@idneon Here's my take as someone who steers the ship 9/10 times as a high-rank reaper:
I move my sails CONSTANTLY during a fight. When you're first learning how to captain a ship, it can be a lot to manage steering and sails at the same time, but it's well worth your time to pay attention to it and practice it. When first starting out, I would recommend leaving sails at one angle with only partial speed as you practice keeping the ship at the right angle to the ship you're fighting.
Many ship fights devolve to chasing one another in a circle, firing at each other a bit. My focus is always to cut the circle smaller to stay on their back quarter. For example, this means that in a clock-wise circle, they are at the 12 o'clock position, going towards 3 and I'm on their back-right quarter at 9 going to 12. One thing that can help me keep this position, once I get it, is to angle my sails to angle my sails to match the angle of the ship I'm chasing. If both ships catch wind at the same time, the shape of the circle doesn't change and I can keep my position.
That's level 1. Level 2 is where fights are a bit more complicated than going in a circle. When your opponent is zig-zagging, running, anchor/harpoon turning, etc, you're fighting to catch up and/or gain a superior position. In this situation, whichever ship is faster on sail changes will come out on top (to some extent). This is much easier in a sloop, where you can do it yourself or have your trusty partner on the sails while you steer, but on a brig or galleon, you really have to have a solid crew that works together to make this happen.
Often, in a brig fight (not a chase), I'll leave the front sail up, back at half. I will focus mainly on steering to keep both of my crew mates some good angles on their cannons while raising and lowering the back sail myself to keep on the enemy's back quarter. Since you have better turning radius with less speed, I don't want 2 full sails anyways. If I can manage both steering and sails, that will give my crew more time to shoot and repair. Basically, the more multi-tasking you can do, the better for your crew overall.