Patch kits
Each Hull Plating level unlocks a slot, and filling it costs 20 gold in port. A Galleon has two more slots and a Ship's Surgeon one more. Nothing refills a slot for you: a kit is property, not a grant, and the rack empties as you spend it.
At sea
Hull does not come back on its own out there. A kit mends it, a merchant sells it, and calm water patches 6% if you were going that way anyway.
Repair
Full repair costs 60 gold per hundred missing hull, rounded up. Every coin you spend buys hull. There is no all-or-nothing cliff, so a short purse still buys a partial repair, and you can put out on what you could afford.