls upon the rudder obliquely, and only strikes it with that part of it's motion which acts according to the fine of incidence, pushing it in the direction N P, with a force 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 fine of incidence. This force is by consequence composed of the square of the velocity with which the ship advances, and the square of the fine of incidence, which will necessarily be greater or smaller according to circumstances; so that if the vessel runs three or four times more swiftly, 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 fine of incidence is more enlarged. This impression, or, what is the same thing, the power of the helm, is always very feeble, when compared with the weight of the vessel; but as it operates with the force of a long lever, it's efforts to turn the ship are extremely advantageous. For the helm being applied to a great distance from the center of gravity, G, or from the point about which the vessel turns horizontally, if the direction P N of the impreflion of the water upon the rudder be prolonged, it is evident that it will pass perpendicularly to R, widely distant from the center of gravity G: thus the absolute effort of the water is very powerful. It is not therefore surprizing that this machine impresses the ship with a considerable circular movement, by pushing the ster from B to b, and the head from A to a; and even much further, whilst she sails with rapidity: because the effect of the helm always keeps pace with the velocity with which the vessel advances (Bourdé, Manoeuvrier).

Plate V

Reference Works