Having a hard time convoy hunting in poor weather.

[Paul Riley]

Interesting CueBall,I had a similar encounter the other day,but only against a single vessel.I was in the N.Atlantic,waves were 15,vis. practically nil with heavy rain.Yet I was determined to nail the ship,as there is a solution to everything.The only practical way was to plot the vessel's course ready for a night attack,and get to about 300m from the target's beam,as any greater than this and you cant see your target as you stated,which also happened to me in the initial phase.A night attack was crucial due to the fact that I would be using the darkness to hide most of the uboat as I struggled to keep a good depth as it bounces up and down in the current.I normally try to maintain a depth of around 9-10m,giving a fairly unimpeded view through the PS and adequate cover.
This tactic COULD work against convoys,but as someone said,radar is a nightmare and your boat would be picked up probably as it bounces up and down at the surface.A possible workaround would be to try and track the lead warship of the convoy just using the hydrophones,and then use the previous mentioned method of depth control as the warship passes in front of your position to get as good a view of the convoy as possible,and then execute your attack as quickly as possible,then make your pre-planned evasion route,if you have one.Again,this of course would only be useful at night,as in the day you would be like a sore thumb,and DC'd to the bottom in seconds.
Also consider the wind direction too,as once or twice before I have had torpedoes that veered off their track and pass harmlessly by the target.If firing into a head wind add +1kt to your torpedo speed to compensate for the drag.Yet,at 300m you are virtually assured a direct hit if all other values are correct,and you dont get a dud.

You probably know much of this already,but hope some of it helps.