ON APRIL 10, 1983, Salcombe's 47ft Watson lifeboat The Baltic Exchange, searching for missing divers at the southern end of Start Bay, was hit by mountainous seas in a force 11 storm.

The first of two tremendous seas, a wall of water about 50 feet high and breaking, knocked the lifeboat down to starboard, washing overboard one of her crew; the second huge sea capsized the boat herself. The emergency air bag with which she is fitted inflated automatically, bringing the lifeboat upright again. The crew at once got under way, recovered their comrade from the sea, and continued with their search. That was a significant milestone in lifeboat history.

Going back a few years, on December 24, 1977, Kilmore's 37ft Oakley lifeboat Lady Murphy, returning from a search, was capsized twice off Forlorn Point in very wild seas about 30 feet high whipped up by a strong gale at about high water springs. She righted each time. One man who went overboard during the first capsize was recovered and three of the four men washed into the sea during the second were safely picked up; tragically the fourth man could not be found. The Oakley's self-righting capability comes from water automatically flowing from one tank to another. As Lady Murphy went over, her mast broke off at its base; that is exactly what it was designed to do so that there should be no possibility, in shallow water, of the masthead hitting the seabed and its foot being driven through the lifeboat's superstructure into the compartment below, thus letting the water into the boat. With the mast, however, the aerials went too and radio communication with the shore was lost. Nevertheless, Lady Murphy's crew were able to bring her back to station with no outside help.

Nearly two years later, in the early hours of November 18, 1979, Barra Island's 52ft Barnett lifeboat R. A.

Colby Cubbin No. 3, and Islay's 50ft Thames lifeboat Helmut Schroder of Dunlossit both launched in winds gusting up to hurricane force 12 to go to the aid of the Danish cargo vessel Lone Dania, whose cargo of marble chips had shifted. Both lifeboats were capsized by steep breaking seas up to 30 to 40 feet high. Both boats righted and returned safely to station, Lone Dania eventually returning to Barra under the escort of another Danish ship. Barra Island's Barnett, like Salcombe's Watson, was righted by an emergency air bag; Islay's Thames is designed to be self righting without auxiliary aids. Although the Barnett's mast was damaged and her MF aerials had been carried away, a six-inch stub of the VHP aerial remained and radio communication with the Coastguard was possible.

Salcombe's The Baltic Exchange, however, was the first lifeboat to continue on service after capsizing and righting, albeit for only a comparatively short time. Before long she heard that a helicopter had taken up the search, and as she needed to pump out her own engine room, she made for the lee of the land, maintaining her search on the way. The lifeboat herself suffered only minimal damage; her engines, steering gear, compass and both clocks were all working and she was able to communicate by radio.

The remarkable story of the Salcombe crew was told in the Winter 1983/84 issue of THE LIFEBOAT. There is a story, too, behind the 'hardware': the boat and her equipment. A hull which will right itself from complete inversion in heaving salt water, machinery and equipment which will go on working after this treatment, do not happen by accident; they have to be designed, built and tested with care. The Salcombe recovery was made possible, as were the others, by hard work done in the design offices and building yards long before the event. The crews achieved the feats; the designers made it possible for them to do so. Let us look further into the designers' work.

Self righting Consider first the matter of self righting.

It is a particular aspect of a craft's 'transverse stability'. Self righting is a quality which has not always been enjoyed by all lifeboats. It can be designed into the hull, but other features may have to be sacrificed to achieve it.

If a vessel floating upright in calm water is forcibly heeled to one side through a small angle and then released, she will in general return to the upright; she is then said to be stable. If the angle is increased, a point will usually be reached where the vessel no longer returns to the upright on release, but carries on rolling until she is upsidedown, and there she stays. She is said to enjoy positive stability up to the critical angle, above which stability becomes negative. The critical angle, known as the angle of vanishing stability, may be of the order of 60 degrees or so in a fishing vessel or merchant ship. Heeling beyond this angle is usually fatal, as water will enter through hatches and other openings, sinking the ship. Even if she remains afloat for a while in the inverted position, the prospects for the crew are grim and there are few records of escape from capsized hulls.

