Patch kits
Each Hull Plating level unlocks a slot; filling it costs 20 gold in port. Nothing refills a slot for you — a kit is property, not a grant, and the rack empties as you spend it.
Repair
Full repair costs 60 gold per 100 missing hull, rounded up. Every coin you spend buys hull — there is no all-or-nothing cliff, so a partial purse still buys a partial repair.