ASDIC sonar

ASDIC sonar is Keeling’s principal means of locating a submerged U-boat, turning sound sent through dark water into bearings and ranges on which the ship’s pursuit depends. Its repeated pings can disclose a contact and track changes in its position, but they cannot provide a complete or continuously reliable picture. The instrument has blind ranges and operational limits; its readings may be confused by countermeasures or layers in the water, and an accurate attack still depends on human interpretation. The hunt makes sonar indispensable without making it infallible.

Detection and interpretation

The sonar’s basic action is rendered as a succession of pings: impulses sent out to search for an underwater enemy. The narrative likens this listening function to the hearing ear, set against radar’s seeing eye. The distinction matters in practice. Radar and visual lookouts survey the surface, while sonar attempts to reveal a submarine concealed beneath it. Neither form of vigilance abolishes uncertainty, and the ship’s understanding depends on reports reaching the bridge and being interpreted there.

The first reported contact in the pursuit has bearing zero-zero-five but no definite range. Later, sonar places a contact at the same bearing at twelve hundred yards. The changed precision gives Krause a stronger basis for action, but even a range and bearing are not a complete solution: he still has to infer the submarine’s movement and choose how to turn against it. A subsequent report gives a distant contact at zero-nine-zero, nearly dead ahead, where an immediate course alteration would serve no purpose. Sonar supplies information that can shape a decision; the captain must judge when that information is sufficient to act.

The instrument also registers disturbances that complicate rather than clarify the picture. At one thousand yards, a contact is reported as a pill: the U-boat has released a Pillenwerfer, whose bubbles interfere with determining the submarine’s new course. Later, another contact is said to sound like a pill. When the attack resumes, the sonar operator reports echoes from cold layers. Krause considers that the U-boat may be hiding in a colder stratum of water, where temperature differences could affect the sound reaching the receiver. In these moments the report identifies a problem, not a settled answer.

The operator and the reporting chain

Sonar’s practical value rests on the person listening to it. Tom Ellis, Keeling’s Radioman First Class, has been aboard since the outbreak of war and trained at the Key West Sound School. His instruction included listening to echoes from a friendly submarine, noting changes in pitch as it moved underwater, and estimating bearing and range. Yet he has never since heard an echo from a submarine at sea. The book therefore distinguishes formal training from tested proficiency: after months of listening without a real contact, Ellis may fail to recognize an echo or to draw the rapid deductions an attack requires.

That uncertainty is sharpened by the precision demanded of the weapon that follows. A depth-charge dropped within ten yards of its target may mean victory; at twenty yards, failure is certain. The difference may turn on a practised operator’s prompt response rather than a slower one. Ellis also has to work under pressure, knowing that the delicacy of his adjustments and his interpretation of changing echoes can determine whether Keeling succeeds. The equipment is not an independent source of certainty: its signal must be heard, understood, and communicated accurately.

Reports reach Krause in short, standardized updates, including contact bearings, ranges, and assessments such as distant, possible, or pill. Those concise calls let the bridge revise its picture while the ship is moving and coordinating with other destroyers. But brevity does not remove ambiguity. When a contact disappears, the report establishes that sonar can no longer hold it in its beam; it does not establish that the submarine has been destroyed. The distinction between losing a signal and resolving the encounter remains central to the pursuit.

Operational limits and countermeasures

The narrative sets out firm limits on what sonar can do. It cannot even roughly estimate the pursued submarine’s depth, produces no results at ranges closer than three hundred yards, can be used only at speeds of twelve knots or less, and is deafened for several minutes by depth-charge explosions. These restrictions create a dangerous gap between detecting a target and attacking it. The destroyer needs sonar to refine its approach, yet its own attack can temporarily silence the instrument, while close range removes the very information needed to judge the target’s position.

The book compares this handicap to a duck hunter trying to swing a powerful gun with weights on his wrists, unable to estimate the duck’s height and forced to shut his eyes before firing and keep them shut afterward. The image captures not just imperfect equipment but a tactical mismatch: the destroyer must act while its means of observation are constrained. A proposed improvement—throwing a depth-charge a quarter of a mile ahead—would create a new danger, since the charge might explode as the destroyer passed over it and blow out her bottom. Better reach cannot simply be purchased without risk to the attacking ship.

A U-boat can further frustrate detection through its maneuvers and countermeasures. The pillenwerfer confuses the sonar picture, and the cold-layer echoes suggest that the water itself may distort the search. When sonar reports no contact, depth-charges may still be released, but the explosion then deafens the system and leaves the next phase of the hunt dependent on renewed contact. Sonar is thus caught in a cycle: it guides the attack, the attack interrupts listening, and the submarine’s evasions make reacquisition uncertain.

Pursuit, expenditure, and significance

Sonar’s reports help keep the U-boat away from the convoy even when they do not produce an immediate kill. During a three-hour chase, Keeling and the cooperating destroyer Eagle have not damaged the submarine, but have forced it away to the flank and out of the convoy’s path. Changes in reported bearing guide the ships’ turns and attempts to intercept. The pursuit is a practical achievement of detection and coordination, though its success is defensive and provisional: the enemy remains dangerous while its position is not decisively settled.

The limits of detection meet the limits of ammunition. In the last twenty-four hours Keeling has expended more than seven tons of high explosive, and after another pattern only six depth-charges remain. Because sonar cannot supply a dependable depth estimate and the target may be difficult to hold, reducing the size of a charge pattern sharply reduces the area searched: halving the pattern leaves only a quarter of the chance of a hit, while dividing it by three leaves one-ninth. Yet even a charge that misses may have a moral effect, compelling a U-boat to evade. Krause must weigh that temporary pressure against the shrinking supply and the possibility of further action.

The sonar’s recurring reports therefore do more than advance a submarine chase. They expose the difference between contact and knowledge: a bearing is not a course, an echo is not necessarily the submarine, and a lost contact is not proof of destruction. The instrument can make a concealed opponent legible only in fragments, through a fallible channel and under conditions the enemy can exploit. Keeling’s response depends on treating those fragments as useful without mistaking them for certainty.