In large ships capsize is a very rare event. It is usually caused by heavy weather aggravated by some mishap such as hull damage or cargo shifting. In fishing vessels and small craft exposed to waves which are larger in relation to their size, capsizes are more common, and are sometimes caused by waves alone. In recent years attention has been increasingly given to the stability of such craft.

Of all the vessels on the sea, however, lifeboats, from the very nature of their duty, are the ones most likely to meet waves capable of turning them over, not only sideways but even endfor- end. Lifeboats cannot be advised to run for shelter when the sea rises; that is the very time when they have work to do, and so often the work is in shoaling water where the seas grow steep and break. Lifeboats have to be able to face up to the very worst the sea can do.

No one can design a boat which can never be turned over by a sea. The forces of nature are such that seas will sometimes arise in which a vessel the size of a lifeboat will unavoidably run the risk of being turned over on to her back. Let us keep the matter in perspective; the risk is very small, but not so small as to be negligible, as the Kilmore, Barra Island, Islay and Salcombe lifeboat crews found.

While the designer may not be able to prevent the boat from turning over, he can still design the hull so that it will come upright again instead of remaining upside down. His first act is to push the angle of vanishing stability up until it reaches 180 degrees; that is, he will so shape the hull that it will roll back to the upright no matter what heel is imposed forcibly, right up to complete inversion.

There are several ways of achieving this end (see 'Self Righting Explained' by Stuart Welford, THE LIFEBOAT, Winter 1974/5). They all boil down essentially to keeping buoyancy high up in the boat, keeping the centre of gravity low and preventing water from coming inboard while the boat is inverted.

The principle has been well understood from the earliest days of lifeboats.

Right back in 1789, when a prize was offered for the best model of a lifeboat, one of the entrants was William Wouldhave, a parish clerk from South Shields.

His model was of a pulling boat with pronounced sheer and high ends.

Although Wouldhave was only given half of the contest's two guinea prize, his design principles were later incorporated in Greathead's Original, the first purpose built lifeboat.

Many of the pulling boats of the last century were built with a heavy sheer, with watertight compartments built into the high bow and stern portions. These boats rolled heavily and a belief grew up that self-righters were not good sea boats. This may have been true of those particular boats, but there is in fact no essential conflict between self-righting ability and sea-kindliness. With care in design, aided by modern methods and materials, the boat can have both qualities.

The approach used in recent designs has been to fit a substantial deckhouse and to build it in such a way that it remains watertight and buoyant with the boat upside down. This feature can be seen in the Waveney, Thames, Arun, Tyne and Brede classes. Of course, if water gets into the wheelhouse the valuable self-righting property will be lost. A watertight wheelhouse is not there just to keep the rain off; it is an integral part of the hull and needs to remain tight in the face of the worst the sea can do.

An earlier approach, adopted in the housed carriage or slipway 37ft and 48ft 6in Oakley lifeboats, was to employ a water transference system. A ballast tank, which fills with sea water within seconds of the boat launching, is fitted in the bottom of the boat, while a righting tank is fitted as high as possible under her port deck. As the boat is pushed over, passing 110 degrees of heel, a valve opens which allows the water from the ballast tank to flow into the righting tank. This transfer of weight to one side of the boat begins a rolling movement which returns her to the upright. The system works, but the tanks and associated valves and piping are troublesome to maintain. To give a righting capability retrospectively to some of the Institution's Watson and Barnett lifeboats, which were not originally self-righters but which still had a number of years station life ahead of them, a different method was devised. A large cylindrical emergency air bag (deflated and housed in a glass fibre cover) is fitted on a lifeboat's after cabin top. If the lifeboat rolls beyond about 120 degrees, a weighted lever operates and a valve discharges compressed air into the bag which is, of course, now in the water under the boat. This newly-created buoyancy renders the boat positively stable and she returns quickly to the upright. It was an inflated air bag which righted the Barra Island and Salcombe lifeboats and almost certainly saved the lives of their crews. It was the sound construction and good maintenance of the inflation systems that ensured that the valves worked and the air pressure was there to generate buoyancy the instant it was needed to bring the boats upright while the crews held their breath.

