BUILT TO SPECIFICATIONS probably not exceeded by any ship afloat; subjected to exhaustive commissioning trials; maintained with jealous care by coxswain, mechanic and crew; watched over by divisional inspector, district surveyor and engineer and submitted to regular survey, the RNLI lifeboat has a long expectation of working life at 100% efficiency. So, when after the Longhope disaster in 1969 and the Fraserburgh disaster in 1970 it was decided to set 1980 as a target date by which time virtually all the RNLI fleet of conventional boats should have a self-righting capability, there were a number of lifeboats, Watsons and Barnetts, still with many years service ahead of them; excellent boats, all of them, but non-self-righting. If, however, a way could be found of providing them with a self-righting ability they could be absorbed into the plan.

A start was made at once. Working together, British Hovercraft Corporation and the RNLI design team developed an air bag system, which would give the 46' 9" Watson, 47' Watson and 52' Barnett boats a 'once only' selfrighting ability. Within a year full-scale trials had proved the system successful; within two years the design of the installation had been worked out in detail and a kit defined, so that conversion could be made in a matter of days, usually while a boat was in a boatyard for survey.

The air bag system has all the merits of simplicity. If the lifeboat rolls past the point of no return, about 120° heel, a weighted lever, responding to the force of gravity, falls. A valve is opened.

Compressed air is released and, so quickly that it is almost instantaneous, inflates a large fore and aft cylindrical bag secured asymmetrically to the after cabin top. This 'lopsided' buoyancy, by making the capsized boat unstable in her unaccustomed inverted position and by exerting a positive upward push, initiates righting. Once the upward roll begins, the reducing negative righting lever of the boat adds to the momentum and at about the 120° point the positive righting lever takes over and the boat returns to her normal trim.

It takes about 30 seconds to read that last paragraph. It takes only half that time, or less, from the moment the gravity lever falls to the time the boat lifts her submerged deck, shaking herself free of the sea. Some 10 to 15 seconds.

In the past three years 38 Watson and Barnett lifeboats have been fitted with air bags. The job is done by British Hovercraft. They have two crews, each of two men. First, a crew prepares all the components of the kit; then, after these have been submitted to exhaustive workshop tests, the crew takes the kit to a boatyard, which may be in any part of the country, wherever a lifeboat is undergoing survey, and completes the conversion. The installation is overseenMany famous yachts, like the early Fastnet contenders or more recent world-circumnavigating single-handers, have been hauled out there; many legendary deep-sea sailors, like Bobby Somerset, Sir Francis Chichester, have sat in the little yard office, discussing over mugs of tea their boats and the sea with the five Mashford brothers who have run the yard for the past 40- odd years, backed up now by the next generation.

Hauled out in the shed with Solomon Browne were the relief 37' Oakley lifeboat Jane Hay and the former Plymouth 52' Barnett boat Thomas Forehead and Mary Rowse, both of which took part in services for which medals for gallantry were awarded last year. The 70' Clyde City of Bristol, which will soon be on station at Clovelly, was moored in the river.

Before installation of Solomon Browne's air bag could begin, there was preparatory work for Mashfords; decking in the after well to make a watertight compartment and blanking off its relieving scuppers in the topsides (at the same time the doors at the aft end of the deck cabin were replaced with a square stretcher hatch); moving the radar scanner from centre to the port side of the after cabin top to allow room for the air bag to inflate; and the blanking off of some deadlights and ports.

The British Hovercraft crew, on this occasion Brian Augustus and Leslie Harris, then took over, helped where necessary by Mashfords' boatbuilders.

First, in the new after well watertight compartment, they fitted the mounting cradles on which rests the Chesterfield compressed air bottle (2,400 Ib pressure); they are sited athwartships, butting up against the cabin after bulkhead.

The heavy bottle was lowered on to the cradle (Fig. 1) and secured with bolted top clamps (Fig. 2).

Next the roof channel is bolted in place (Fig. 3), the deflated bag already attached to the two inlets through which the air would be driven from the manifold which can be seen beneath the channel. This channel's position is critical. Exact specifications vary slightly for the three classes of boat, but for a 47' Watson the position of the channel is such that when the bag is fully inflated (2 Ib pressure) its axis will be 27* ± 2* from the boat's centreline (Fig. 4).

Once the bag is in place and inflated, the three straps which span it are spread out and correctly angled. Then the brackets to which they will be attached with retaining pins (Fig. 5) are positioned and bolted to the cabin sides.

All external fittings are of alloy which has first been anodised, then etchprimed and finally painted with anticorrosion paint. Each is bedded down on chromated calico, spread on each side with Evomastic sealing compound which, while waterproofing the joint, remains flexible so that, if necessary, thefitting can easily be removed. AH fixings are stainless steel.

The bag for the 47' Watson is 5' 4" diameter by 7' 6" long, and it is made of the same material as is used for hovercraft skirts: two layers of neoprene sandwiching a reinforcing layer of nylon. The straps are made of a double thickness of this same material vulcanised together. In fact, all joints of bag and strap are vulcanised by a method perfected by British Hovercraft.

When the bag is finally folded down on top of the channel it will be covered by a lid of curved, glass-fibred marine ply between two semi-circular end boards. As the bag inflates on capsize, this lid would be thrown off and jettisoned.

A flexible 1" diameter hose, through which the released compressed air will be driven to inflate the bag, is led from the manifold through a watertight flange in the cabin top, down the after bulkhead and through to the well compartment, to be attached to the air bottle.

So far so good. The mechanism is there. But the brains of the business are in a watertight control box sited on the starboard cabin side (Fig. 6) and connected to the compressed air bottle by high pressure small diameter hoses. In this box an anti-bounce device swings with the roll of the boat. Only when the roll reaches about 120° will a gate in the device allow the lead-weighted gravity lever to fall, thus opening the valve which in turn opens the flood gates of the compressed air.

So much thought, so much care, so much strength. Yet the working life of the air bag, if it is put to test by storm at sea, will be the few seconds between the time the lifeboat capsizes and the time when, as the bag starts to right her, she regains positive stability. In those few seconds, control box, air bottle, hose, bag and straps will take tremendous strains; the strains for which they have been designed. Their performance in that fraction of time will justify all the work that has gone into their preparation, for on them may depend the lives of a crew of seven lifeboatmen, and possibly also the lives of men and women just saved from the sea.'—J.D..