Convoy Zigzagging

Convoy Zigzagging is the practice of changing a convoy’s course to frustrate a submarine’s approach and reduce its chance of landing a torpedo. In the book, the procedure is more than a general precaution: it is a tactical rule whose value depends on timing, coordination, and obedience. Krause applies it to a ship rejoining the convoy and builds a separate, conditional course-change instruction into his response to a suspicious contact ahead.

Purpose and Tactical Logic

The underlying logic is presented through a practical exercise in attack geometry. A few minutes spent with a manoeuvring board and parallel rulers, working through an approach problem, would show why zigzagging makes a submarine’s task considerably harder and postpones the moment when it can obtain a firing position. The defense does not rely on a course that is permanently safe; it makes the convoy’s movement harder for an attacker to predict and use.

The decisive moment is the shot itself. An unexpected course change as the submarine fires can mean a clean miss. Zigzagging therefore changes the attacker’s problem twice: it complicates the approach and can invalidate the firing solution at the point when a torpedo is launched. Krause’s experience at antisubmarine school, including time in a submarine’s conning tower planning an approach, reinforces the reasoning, but the narration presents the principle as one a careful observer could understand from the manoeuvring problem alone.

That tactical value conflicts with a common preference among merchant captains. They generally resent zigzagging and tend to believe the safer course is to pass through a dangerous area as quickly as possible. The rule asks them to accept a less direct route because speed toward the destination is not the same as safety from attack. The contrast matters: the procedure can be understood on paper and still be unwelcome to the people ordered to follow it.

Modified Zig Plan Number Seven

Krause applies a specified version of the procedure when he orders a ship to make its best speed back toward the convoy. He says he will screen it ahead and directs it to use Modified Zig Plan Number Seven. When the other party questions the instruction, Krause makes clear that it is an order and identifies the start as zero minute. The exchange establishes both a named plan and a defined point at which its execution begins; the text does not, however, give the plan’s complete sequence of turns.

The order links the ship’s return to the convoy’s protection rather than treating rejoining as a simple matter of taking the shortest course. Krause will screen ahead while the returning ship works toward the formation, so the movement is coordinated with an escort’s position and task. The merchant captain’s reluctance does not alter the instruction. For Krause, the plan’s defensive purpose and the need for coordinated action outweigh the preference for a more direct passage.

A later instruction repeats the rule in practical terms. As Keeling draws past Hendrikson and the ships approach the limit of hailing distance, the other ship is told to maintain its course under Modified Zig Plan Number Seven and to watch for Cadena coming up astern. The instruction joins three operational concerns: keeping to the prescribed maneuver, maintaining contact between ships, and anticipating the arrival of another vessel. The plan is not presented as an isolated evasive gesture; it is part of managing movement within and around the convoy.

Contingency Courses and Communication

Krause also uses course changes as part of a conditional warning system. When he reports something suspicious several miles ahead, at approximately bearing zero eight six true, he does not claim to know what it is. He tells the convoy to maintain its present base course and keep a good lookout ahead while Keeling goes forward to investigate. If Keeling fires a gun, the convoy is to make a radical change of base course to zero-four-two true.

The instruction separates the normal course from the emergency response. The convoy is not ordered to turn immediately on the basis of an unexplained suspicion; instead, the signal of gunfire is the specified trigger for a sharp alteration. Krause further directs the convoy to hold that course for half an hour, then return to zero eight seven if no further warning has been received. The defined trigger, temporary course, duration, and condition for resuming the earlier course make the instruction a compact procedure rather than an open-ended alarm.

The plan depends on the message being heard and acted upon. Krause hopes Cadena has understood, and recalls that the Polish captain and British liaison officer aboard her have also heard the exchange and can keep the captain in line. The detail places communication and authority alongside maneuver: a contingency is useful only if the relevant ship receives it and follows it. The passage does not establish that Cadena makes the emergency turn, only that Krause gives her the instruction and relies on the listeners aboard to ensure it is understood.

Formation, Discipline, and Limits

These maneuvers take place within a screen already constrained by the need to protect a large convoy. The escort has only four fighting ships, just enough, when all are in station, to cover the convoy’s broad front with sonar searches. Detaching one or two would leave gaps through which other submarines might pass. That context distinguishes a convoy’s evasive course from an escort’s freedom to pursue a contact: the ships must respond to danger without casually sacrificing the formation’s wider protection.

Zigzagging consequently operates at more than one scale. A ship returning to the convoy follows a prescribed modified plan; the convoy as a whole can alter its base course in response to a clear signal; and the escort must still account for its station and the other ships around it. The procedure’s effectiveness rests on those movements being coordinated, not on unpredictability for its own sake. Its recurring tension is between the merchant captain’s wish to make direct speed and the escort commander’s obligation to keep the convoy difficult to attack. In the book, course changes are both a defense against the submarine and a test of whether a scattered group of ships can act as a formation.