Having a hard time convoy hunting in poor weather.

I'm not far off the coast of Ireland, hunting a convoy that BdU sent me a contact for. I've been able to home in on the convoy by plotting an intercept to its route and following hydrophone contacts, but I'm having one hell of a time actually scoring any hits! The seas are very heavy with 15 mps winds, rain, and high waves. It's nighttime and visibility is very poor. I thought that this would make for perfect hunting weather, since I've been able to get inside of the escorts without even trying (and without knowing it). The problem is, I can't get close enough to see anyone, much less sink them! If I run underwater, the waves cover the scope and I can't see the ships until they're right on top of me. If I run on surface I get spotted long before I can see the ships I'm hunting. I dont really understand this, since my Uboat should present a much smaller visual target. I've got my officer on deck and a qualified watchman also up there, but they can't seem to see ships until they are *right there*. I was surfaced, looking at the chart while my men were standing watch up top. I got the "ship spotted" message and when I got on the bridge to look, there was a tramp steamer clearly visible just 300 meters off starboard! Two other times I was running surfaced and was fired upon by destroyers without my men even seeing them!

So, why the heck can't my men see ships when the ships can see me just fine? It's especially frustrating considering how poor the weather is and how big the waves are. I've tried tracking targets using just the hydrophone, but can't figure out how to get a firing solution. "Long range", "Short range", etc. just doesn't cut it.

Any tips?