Basically the TDC keeps altering the firing solution over time and takes your sub's movement into account, so each torpedo is fired to that solution despite the delay between shots. In theory, if you knew that your solution data was 100% accurate, you would never fire a spread - you would fire each torpedo in sequence at a zero degree angle to your solution.
However, it is highly likely that your target speed, AOB, or even target ID (and hence range) may all be a bit off. This means your solution will be WRONG, so if each torpedo is fired at the native TDC solution they will all miss.
A spread is an insurance policy against small errors in your TDC data input. As an example, let's say you have estimated the target's speed at 5 knots, and it is moving from left to right.. Fire one torpedo as per normal - let's call that the "5 knots target speed" torpedo. For your next shot dial in 1 degree to port- that will be your "4 knots target speed" torpedo. Then for your third torpedo dial in 1 degree to starboard, and that is your "6 knot target speed" torpedo. There is virtually no chance that they will all hit, but at least one probably will... This is the way to go if you think it is unlikely you will get the chance to set up for a second attack if the first one fails.
Obviously the exact degree of spread deviation is a bit of a guess. I always use 1 degree simply because from reading actual patrol logs that seemed like a pretty standard approach... I find it works quite well. Plus it's realistic!
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