As previously stated, you must get inside the TDZ to even have a chance of hitting. For our purposes the TDZ must meet two criteria:
a. The torpedo warhead is armed. This requires the torpedo to have traveled its minimum arming distance, generally 450-500 yards.
b. The torpedo can reach its target, i.e., it does not run out of fuel before target interception.
The inner size and shape of the TDZ depends on three factors. They are:
a. The distance the torpedo must travel to arm.
b. The chosen torpedo speed setting.
c. The target speed.
The outer size and shape of the TDZ depends on three factors. They are:
d. The maximum distance the torpedo can travel at the chosen torpedo speed setting.
e. The chosen torpedo speed setting.
f. The target speed.
The size and shape of the TDZ holds only as long as these factors remain constant. The moment any one of them changes is the moment the size and shape of the TDZ is redrawn. Also, note that the TDZ is relative to the target ship. You might be in the TDZ when a change of course or speed by the target throws you out of the TDZ.
TDZ – Stationary Target
Constructing the TDZ around a stationary target is very simple. First, since the warhead must arm, draw a circle around the target with a radius equal to the arming distance. Don’t shoot from inside this circle! If you do (and you hit), nothing will happen as the dud torpedo impacts without exploding. Second, since the torpedo must reach its target, draw a circle around the target with a radius equal to the torpedo range for the chosen speed setting. Don’t shoot from outside this circle! If you do you won’t hear anything because the torpedo will never reach the target.
TDZ – Moving Target
Things get a bit more interesting when the target is moving because the TDZ gets shifted along the target’s line of motion.
At a chosen speed setting, a torpedo has a finite amount of run time before it exhausts its fuel. Let’s assume we are shooting a hypothetical torpedo with a speed of 60 knots (!) and a range of 6,000 yards. This torpedo has a maximum run time of 3 minutes. In addition, let’s assume our target is travelling at a speed of 25 knots.
If the torpedo is fired from dead astern of the target, the torpedo obviously has to catch up to the target. At a range where the torpedo must use its maximum run time (here 3 minutes), the range to target at the time of firing will be considerably less than the maximum distance the torpedo can travel. During the 3 minute run time, the target will travel 2,500 yards and the torpedo will travel 6,000 yards. This means the maximum range we can fire from astern and still hit the target is 3,500 yards astern of the target.
Likewise, if the torpedo is fired from dead ahead of the target, we can fire from as far as 8,500 yards ahead and still hit the target.
The standard method of constructing a TDZ is as follows:
- Compute the distance (PP-AP) from the target’s present position (PP) to its advanced position (AP) as follows:
- Plot point AP along the target’s line of motion at a distance = PP-AP from the target’s present location.
- Draw a circle centered at AP with a radius = Torpedo Range. This circle defines the outer limit of the TDZ. Don’t shoot from outside this circle!
The inner edge of the TDZ is drawn in the same manner, except the radius = arming distance. Again, don’t shoot from inside this circle. In general, it will be very rare indeed that you will need to worry about arming distance. Note that the inner TDZ boundary is not necessarily within the circle describing the outer TDZ boundary.