The longer the ship has been up and running it will start to form barnacles. I.e its been up and in use for and hour without being sunk.....a few batnacles will form on the hull. Its been up for 3 hours without sinking used by 2 or 3 chainging crews, multiple barnacles will form (maybe by the hour).
These will put holes in your ship, the less you have - say an hour will put 3 patches of about 10.....this may pop the odd hole every 20 mins. Play for 6 hours on the same ship 18 patches of 10 that would put more holes in your hull more frequently.
These can be removed at any shipwright though at a cost.
I also think scurvy and lemons should be added, in a similar fashion.
