WHAT ARE THE FORCES WHICH, WHEN HARNESSED, WILL MAKE A BOAT RIGHT HERSELF? by Stuart Welford, MiMechE MRINA Research and Development Officer, RNLITHE RNLI FLEET has included selfrighting lifeboats for well over one hundred years—since 1851 in fact, when the Beeching and Peake boats were first built and their self-righting ability assessed. The test was simple and thorough: 'Capsize the boat and check that she returns to an even keel'. This is something which does not change: today, as in the past, every new selfrighting lifeboat must prove herself in just such a test.

In calm water the self righting principle is fairly simple. It depends on the relative positions and magnitudes of the forces acting through the boat's centre of gravity (G) and her centre of buoyancy (B). The weight exerts a downward force through the centre of gravity, while the water displaced by the hull Fig. 1 Capsized boat, with her weight acting down through the vertical centre of gravity on top of the centre of buoyancy, is in a knife-edge, unstable situation.

As soon as there is any leverage between the boat's weight and buoyant force she will right herself.

exerts an upward force through the centre of buoyancy. The diagrams in Fig. 1 illustrate how the leverage between these forces rights a capsized boat.

In the old pulling and sailing self righters, the main buoyancy for righting was provided by high end boxes, while the vertical centre of gravity was kept fig. 2 Early self righter: high end boxes provided the buoyant force, heavy keel and drop keel the weight, to right the boat.

low with a heavy keel (Fig. 2). However, there was a practical limit to the height of the end boxes: if built too high, they could obstruct vision as well as making it more difficult both to handle the boat and to take off survivors from ships in trouble. This limitation to the end boxes meant that, to ensure self righting, boats had to be kept fairly narrow; their initial stability was thus reduced and they tended to capsize more easily. They became known as self-capsizing 'rolypoly' boats, which explains their loss of popularity from around 1890 onwards.

In fact the percentage of self-righting boats in the lifeboat fleet had risen to a maximum around 1890 but it then fell to a minimum by 1950.

The modern self-righting lifeboat, such as the Arun or Rother, has the same large beam/length ratio as the very popular non-self-righting boats designed during the years between the wars; the buoyant force, in a capsize, is provided by the watertight compartments including engine casing and superstructure.

The first boats to have this sort of righting arrangement had, in addition, a system of water ballast transfer since, at that time (1952), it was not thought wise to rely on too much elevated midships buoyancy. These boats bear the name Oakley after their designer and there are more than 30 of them in service today.

Modern design has, therefore, made it possible for self righting ability to be achieved without loss of initial stability, and the old adage of a self righter being a self capsizer is now a myth.

The mechanism of capsize and self righting in storm conditions is perhaps slightly more difficult for the layman to understand than the simple calm water test situation. It is best explained with the aid of the five diagrams and comments in Fig. 3.

To some, diagram 3d may appear to result in an unstable situation; however, diagram 3e explains how the hull shape provides a restoring force as soon as the boat is heeled. A final word about the centre of gravity being above the centre of buoyancy. It is obvious that a boat with the centre of gravity below the centre of buoyancy will be a stable (but stiff) vessel: for example, a yacht with a heavy deep keel. However, in lifeboats, and indeed most ships and motorboats, the shallow draft requirement and the equipment and machinery weight, and their essential location, determine the perfectly stable relation between the two forces, with the weight acting down on Fig. 3 Capsize and self righting in storm conditions: a. As the peak of a violent, probably breaking wave throws the boat on her beam there can be a time when the buoyancy (B) is moved to the 'wrong' side of the of gravity (G) and the boat capsizes.

b. In the inverted position the boat floating mainly on her superstructure (which is narrow) and the centre of gravity becomes high out of the water: this creates an unstable condition and the boat lolls away from it.

c. Boat righting is due to the couple or lever between the weight and the buoyant force.

d. Boat is now back in a stable position.

e. Boat stable since even a small angle heel produces a righting lever which brings boat back to an even keel.the buoyant force (Fig. 3d and e) A yacht needs her keel to balance the wind in her sails; her propulsion is through a point well up the mast, and with the wind on the beam a capsize could more easily occur without the keel. Motorboats and ships, of course, have their motive power fore and aft and below the waterline, so that they do not need deep keel, and in practice the centre of gravity invariably ends up above the centre of buoyancy.

Since the Fraserburgh disaster the aim of the RNLI has been to provide virtually self righting fleet, if possible by 1980. Already, about 30 of the very popular Watson and Barnett non-self righters have had emergency air bags fitted to their after cabin-tops; should these boats capsize, the bags are automatically inflated and this extra buoyancy initiates righting. In addition to the offshore lifeboats, of course, there are six Atlantic 21 inshore lifeboats in service with a righting potential providedby air bags, and all the new ones being built will also have bags.

The programme of building new selfrighting boats to modern designs is well under way. With the old and the new, nearly 60% of the conventional lifeboats in the RNLI fleet are now capable of self righting. Thus, a good start has been made towards the 1980 target of a selfrighting fleet of lifeboats between 35' and 55' in overall length..