One of Britains Giant Watch-Dogs That Keep The Convoys Safe. In the convoying of merchant ships, the Coastal Command of the Royal Air Force co-operates with the Navy to splendid effect. The planes employed are chiefly the Short Sunderland four-engined flying boats, 112 ft, wing span, one of which is here being drawn, stern first, up a slipway to undergo its periodic overhaul. Note the guns protruding from the boats tail.