Why do ships zig zag to avoid submarines?

Answer 1

Zig-Zagging was used in older ships to keep any tracking submarine from gaining a viable target solution, which is vital information required for the torpedo settings prior to shooting. It takes time to calculate specific targeting solution information, and if the ship changes course often enough it makes it extremely difficult for the submarine's tracking party to effectively gain a proper solution for firing.

There are 3 things required for a solution:

1. Target Angle on the Bow (the angle its bow is facing)

2. Target Speed

3. Target Range

1 & 2 are easily obtained by visual and sound methods. However, range is an estimate, and is only calculated after the target has been tracked for a certain amount of time on a steady course and the submarine changes its course. This gives the attacking submarine a known variable in the equation for range.

However, if the target changes course at random intervals, it resets the tracking solution to the beginning. While using Radar (many US boats had Radar capability in WW2) can give you range, you have to be surfaced to use it, and if you're running a submerged attack, that's not an option. Also, WW2 submarines, both Allied and Axis, and today's submarines, employ Radar detection to determine if Radar is being used. This means that anyone using it can also be detected.

Most of the public only know WWII ships using solid colors for their exterior paint, but the reality was that most ships transiting hostile waters had camouflage paint schemes that made it more difficult for an attacking submarine to determine range and AOB.

Older torpedoes were "John Wayne" style - point and shoot while leading the target. Simplified, but essentially correct, though they in fact used a lot of math to get the proper settings for the torpedo. Range was the most vital element in the solution - too short or too long and you'd miss, giving away your position in the process.

Modern torpedoes are not only wire guided, they use active/passive sonar to find, lock on and track down their targets. Range is still a factor, as all torpedoes have a range limit. However, with today's torpedoes, you merely need to get it close. If the torpedo misses and the wire is still intact, Fire Control can manually steer the torpedo to the target. Its Sonar though, is its best asset in finding and running down a target.

Even today, all modern submariners involved in tracking solutions are required to learn how to do a solution manually using the old methods, in the event the computers aren't available or are damaged. I had to learn it myself, as have thousands before me and after me.

The reality is that if a modern torpedo gets an active sonar lock on its target, the chances of evading it successfully are slim and none. If you ever saw Red October, you've seen a homing torpedo simulation and how it works, but the reality is they aren't that easily fooled with countermeasures.

But Hollywood and reality aren't usually on the same page.