Why are you wasting a valuable Kriegsmarine resource on a lousy trawlwer? The torpedo probably cost more than that derelict barge. The renown is so puny, as is the tonnage. And you wasted at least two torpedoes on that thing?
Chastisement over.
In such foul weather I run submerged as much as possible, unless I am trying to make haste to get to my patrol zone or base on the return leg. The chances of running into something while surfaced is so low it's not worth it. As it says in Das Boot, "You can hear more down here than you can see up there." I only surface to recharge and then down again. By running at ahead slow I maximize my battery time and only need to surface for air and to recharge.
P.S. Regarding minimum running distances to arm, for the normal torpedoes it is 300 metres, but for the new acoustics I believe it is 400 metres. You also have to take into account the relative motion of the target, which could increase or decrease the distance. A target moving toward you will eat up some of the distance, so you need to allow more than 300 or 400. A target moving away will increase the effective distance, so you can fire at less than 300 or 400 and still get a detonation. The things we Kaleunen must know!