[Robert A Cook PE]

BTW, zig-zagging was intended to throw off pursuing submarines, not torpedoes. A slower ship (say a submerged sub) can intercept a faster ship if their courses can be made to intersect. The pursuer maintains a course in which his line of sight, measured off the bow, to the prey is constant. If line of sight between the pursuer’s course and the prey is opening (getting wider) the pursuer will pass in front of his intended prey, if it is opening, he will pass astern, if it is constant, they are on an intercept course. (In peace time navigators use the same logic to avoid collisions.) Constant line of sight produces the most efficient intercept course when both vessels are moving at constant speed.

Ah, but in those days of line-of-sight torpedo runs, VERY, VERY short (<1000 yard US torpedo ranges - if you were going to get a hit at all), aimed by a single 1-2 second long eyeball-in-optical periscope siting tracked by a manual plot using magnetic bearings, a GOOD submarine skipper over the course of an entire WAR “might” get 100,000 tons of enemy shipping.

Now, that 100,000 tons was in the form of 2,000 ton, 4,000 tons, or 6,000 ton ships, so the “good skippers” did hit many ships - and could actually start sinking them after the crappy early-war torpedo detonators (magnetic exploders detonated early when they passed under waves, and contact exploders FAILED when they hit a target perpendicularly!) and bad depth-keeping (the torpedo ran deep, so they ran under a ship, and didn't explode at all.)

So a sub had to get very, very close to the targets, aim 3 or 4 torpedoes to “maybe” get one hit on one ship (they aimed ahead of the target's course, at the target bow, at the target stern, and slightly behind the target), and

But today?

EVERY target is 100,000 ton merchant.
EVERY target has one propeller, one engine room, one bridge, one rudder.
ANY hit anywhere aft will disable (but maybe not sink!) EVERY target.
EVERY torpedo has a sonar guidance package, aimed from 24,000 to 36,000 yards away using digital sonar tracking and aiming that can be plotted for hours to the exact course, range, and speed by EVERY submarine.
Silently, without EVER coming to the surface/near-surface to to have “see” the target visually. Which, by the way, is over the horizon anyway.

So, why ever zig-zag now? You're dead. Or dead-in-the-water - which is the same ting as far as a blockading navy is concerned.

[Lonesome in Massachussets]

Zig-zagging was worse than useless during World War II, as well. Submarines would attempt to intercept ships beyond visual range, if they could detect their smoke trails. It might be hours or days before they even sighted their prey.