A similar type of system is used to right an Atlantic 21 rigid inflatable lifeboat. A deflated cylindrical buoyancy bag is housed on a tubular alloy roll bar, or gantry at the after end of the boat; should the boat be capsized, once the crew have all assembled in the water at her stern and taken hold of the lifeline, they can, by pulling a cord, inflate the bag with CO2 gas. When this is done the boat rights herself with impressive speed. The roll bar itself is intended to protect the crew from being crushed beneath the boat, should she be turned over in shoal water or among rocks.

To complete the picture and going down the size scale, equipment has now been fitted to the 17ft 6in twin-engined Mk IV Zodiac D class inflatable lifeboat so that should she capsize her crew can, using their own weight and the correct drill, man-handle her back to the upright.

Drills for righting all classes of inflatable lifeboats have been developed. Checking stability Stability is the property of a vessel which returns her to the upright after heeling. As already indicated, most vessels are only stable up to a certain angle, beyond which they become unstable and turn over. Modern lifeboats are exceptional in that they can self right from any angle. How does the drawing office know this in advance? Without going into too much detail, it will easily be appreciated that to predict a ship's stability an essential starting point is to know the position of her centre of gravity (CG). The higher the CG, the less is the stability. The designer can calculate with fair confidence the CG position of a new vessel, provided that she is built exactly to the approved drawings and there are no unauthorised additions. When the boat has been in service for a few years, however, the CG tends to rise; even coats of paint accumulating on the upperworks can cause it to creep upwards. It is wise to check from time to time that the precious stability is not being 'eroded'.

The check is made by a simple operation known as an 'inclining experiment', in which a known weight of ballast is transferred from one side of the deck to the other, the steady angle of heel which results being measured.

From the known weight and the spread of ballast and the heel angle, a quantity known as the metacentric height (M) can be calculated; from this and from the boat's lines plan the position of the centre of gravity (CG) can be calculated.

Once having located the CG, the stability at any angle of heel can be worked out.

The designer of a lifeboat can satisfy himself that the stability is positive all the way up to 180 degrees; if it is not, the shape of the hull must be altered until it is.

Although the inclining test is simple in principle, measuring the heel angle can prove tricky in practice on a boat as small as a lifeboat, with disturbances from wind, waves and the movement of other nearby boats. The traditional apparatus is a long string carrying a weight which hangs in a bucket of water to damp out the swinging motion. The angle is shown by the string's movement across a foot rule fixed to the boat. This works well in a big ship but the human operator is hard put to it to judge the average reading of a lively boat.

When it comes to measuring quantities like angles, particularly when those quantities are changing all the time, a machine can always do better than a man. With this in mind a microprocessor controlled automatic inclinometer has been developed for the Institution.

This instrument measures heel angle very accurately, records its value digitally several thousand times each minute and then uses its inbuilt computer to calculate the mathematical mean value of the heel angle. This inclinometer will thus enable stability checks, to be made throughout the life of a lifeboat more easily and with greater accuracy than in the past.

The measurements given by the inclinometer, used in conjunction with an Apple computer the Institution has recently purchased for the design office, means that stability calculations, among others, can be made much more quickly than before. Thus more alternatives can be explored in the early stages of a new design and the stability features of an unbuilt boat can be set out in more detail than was previously possible. The new computer has already proved its worth in analysing the stability of various alternative designs being considered for a new fast carriage lifeboat.

As a final check on all calculations and predictions, each new lifeboat, 10 metres and over, is, as for many years, subjected to an artificial capsize in harbour as part of her builder's trials.

The boat is hauled over by a crane which pulls upwards on a strop wrapped round the hull. The righting is carefully watched and timed. Not only does this righting trial establish confidence in the stability calculations, but it also checks the operation of self-closing valves and gravity switches, the watertightness of doors and so on. And it is psychologically very reassuring to see the actual boat go over and come up again, even if in harbour in a flat calm! (to be continued).