According to US escort of convoy instructions zig zagging not always considered effective

http://ftp.ibiblio.org/hyperwar/USN/...-Convoy-4.html

1. Zigzagging for slow convoys (speed less than approximately 12 knots) with a broad front is considered to be ineffective in avoiding attack of submarine as well as having the serious disadvantage in the loss of distance made good, and may result in confusion and straggling.
   
2. Zigzagging is considered to be effective and should be prescribed for high-speed convoys and slow convoys of relatively small numbers of ships in which the front is fairly narrow. Escorts conform in general to the zigzag courses of the convoy, but their stations remain relative to the base course.

Of course as war went on most convoys were of a large number so would be broad at front
And most convoys in game less than 12 kts

Adding more zigzags can increase file size by massive amount too

If Japanese convoys always regulary zigzagged then there needs to be some form of course changes added

Japanese convoys were all (with a few early invasion forces) small by allied standards.

If you read US submarine books, you'll see that the skipper almost always ends up plotting the ZZ courses of the ships before attacking.


tater

I have another question, then. When reporting the speed of a zigging Japanese convoy, would the speed reported be the speed of the individual ships, or the net speed along the base course?

No you tail and track the convoy
work out the ship speed
work out the base course around which
the zig zagging is taking place,
make an estimate of the formations
speed of advance along the base
course, and phone in the estimated
course and speed of the formation
along the base course.
tactical maneuvering zig zagging
wont effect this report
but strategic routing zig zagging
would through it off completely
so you stay with em till the
turn and phone it in and hope for the
best.

US boats would almost never radio a contact report though—only if they were completely unable to attack, and even then, I'd imaging they'd let the convoy get away a little so if they were DFed it might not be linked to the convoy/TF.