Four Royal Naval auxiliary patrol boats. The Auxiliary patrol of the Royal Navy is one of the most important duties undertaken by the senior sevice . Not only has the sea to be constantly patrolled , but the rivers and open waters of Britain must also be closely guarded and watched . The fleet of the river patrols consists of peacetime yachts and motor boats whose owners have generously given , loaned or hired their craft to the government . These little ships are manned by volunteers who have joined the RNAP for the duration of the war . With their white ensigns worn at the stern , their hulls painted a service grey with their guns mounted on their decks , the ships of the river patrol become his majesty ' s ships , and are subject to naval discipline and routine . The duties of the patrol are many , but first and foremost they must keep a sharp look out for the enemy ' s activities , such as observing the dropping of parachute mines , low flying enemy aircraft on raids over their patrol area .