The ability of a modern lifeboat to self-right after a capsize is a valuable safety feature, but just how is this achieved? Mike Floyd sets out to explain the principles behind a modern self-righting lifeboat without recourse to diagrams or too much technical language… Standing on a storm-lashed shoreline it is possible to begin to appreciate the immense power of large waves.

These are waves which can batter a concrete pier into oblivion, smash brick buildings and erode a granite cliff - yet a lifeboat must be designed to go to sea in them. No vessel can simply resist such forces, she must live with them, rolling with the punches and accepting that the time may come when she is simply overwhelmed by the might of the sea. The trick is to ensure that a lifeboat survives when she is overwhelmed, returning to the fray in a fit state to carry on.

Strength and reliability are part of the equation, but the ability to come out fighting relies on being upright, an aspect of design which is very special to lifeboats and which is not as simple as it might seem.

A boat floating quietly in still water may look tranquil, but in fact she is carrying out a complex balancing act. Her weight, acting downwards, is being balanced exactly by another force pushing upwards — a force which prevents her from sinking further into the water and acts to keep her upright and floating level foreand- aft.

Eureka! It was Archimedes who first postulated that a floating object displaces its own weight of water (hence the term 'displacement' when describing the weight of a boat) and it is this displaced water trying to return to its previous position which provides the upward force, known as 'buoyancy'.

Just as weight can be 'averaged' and assumed to act through a single point called the 'centre of gravity', so buoyancy - essentially the immersed volume of the boat — can be 'averaged' in the same way and be thought of as acting through another single point called the 'centre of buoyancy'. It is the relative position of these two points which determines whether a boat will float level and upright, and how much force will be required to heel her over, or capsize her.

Although the centre of gravity of the boat is fixed once she is built (determined among other things by her construction and the weight and position of engines and equipment) the position of the point through which buoyancy acts is constantly on the move when different parts of the hull are immersed by heeling or wave action.

The total volume of boat in the water will remain roughly constant, in order to provide a balancing 'returning push', but the ever-shifting relationship between the two centres - one fixed, one moving - affects the way in which a boat behaves in waves, and also determines whether she will be self-righting.

The relationship is extremely complex, but in essence the centre of gravity will always try to tip the boat one way or another until it is directly above the centre of buoyancy, and boats are designed so that this is the natural position when they are safely upright in the water, level fore-and-aft and from side to side.

Should a boat capsize, the centre of either force could be moved artificially to bring her back onto an even keel. The centre of gravity could be moved temporarily by shifting ballast (often water) inside the hull, or the centre of buoyancy could be moved artificially by, for example, inflating an air bag under the water.

However both systems have their disadvantages.

Moving ballast involves a complicated system of tanks, pipes and valves, and also has the practical disadvantage of bringing corrosive seawater inside the boat. Air bags are relatively simple to add to many types of boat but, although the self-righting ability remains so long as the bag is left inflated, they are basically a once-only system and have to be stowed and the system recharged with gas before re-use.

Automatic A more elegant solution is to make the boat inherently self-righting, by ensuring that the centres of gravity and buoyancy move automatically to positions that bring the boat upright.

The principle used for modern lifeboats was well known even in the 1850s: keep the centre of gravity low, and ensure that the centre of buoyancy moves outward as the boat heels to exert its returning push up on the 'downhill' side of the boat. This can be achieved by positioning buoyancy high up, so that when the boat is heeled it is further 'out' and therefore has greater leverage. When the boat is upside down the weight is high above the buoyancy and the boat is unstable and will 're-capsize' into an upright position.

This aim has exercised the minds of lifeboat designers for the past 200 years, for every aspect of boat design is linked and, as the early designers discovered, making a boat self-righting from a capsize could make her 'tender' (easily heeled), uncomfortable and difficult to work aboard when upright in normal service.

The 19th century lifeboats used heavy keels to keep the weight low and had raised fore and aft end-boxes (which gave them their distinctive appearance) to raise the buoyancy as far as possible when inverted. These boxes could not be too high, as they would obstruct visibility and provide too much wind resistance, so to be sufficiently unstable when upside down the boats had to be relatively narrow - with the disadvantage that they were also less stable when the right way up.

To retain relatively wide, and therefore stable, hulls which are still self-righting the buoyancy must be pushed still higher to give greater leverage, in fact so high that it cannot be accommodated in the hull at all.

This is the clue to the modern breakthrough which gives stable, yet still self-righting, hulls - thinking beyond the hull itself and using the superstructure to provide this extra buoyancy.

It is, therefore, tall and bulky superstructures which give lifeboats their ultimate stability, even though the seemingly top-heavy look leads many to assume that it detracts from it.

Photographs of self-righting trials - which all new boats have to undergo - show clearly that the sealed superstructure of a modern lifeboat provides so much extra buoyancy that it is impossible for them to stay capsized. The huge extra volume of the superstructure simply refuses to be pushed far underwater by the fixed weight of the boat, and the lifeboat is so unstable when inverted and trying to float on the relatively narrow superstructure that she rights herself within a matter of seconds.

The key words are of course 'sealed superstructure', for if the cabin floods, the buoyancy, and hence selfrighting ability, is lost.

Strong, modern materials have made it possible to give quite high superstructures the required strength without excessive weight, and the need for an effective seal has led to such things as the watertight doors which are now a prominent feature of all lifeboats. The doors must be quick and simple to operate, yet must seal perfectly when closed.

Cabin and wheelhouse windows are also part of this seal, and much work has been carried out to ensure that water pressure during a capsize will not push them in, nor the force of a breaking wave smash them.

Neither can the windows be opened, a further safeguard against breaching this essential watertight seal.

It is only relatively recently that materials such as the immensely strong polycarbonate now used for the 'glass' area have made such designs a practical and safe proposition.

Other aspects of the design of an inherently selfrighting lifeboat must also come under close scrutiny.

The superstructure must be sealed during a capsize, yet both engines and crew need fresh air (the engines in very large quantities for combustion of the fuel), and stale air and diesel exhaust have to be vented outside the boat when running normally.

Special vents have therefore been developed to ensure that neither crew nor engines suffocate in normal service, yet which prevent water entering should the boat capsize. The very hot exhaust of a powerful turbocharged diesel engine presents its own problems in this area, but this is solved by ensuring the exit is in the small area of the boat's stern which is clear of the water when upright, upside down or at any angle in between.

Specialised Special engine cut-outs have been used since the earliest self-righting motor lifeboats, and today's versions have been developed to either cut the engines or return them to idle during a capsize, warning lights coming on to remind the crew that the cut-outs have operated.

The modern lifeboat is therefore a very highly developed and extremely specialised vessel. Not only do her designers have to balance weight and strength, speed and seaworthiness, handling and ultimate security but they also have thrust upon them the additional task of ensuring that she will self-right after a capsize. Hardly surprising then that such designs cannot be bought offthe- shelf and that the RNLI's technical and operational sections expend so much time and energy in designing and proving each new class of lifeboat..