When i posted this spread angle calculation, the last part of it was cut off. I've now restored the rest of the calculation.
@Tarjak. Yes, you do the longitudinal spread (as the Americans call it) too, and it is by far my preferred spread technique, but doesn't use the spread control dial at all. Wanting to finally use that effectively was what motivated me to convert the equation to the metric system
In terms of analysing the advantages and disadvantages, i could say that the longitudinal spread has the enormous advantage of requiring no calcualtion, and being very simple to implement. It's disadvatage is that if a ship detects the attack and attempts an evasion, then if it can evade the first torp, the chances are great that it can evade all the torps. This is not the case for a true fan shot. However, even when using steamers against destroyers I don't think i can recall a single successful detection-evasion manoever except in the case of a stationary target suddenly accelerating.
The more realistic disadvantage of a longitudinal, or indeed any manual spread over a programmed spread, is the longer periscope exposure required to make the spread, unless you are prepared to shoot blind for some of the shots.
That notwithstanding, i don't think this technique is going to be your number one choice. Still, give it a go. I got the courageous with three out of four on the first attempt