ONE OF THE CHARACTERISTICS of inflatable and semi-rigid boats which makes them particularly suitable for rescue work at sea is their inherent stability; and the lower in the water they are, with their wetted beam increased, the greater their stability. Such increased stability has obvious advantages when lying alongside a casualty or taking a survivor from the water in bad conditions.

Ballasted down, moreover, an inflatable heading straight to windward in rough weather will have greater grip on the sea, the bow going through the waves rather than being thrown up by them, thus increasing the safety of the boat and reducing the slamming so fatiguing to the crew. For an Atlantic 21, with a radius of action of about 50 miles, returning 'light ship' from a long service with most of the 24 gallons of fuel she carries in her two lateral tanks burned up, there are also obvious advantages if she can take on board compensating ballast, especially if the weather is wild and she has to cross a bar on her way back to station.

First class fluid ballast—sea water—is there for the asking. For the RNLI team at the Cowes Base, under Trevor Evans and Mike Brinton, the problem was how to make use of it most efficiently? A programme of development work brought an answer for both inflatable and semi-rigid boats. While the principles are the same for both, their application, had, of course, to be modified to meet the design differences of the two types of hull.

The first problem was the scoop. Four or five different ideas were considered before the final answer was found: a vertical tubular scoop (in principle and elementary appearance reminiscent of the tubular self-bailers many people will have at some time fitted to their dinghies), the position of which can be altered to take in rapidly, hold, or let water out while the boat is under way.

The tank can also be free flooded when stopped alongside a casualty.

Zodiac Mark V Experiments in water ballasting have been made at Cowes on three 19' Zodiac Mark V inflatable inshore lifeboats.

One is stationed at Minehead, one at Silloth, and a photograph of the other can be seen on the cover of this journal.

Originally there were two scoops, manually operated by a crew member, fitted on each side of the transom, each supplying a tank beneath the boat's floor on its own side of the central inflatable keelson.

The lower end of each outer, stainless steel, tube is sliced off at an angle of 27 degrees, and the tube mounted on the transom with the sloping aperture facing aft. Higher up the tube, facing forward, is an outlet passing through the transom to the tanks (a general idea of the shape can be gained from the photograph of the Atlantic 21 scoop, different in detail, on the opposite page).

Inside this outer tube is a tubular sleeve, its open foot also sloped at 27 degrees. When the sleeve is fully down the open foot faces forward, projecting below the transom, and water, forced into it by the forward movement of the boat, passes up and through a port on the inner tube and thus through into the tube which feeds a tank beneath the floor of the boat. Each of the two tanks holds 195 Ib of water.

If the sleeve, which comes up in a spiral, is raised by the crew a half turn to the next position, its open foot will lie within the open foot of the outer tube, facing aft, and another aperture will have been turned opposite the outlet. As the boat moves forward at speed, water will now be sucked out by the negative pressure of the sea, emptying the tank. If, however, the crew member had brought the sleeve up a whole turn to its third position, so that it was fully raised, blank tube would have been aligned with the outlet, the valve closed and the water held in the tank.

As was said above, there were originally two scoops and a soft neoprene tube led from each outlet to its own tank, one each side of the central inflatable keelson. Further development, however, has made it possible for one transom scoop to serve both tanks.

A soft neoprene Y tube passes through a sponge section which takes up the inflated shape of the inflatable keelson; the trunk of the Y tube is attached to the outlet pipe and the two branches each feed the water to one tank.

The tanks (see illustration) are made up at the Cowes base. They are designed to take up the boat's shape in beam and depth measurements but are not attached in any way to the 'hull'. They are suspended from the underside of the marine plywood floor so that the flexible skin is left free for its own characteristic rippling, which helps the boat to grip the water. Also, if the tanks are independent of the bottom they are less likely to be damaged when beaching. They span the hinge in the floor (itself there to add flexibility) but are supple enough to give with the hinge.

The tanks, made of RFD neoprene coated nylon, are given their 'body' by longitudinal formers made into a threedimensional grid with baffles, all breached with water passage holes. The adhesive used is Bostic. After a tank has been glued to the underside of the floor, the join is finally sealed all round with a neoprene flange.

Necessary air vents from .the tanks, which must be open when the tanks are filling or emptying, are led up to the forward side of the Zodiac Mark V standing steering position console and can be controlled by the helmsman; when water comes through the air vents he knows the tanks are full. There are also hose filling points so that the tanks can be filled ashore before launching, and also so that, if necessary, they can be cleaned out.

Atlantic 21 The Atlantic 21, for its water ballasting, has just one vertical transom scoop, hydraulically operated. The controls are sited on the console, close at hand for the crew who, therefore, do not need to leave their seats to operate the system. Once again, the tubular valve has three positions. Fully down, the scoop faces forward and, at speed, can fill the two ballast tanks, either side of the fuel tank compartments, which together hold 440 Ib of water, in a matter of seconds. Partly raised (straight up this time) the valve is shut and the water contained in the tanks. Fully raised, the sleeve leaves the fixed tube's aft-facing aperture open, and the water can drain away, sucked out as the boat drives forward.

Of course, in normal conditions, when extra ballast is not needed, the valve in both Zodiac Mark V and Atlantic 21 would be left in the closed position.

From the scoop valve, the water goes through the transom and into a manifold, from which pipes take it to port and starboard glass fibre integral ballast tanks under the floor, shaped to fill the wing longitudinal compartments outboard of the fuel tanks.

Atlantic 21 B537, stationed at Blackpool, has a special problem. She has a long run out to sea through shallow water, which can be whipped up into short, steep waves, each of which would throw her bow up; a rough ride, indeed, and hazardous. To hold her head down, so that she can forge through these steep, hollow seas, caused by extensive sandbanks, without reducing speed, she has been fitted with water trim tanks forward; they hold an additional 380 Ib. Made of neoprene, and held in shape by internal, holed baffles, they fit into the two forward central longitudinal compartments under the floor.

They can be filled and emptied, controlled by the manifold valve operated from the console panel, which also indicates with lights the exact position of the scoop.

Once again, of course, there are air vents, one to each tank, as well as hose filling points.

Perfection is hard to achieve, and possible improvements are always under review. The ballast tanks of the Zodiac V for instance: there are some indications of wood fatigue in the floor— perhaps if the position of the tanks were moved slightly it would make the difference . . . and so the development work goes quietly on ….