Fischer

While your comments would be true for individual ships, we must remember that the convoy is made up of lines of the order of at least 5 ships long. Each line would be about a 1km long. The turn would be about a single piont, so the rear ships would have to travel to the turn point before turning. At 12 kts that would take at least 3 to 4 minutes. Moreover the ships do not turn on a sixpence, so the act of turning would take some time. Also, at night, such rapid maneavers would lead to collisions.
I found this on another forum
"The Zig-Zag patterns used by Allied convoys were VERY strict and difficult navigational evolutions... in fact, the Navy sailors used to joke about worrying MORE about getting run over by a freighter than hit by a torpedo.

Each Ships Captain would receive a plan with a series of "Zig-Zag Plans"... so once underway, the Convoy Commodore could order by flashing light a specific "plan" based on a base-course. For example: "Zig-Zag Plan 10, 110* " would mean a base course of 110 degrees, with a 15 degree turn to port after 20 minutes, a 20 degree turn to starboard after 30 minutes, a 25 degree turn to port 15 minutes later, and then a 10 degree turn to starboard after 25 minutes... REPEAT"

One of the KEY problems with the Convoys, is that they moved at little more than 10 knots (11.5 mph), and more often at 6 knots !! A German submarine could move on the surface at almost 18 kts (20 mph)... twice as fast as the convoy.

SO, the Zigzag was used to confuse the fire-solution of the U-Boat... a steady course is EASY to determine when to shoot, it's like leading with a shotgun. IF the target(s) are making random (but coordinated) course changes it either throws the solution out the window, OR forces the sub to approach closer. THIS makes the sub more susceptible to detection and counter-attack.

The freighters move in a "tight" group, zigzagging, while the ASW escorts (Corvettes, Destroyers and Frigates) roam about like boarder collies. The submarine plots it's fire-solution by basic navigation skills, and since this in the days prior to computers, a well-timed "ZAG" can negate the weapons officer's computations.

Zig-Zag navigation went out the window pretty quickly in World War II as a result of:

Faster cargo ships
More escort ships
Better equipped escort ships with ASDIC (sonar) and Radar
Escort Carriers (subs HATE airplanes !!)
British cracking the German codes - NOW you just AVOID the subs."

So it may be possible that our convoy might not even ZigZag at all. However, assuming it does, the abovesuggests that the periods are 10 to 25 minutes and random. also, I believe we would be unlikely to zigzag during the night - there was too much of a risk of collisions.