The Invisible Enemy

[Fisherking]

The convoy should zigzag at five minute intervals. Sub commanders have to observe, target, and fire. It takes time to do the trig once the course is plotted and run times for torpedoes is typically about 2 minutes. Frequent changes of 45°-60° would make targeting of the ships difficult. If submarine contact is made then the convoy should make a 90° turn away from the contact as directed by the escort commander and make course changes every two minutes while contact is ongoing.



An example:
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We should make a turn to the west at once and continue for about 12 hours. then turn North West for 24 hours before settling on our homeward course.

[Franconicus]

I am not an expert on this, but it seems hard to change the course of a big ship every two minutes. Even harder to change the course of more than 30!

[Fisherking]

The ship may take a little time for the ship to respond and to settle on course but it is a matter of changing the wheel the number of degrees to bring about the settings.

If you think it is too frequent we can double the times but it dose diminish the effect.

[King Kurt]

I think 2 minutes is too quick, especialy with an ungainly convoy. I think every 20 -30 minures is more in keeping.

[Fisherking]

The idea is to make it difficult to target the ships. 5 to 10 minutes may do that but 20 to 30 minutes is a very minor inconvenience at best.

In that amount of time a Skipper could set up complex shots, targeting 5 ships, launch all his torpedoes, and be almost reloaded before we changed course to foul any shots.

While 2 minutes is difficult, it is only while the convoy is under attack. If you think it is too frequent leave it at 5 minutes and 10 when not under attack.

It is stressful on the helmsmen but not so stressful as being sunk. We want the convoy to have a plan of action so that the ships don’t panic. It is as much a morale factor for us as it is a deterrent to the Germans.

It also means that the subs have to close to a much closer range to get a good shot which also means we have a better chance to detect and kill them.

To get a good shot the U-boat might even want to surface and risk a couple of quick launches before diving and running away. That puts him in our sights and even the gun crews on the merchant ships might get in some shots.

The frequent changes keep helmsmen alert and under attack it keeps there minds off worrying about being hit. Otherwise they will be watching the water for wakes or imagined wakes and zigzagging on their own, which dose not keep the convoy together.

[King Kurt]

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.

[Franconicus]

Deadline on Sunday, that's alright?

[Fisherking]

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.

I guess we will go with daylight hours. But if attacked at night we had better make some sort of change.