FREE PASSAGE OF AIR into engine room and cabins of a motor boat under way is essential to both machinery and man.

Without it, engines cannot run and the well-being and efficiency of the crew would soon be impaired. It follows that the ventilation system of a lifeboat for which a watertight wheelhouse is an integral part of her self-righting capability is of first importance. There must be vents and a trunking system; but where air can pass, so can water. What will happen should the boat capsize? To meet this eventuality the design office of the RNLI has produced a gravity air intake valve with minimum mechanical action which, with all the virtues of simplicity, ensures not only that the sea is excluded as the boat goes over, but also that ventilation is interrupted for as short a time as possible.

To deal with the last point first, the valves are sited to port and starboard at the highest possible point in the boat's structure (fig. 1). Should the boat be .

rolled to starboard, while the starboard! valve closes, the port valve, still clear of the water, remains open and continues to supply air to the engines. It is not until the angle of heel approaches 100° that both valves close, and it is then only a matter of seconds before, as the boat rights herself, the water drains •away and the valves open once again.

The air intake valve (see fig. 2) is an ante-chamber to the ventilation trunking system (A), the upper and lower compartments being separated by a hinged flap (C). Beneath the hinged flap is a perforated control box (B) through which air and water can pass but which is designed as a runway for the gunmetal ball (D) which closes the flap.

In normal conditions the ball lies quiescent at the base of the control box, the flap is open and air passes freely through the perforated box to the trunking system (fig. 2). If the boat should capsize, as she heels over the ball responds to the pull of gravity and rolls outboard to close the flap (fig. 3).

By the time she is on her beam ends andthe compartment is filled with water the flap is fully closed by the ball (fig. 4), and it is held even more firmly shut by the head of water which fills the lower compartment (fig. 5). As the boat rights herself, the water drains away, the ball rolls back into its resting place, the flap opens and fresh air passes freely once again.Fig. 1: Air intake valves sited in turn of wheelhouse roof, port and starboard.

Fig. 2: Boat upright; air enters freely.

A, ventilation trunking into boat; B, perforated box through which air (and water) can pass; C, valve flap; D, 3%" gunmetal ball.

Fig. 3: Heeling, 60°: force of gravity rolls ball outboard closing valve flap.

Fig. 4: On her beam ends, 90°: valve closed before water can enter ventilation trunking.

Fig. 5: Capsized, 180°: head of water adds strength to valve seal.

Fig. 6: Righting: water drains away and valve opens..