The UK's near total dependence upon seaborne commerce has proved a weakness often exploited by her enemies. During World War I the Germans were able to keep their High Seas Fleet as a threat 'in being' while using their submarines almost without restriction to damage the British economy, to the extent that victory was lost only through the rediscovered art of convoy.

Hitler, with an even weaker surface fleet, looked again in 1939 to his U-boats to blockade the British into submission. The courage of the submarine crews was, however, to be matched by that of the British and their allies, backed by technology that was eventually to prove decisive.

Science could assist, not only in developing the familiar 'black boxes' but also in passive assessment of the struggle to come. A convoy system was obviously required but, bearing in mind the enemy's weaknesses and strengths, answers to fundamental questions were also required. What was the optimum convoy size and how best was it to be arranged? How many escorts were needed and where were they to be placed? In terms of cargo ship/days, was it more effective to run 'independents', with their higher loss rates, than to keep ships hanging around waiting for a convoy? What routes and convoy cycles best served the war effort, while taking into account the limited escort capacity? The science of operational research provided the answers to these questions and many more.

The basic weakness of the U-boat was its slow speed and endurance once submerged. Even surfaced, its horizon was normally limited to about 16 km ( 10 miles), and it could be simply demonstrated that a single group of ships was far less likely to be detected than a number of independents. Also, to devote warships to convoy escort duties was not totally defensive warfare for, once sighted, the convoy acted as the bait to bring the U-boat to the hunter saving much fruitless search. Only by directly patrolling the submarines' transit routes could anti-submarine ships be employed as effectively.

Independents suffered losses at two to three times the rate of ships in convoy, but the Admiralty's belief that the latter should not exceed 40 ships came under scrutiny. Escorts were allocated (ideally, at least) at three to a very small convoy, plus one for each additional 10 ships. This principle was shown to be anomalous for, applying it, say, to two separate convoys, each of 20 ships, each would need to be accompanied by five escorts, yet by combining the two convoys into one of 40 ships, only seven escorts would be required.

With escorts in desperately short supply, the reasoning was attractive. It was also correct, for though twice as many ships occupied twice the area, their perimeter (assumed circular) increased by less than 50 per cent.. To extrapolate

late further, 60 ships had less than twice the perimeter of 20 and 100 less than 2 1/2 times. Thus to keep the same spacing, all else being equal, only 12 to  13 escorts were required for a 100-ship convoy, where 25 would have been needed for five 20-ship convoys, together witl7 a higher risk of detection. Larger convoys were instituted in early 1943 and halved the loss rate.

A surfaced submarine could extend its limited horizon by listening for a convoy's hydrophone effect or by sighting its smoke, but a group of submarines could increase its chance by forming a line of bearing across a suspected track. Should a submarine make a contact it would report to its shore-based control on HF, the latter rebroadcasting on MF to all boats to close in, the well known 'wolfpack' tactic (known to the Germans as Rudel Taktik). The weaknesses of the system were the amount of wireless traffic generated and the need for U-boats to run on the surface, failings both exploited by the British.

Rapid and encyphered, the transmissions tended to have a pattern and, even before they could be decyphered, they were useful because their very direction indicated a convoy under threat and requiring reinforcement. The breakthrough was the development of a shipborne high-frequency direction-finder (H/F D/ F or 'Huff Duff ). Two or three ships so fitted could accompany a convoy and take bearings on any transmissions, which if close enough or on a bearing that posed a threat, would be aggressively run down by the best-positioned escort. To sink the submarine was a bonus but merely to make it submerge was sufficient to for it to lose touch, sometimes for good, as the

German boat's submerged performance at that time was poor.

U-boats were, indeed, surprised and sunk but, as H/F D/F appeared at about the same time as escort-borne radars, the latter tended to be suspected by the enemy as the cause.

Asdic (now termed 'sonar') was widely fitted in Royal Naval escorts, but its effectiveness had been rather overrated before the war, and this factor was compounded by the common practice by surfaced U-boats of attacking a convoy at night, capitalizing on their small silhouettes. The answer was radar, the escorts being fitted with modified metric-wavelength ASV sets from aircraft, these being gradually replaced by centimetric sets that could detect a surfaced submarine out to about 6.4 km (4 miles).

It was the aircraft that proved the greatest nuisance to the surfaced U-boat, but its presence with a convoy did have the drawback of necessitating a degree of radio traffic where, otherwise, silence was observed. Even with radar, aircraft took a while to develop the art of submarine hunting, partly because the Germans had a radio receiver that very effectively detected radar transmissions, enabling rapid avoiding action to be taken. Not until November 1941 was the first 'kill' ascribed to radar, By early 1943 shorter wavelengths confounded the German equipment and, for a space of three months, U-boat losses ran at an average of one per day.

Night-running on the surface, even to charge batteries, became prohibitively dangerous

when aircraft received the 80-million candlepower Leigh Light, extra guns and improved depth charges with which to mount sudden and unannounced onslaughts from out of the darkness.

The enemy survived by developing the Schnorckel (snort) but slow, submerged transits now occupied most of a boat's endurance and even on station high surface speeds could not be used. Efficiency fell off rapidly. Desperately, some U-boats carried enhanced AA armament, electing to fight it out on the surface. This suited the aircraft very well, and spiralling U-boat losses soon discouraged the practice, with air-to-surface rockets proving particularly effective against the submarine's thick hulls.

With the U-boat now attacking submerged, Asdic came back into its own. In 1943, if she kept her speed down to about 15 kts, an escort could obtain a contact at up to about 2.4 km ( 1.5 miles) but, because of the geometry of the pencil beam, contact was lost on the run-in, some 2Z5 m (300 yards) short of target. As charges were still dropped or fired over the target's position, this gave an alert submarine commander a precious half minute to take the evasive action. Ahead-firing weapons were therefore developed. Mousetrap and Hedgehog and, later, the Squid, were able to reach the target before contact was lost. The Squid, in fact, was soon used in conjunction with an Asdic that could measure depth as well as range and bearing.

Forerunner of today's countermeasures, the Foxer was only one such item, American contributions included Magnetic Anomaly Detection (MAD) as early as 1942, with sonobuoys a year later. The British had an air-launched anti submarine homing torpedo, FIDO, by mid-1943

Ultimately the Germans introduced the highspeed submarine in the Types XXI and XXIII. The former had a 16-kt submerged speed, underwater fire control and advanced torpedoes but, fortunately for the Allies, was brought into service too late to have much impact.

Through all causes, the Germans lost nearly 800 U-boats during the war. These had accounted for over 14 million gross registered tons of mercantile losses, over two-thirds the total from all causes. Science assisted the Allied victory immeasurably in both anticipating and responding to enemy developments.