IN this article it is proposed to explain the nature and action of stability as con- cisely as the subject permits, without introducing technical terms, beyond those essential to definition, or attempting to go beyond the purely mechanical, or what is understood as unscientific, demonstra- tion of the matter in hand.

Stability is a quality possessed in some degree by all boats and vessels, and is absolutely essential to their safety and utility; a boat, as we are mainly interested in boats, is said to be " wanting in sta- bility" relatively only to the amount desirable, or possessed by other boats of similar form.

The Life-boats of the Institution are constructed to exhibit an unusual amount of this quality. The term stability, as generally used, is taken to refer to lateral stability only. Longitudinal stability, which is governed by the same principles, is referred to later on.

Stability, then, may be defined as the endeavour made by boats to regain an upright position when thrown out of this position by any extraneous forces, such as the wind or sea. In order to account for this endeavour, and to measure its force under the different conditions through which the boat passes when ex- posed to wind or sea, it is necessary to define and locate two points.

Firstly, the Centre of Gravity of the boat; secondly, the Centre of Gravity of displacement or, as it is more frequently called, the Centre of Buoyancy.

The Centre of Gravity of the boat is the common centre of the weight of the boat and of all its contents; the point at which it would balance if hung in air.

The Centre of Buoyancy is the corre- sponding point in the body of water dis- placed by the boat.

The boat, when floating, displaces a mass of water equal in weight to that of the boat and its contents, and is subject on every portion of its immersed surface to pressure from the water so displaced acting in various directions according to the shape of the immersed surface. The sum of this pressure may be taken as acting upwards in a direct vertical line passing through the Centre of Buoyancy.

The weight of the boat and its contents is acting downwards in. a direct vertical line passing through the Centre of Gravity of the boat.

These forces are equal, and when the boat is upright, and at rest, fall in one and the same vertical line, and exactly counterbalance each other, as in Fig, 1.

Fig. 1.

W.L Here G represents the supposed position of centre of gravity, B the supposed posi- tion of centre of buoyancy, and W L the water-line. The positions are selected with a view to illustration, without re- ference to the actual relative distances that they would occupy in an Institution Life-boat.

In Fig. 2 the boat is represented as heeling to an angle of 15°, and, while immersing a portion of one side, has lifted out of the water a part of the other. It is evident that the immersed portion has altered its shape, and that the centre of buoyancy has moved out to accommodate itself to the changed displacement, and may now be taken as at B.

The forces remain equal, but are now acting at different points and in contrary directions. Joining G with the vertical passing through B, there appears a lever,or righting couple. At one end a force equal to the weight pf the boat pressing vertically downward, at the other an equal Fig. 2.

In the Life-boats of the Institution this condition exists at all angles of heel, and indeed stretches itself into a self-righting property; but it is not now proposed to follow our boat round beyond her beam ends.

In Fig. 4 we have a section of a Life- boat on its beam ends and, owing to the Fig. 4.

force pushing vertically upwards. These forces are obviously assisting each other to raise the boat to an upright position.

In Fig. 3 the boat has still further buried her side, heeling over to an angle of 30°. The centre of buoyancy has again moved outwards, lengthening the righting Fig. 3.

W.L.

extreme lowness of its centre of gravity, still exhibiting a righting couple.

In Fig. 5, with a higher centre of gravity and deeper body, the centre of buoyancy Fig. 5.

lever.and increasingthe effort to straighten up. Thus the length of the righting couple becomes a measure of the boat's stability, as the righting force at work equals the weight of the boat multiplied by the length of the lever.

In Figs. 2 and 3 the boat is stable or possesses stability, because, on removing the pressure of wind or lift of sea, she will fly back to an upright position and there remain.

has swung into the same vertical as the centre of gravity, and the boat is in equilibrium as in Fig. 1.

If, as in Fig. 6, the centre of buoyancy swings on and passes from lee to weather side of centre of gravity, a lever is instantly developed and becomes an up- setting couple, when nothing can save her.

Pig. 6.

On further reference to these figures, it will be observed that a vertical drawn through the centre of buoyancy cuts the vertical drawn through the centre of gravity at various distances from the latter. This point of intersection is called the meta-centre.

When it falls above the centre of gravity as in Figs. 2, 3, and 4, the boat is stable; when below, as in Fig. 5, unstable. Life-boats are necessarily of full section and light draught, having to retain their carrying power without much increase of draught, and to enter the water from flat beaches. They are consequently heavily ballasted at the lowest obtainable stowage, carrying from one-fifth to one-sixth of their entire weight in iron on the keel. They are extremely lively, springing back from the blow of a sea with surprising quickness. This sprightliness, though puzzling to un- trained oarsmen, is a great element of safety; the boat is seldom caught at a disadvantage by a second sea, and if upset is usually knocked over at a blow, and not by the cumulative mischief of succes- sive impulses. It has been assumed that G has throughout remained in the same position, that is, in the centre vertical.

In Life-boats and in yachts carrying lead ballast outboard this is practically the case. In open boats, however, with shift- ing ballast, or where the crew tumbling to leeward materially affect the position of G, a new danger arises, which will be readily explained by reference to Fig. 3, 6 representing the centre of gravity after shifting to leeward of weights and crew.

No righting couple is developed, and the boat will be instantly pressed further down, and so on with each shift to lee- ward of G.

On the other hand, the judicious hand- ling of shifting ballast is obviously a security. By moving G to windward the righting couple is proportionately lengthened.

It may be concluded, therefore, that lateral stability is increased by lowering the centre of gravity.

Initial stability is due to beam, and increases with it.

Longitudinal stability is the resistance shown by the boat to the immersion of her bow or stern. In this case the centre of buoyancy has moved fore or aft of the centre of gravity, and a longitudinal or tipping couple has been created.