As in all things, intelligence not brute force, will hold the key. We must get to know our enemies.
I can't see them arming merchant ships, I think it's a good idea, but I can't see it. Daylight attacks could easily be fought off, but at night would be a whole new story, especially if they adopted wolf pack tactics mounting multi pronged attacks.
"I am totally responsible for what I write,... however I cannot be held responsible for your complete inability to understand"