ships had been armed by the end of 1940; and all ships were armed by 1943. The low-angle guns were typically in the 3-inch to 6-inch range (75–150 mm) depending on the size of the ship. Rifle-calibre machine guns were augmented or replaced by Oerlikon 20 mm cannon as they became available. The high-angle QF 12pdr Mk V mount was the most common anti-aircraft gun and later ships sometimes received Bofors 40 mm guns. Untrained gunners posed significant risk to friendly aircraft in the absence of efficient communications. DEMS guns were manned by 24,000 Royal Navy personnel and 14,000 men of the Royal Artillery Maritime Regiment. 150,000 merchant sailors were trained to assist by passing ammunition, loading and replacing casualties. Initially, Royal Artillery personnel provided anti-aircraft protection by bringing their own machine-guns aboard ships operating close to the British Isles. DEMS gunners were often retired military personnel and young Hostilities Only ratings, commanded by a petty officer or Royal Marine sergeant. Large ships sometimes embarked a junior naval officer to command the DEMS gunners. Canada placed guns on 713 ships, while the Royal Australian Navy provided gun crews for 375 Australian and other Allied ships. D-day landings and the Royal Observer Corps In 1944, during preparations for the invasion of France called Operation Overlord there was deep concern over the danger to Allied aircraft from the large number of DEMS involved in the landings. A request for volunteer aircraft recognition experts from the Royal Observer Corps produced 1,094 highly qualified candidates, from which 796 were selected to perform valuable aircraft recognition duties as seaborne volunteers. These Seaborne Observers were organised by Group Commandant C. G. Cooke and trained at the Royal Bath Hotel Bournemouth before the volunteers temporarily joined the Royal Navy with the rank of petty officer (aircraft identifier). The volunteers continued to wear their ROC uniforms, but wore seaborne shoulder flashes and a Royal Navy brassard with the letters RN. During the D-day landings two seaborne observers were allocated to each of the defensively equipped British and American merchant vessels. The ROC volunteers were given direct control of each ship's anti-aircraft batteries, immediately reducing the previously high level of friendly fire incidents. Their success is measured by a signal from Wing Commander P. B. Lucas, air staff officer who reported: Twenty two seaborne observers survived their ships being sunk, two lost their lives and several more were injured