Not sure if it can be, or is coded into the game, but an armed trawler would never launch DCs to explode at periscope depth. They simply weren't fast enough to get out of the way of their own ordnance.
It's all math and a formula involving the DC's sink-rate, the launching ship's speed and the blast radius of the DC.
DEs with a speed of 20 knots covered 10.2 metres per second. Depth charges varied in their sink-rates from the 1.8 metres per second of the Mk 2, 3 & 4 to 3.5 m.p.s of the Mk 8. If we assume a generous average sink rate of 2.5 metres per second, it would take a DC 5 seconds to reach ( and explode at ) 12.5 metres. During that 5 seconds, a vessel travelling at 20 knots would travel 51 metres. Hairy and the ship would be deluged with spray from the blast, but just about safe from the blast radius of a DC ( Don't forget that water dampens blast effect.)
At a sink rate of 1.8 m.p.s. a DE traveling at 20 knots would be 67 metres clear of a DC set to explode at 12 metres. Safe enough.
Launchers were developed a) to enhance the DC pattern, and b) to throw DCs clear of slower moving ships, so that they weren't damaged by their own weapons.
We have to accept that escort commanders, including armed trawlers, knew their maths and could easily calculate a safe depth for their DCs to explode and leave their own vessels clear. A speed of 8 knots = 4 metres per second. To get 50 metres clear from the explosion point would take a vessel moving at 8 knots 12, or 13 seconds. In turn this means that - to be 50 metres clear, a vessel travelling at 8 knots would need to set DCs to explode at a depth of around 32 metres.
So, basically, if escorts do blow off their own sterns, enjoy it. It's fun, but nowhere near realistic.
