Welcome Aboard!
I'm sorry to answer your question with more questions, but first we need a little information.
1. What version of the game are you playing - Stock or one of the supermods?
2. What in-game year is it?
I ask these because the supermods don't only change graphics and gameplay, but also when, where and how objectives are assigned. As to the date, in 1941 all bases are switched to the French Atlantic coast and you won't see the North Sea again for a long time.
"Patrol Grid for 24 Hours". This is a holdover from the original design, which was to be a branching set of missions, each one based on how you performed the previous one. When we (the community) rebelled against that, the devs changed it and the "24 Hours" was all that was left. You don't have to go to your patrol grid at all, and you don't have to stay there. Doing so gets you extra renown, but many players make up for that by going to previously discovered hunting grounds and sinking lots of tonnage.
No, the ships you sink have no effect on the outcome of the war, or even on shipping itself. There are only two
Nelson-class battleships, but players have sunk many more than that in the same campaign.
"Go out and sink stuff" is the name of the game in the first couple years of the war. "Just try to survive" is what you can expect later. I would recommend picking a supermod and building on that. It will totally change the feel of the game.
I also recommend using SH3 Commander, as it changes the feel of the campaign, doing things you wouldn't think possible from just playing the basic version.