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Introduction to the Convoy system in North Atlantic
Merchant ships in the early part of WWII travelled alone across the Atlantic to Canada and the USA. They returned with various merchandise, grain, timber, tanks, etc. However, they were easy prey to German U-boats and the convoy system was instigated.
Naval vessels such as destroyers and corvettes protected the convoys. The destroyers had the speed to catch up with the U-boats and force them to dive below the water. They would then drop depth charges around the U-boats, sometimes successfully disabling or sinking the U-boats.
The presence of this naval protection meant that the U-boats would be prevented from surfacing and locating the targets in the convoy during daylight hours. Additionally a submerged U-boat was very slow, only about 5 knots per hour, whereas the slower ships in the convoy sailed at about 9 knots. The convoy could thus outpace a submerged U-boat. However, when on the surface, during the hours of darkness, the U-boats had a speed of about 12 knots and could catch up with the convoy.
Each merchant ship in a convoy was typically assigned a station so the convoy formation consisted of several columns each of three to five ships. The lead ships of the columns were spaced at intervals of 1,000 yards (910 m) along a line perpendicular to the convoy course. Each ship in the column followed the ship ahead at a distance of 800 yards (730 m). The typical convoy would be approximately 5 to 6 miles (8 to 10 kilometres) wide and 2 miles (3 kilometres) ) long. The convoy would thus be up to 10 columns wide with each column containing 3 to 5 ships.
The rescue plan for early convoys was to have the last ship of each column rescue survivors of other ships in that column. If the last ship in column was hit, the rescue task fell to the escorting warships. In practice, the escorting warships performed rescue tasks more often than the 25% suggested by random hits on a four-ship column; because some merchant ships refused to leave the protection of the convoy formation to fall back and remain a stationary target while rescuing survivors. Merchant ships were not well suited to manoeuvre to pick up survivors, and those attempting rescue were hampered by lack of suitable rescue equipment.
Warships attempting rescue were diverted from the task of defending the rest of the convoy from the attacking U-boats. The first specially equipped rescue vessel went into service in January 1941. When rescue ships were unavailable, large, ocean-going tugboats or converted trawlers were sometimes designated to perform rescue duty.
The rescue ship was normally assigned a convoy position at the rear of one of the central columns of ships. From this position it could observe damaged ships falling astern of the convoy and quickly rendezvous to rescue survivors.
Rescue ships were typically small freighters with passenger accommodations. Conversion to rescue service involved enlarging the galley and food storage areas and providing berthing and sanitary facilities for approximately 150 men. Scrambling nets were rigged along the sides, and boats suitable for open sea work were substituted for normal lifeboats. Rescue ships normally included a small operating room for an embarked naval doctor and sick bay staff.