I read a lot of nautical fiction (O’Brian and Forester), and here’s what struck me -
A two masted schooner has a foremast and a mainmast, not a main and a mizzen.
For the time period you’re going for - use points instead of degrees. This:
“When the moon is gone again, wait 60 seconds and then come right about 90 degrees.
Should be southwest. Got it?”
sounds jarring. “Wait 60 seconds, then turn to starboard and come to southwest by west” or something like that would fit better with the technology/time period you’re going for.
I’m not really buying that the clouds are thick enough that ships are completely invisible, but has enough breaks that they can see each other.
Big issue on the horizon, and him trying to escape. You say:
“Now it was a race to the horizon. He had learned that on a clear day, from the deck of a
ship the size of Seaspray, the horizon would be about six or seven miles away.”
First - a horizon of 6 miles means a height above sea level of 25 ft (assuming you’re on Earth), which seems awful high for a schooner.
Second - horizon distance from the deck isn’t what’s important when searching for another ship. You take the distance to the horizon from the top of each ships mast, add them together, and that’s how far apart the ships have to be to be over the horizon from each other. Remember, you only need to see the other ship’s mast to find it, not the hull. So if the schooner has 40 ft masts, and the brig has 60 ft masts, it’s more like 17 miles, not 7, to be safely out of view.
I’m not buying that chainshot took down both masts. Maybe have it that one idiot loaded ball instead of chain and took down one mast.
Ships at sea would never use a gangplank to connect them. You have to have them keep course perfectly aligned to avoid breaking the gangplank. Just lash them together.
Plot wise - why didn’t the traitor tell the other captain that there were another 12 sailors hiding? Change of heart, regret?