Currently you can repair your ship back to full hull if your hull is at 50% or higher if it’s below 50% you can’t do anything. This seems backwards from both a "realism" perspective and a gameplay perspective.
From a realism perspective this is saying that as long as your hull isn’t too badly damaged you can repair it to the same level as the professional shipyards in London. In fact, you can repair it better than the Leadbeater & Stainrod shipyard and just as good as the high-reputation Cotterell & Hathersage shipyards. This doesn’t make a lot of sense. It seems more likely if your ship was very badly damaged that you could make at least some attempt at repairs, jury rigging some things together that at least let you limp along, but that it wouldn’t be near as good a job as what could be done in drydock.
From a gameplay perspective allowing badly damaged ships to get somewhat repaired presents the player with an interesting choice. For example suppose a player can get their ship back up from 25% health to 50% health, if they’re far from London, maybe that lets them continue on their journey to whatever their planned objective is, but it’s a big risk. Or maybe their at 15% and low on supplies, are they better of spending some supplies and getting the health back up to 25% or heading back to London at 15% and hope they don’t run into anything. Repairing a ship that’s in pretty decent shape to 100% presents no risk (other than consuming supplies) and therefore doesn’t make for a very interesting gameplay choice.
I think low hull health ships should be able to do repairs, but only to a limited extent, slightly damaged ships should not be able to do repairs at all. You can imagine a simple system which is the current system simply inverted – ships below 50% can repair to 50%, ships above 50% cannot repair at all. You could also come up with a fancier system where a ship can repair some percentage of damage suffered while at sea, but never close to 100%. For example suppose you allow repair of 50% of the maximum damaged suffered, but never more than that (although this would require keeping track of how badly damaged the ship had been over time) – in this system if a ship went to 90% health, it could repair to 95% (half of the 10% loss), if a ship went down to 50% health it could repair up to half of the 50% loss, so up to 75% but no more, or if a ship went to 10% health it could repair up to half of the 90% health loss so it could be repaired up to 55%.
Regardless of the system devised, in my opinion, a ship at sea should never be able to repair to the level of the professional shipyards.