A simple combination of ideas here would be to add a research item that would increase the organization of the units, making them harder to defeat. Better organized thus equals stays in fight longer.
the issue has nothing to do with staying power in a fight, its being able to float while resembling a sieve thats the problem. If strength is down to under 10%, i'd think it fair to assume there is some critical hull damage at or below the waterline by this point. Some ships sank from very little apparant damage, some took obscene poundings and still got home.
while i agree it may be more realistic, i'd be foaming at the mouth i think if i waited a couple years to deploy a BB, only to have it sink after taking 1% losses. in fairness to the concept however, i can't disagree that its possible.
A viable option then might be to have the chance of surviving equal to the ships strength, add to this a bonus from researched damage control, pushing this chance of survival over 100% base, add to this possible related leader traits, add crew experience to this as well. then possibly if doable, have ocean state(calm, rough seas, stormy) detract from this, as a damaged ship is more likely to sink in stormy weather. Im gonna consider a hypothetical here.
DC bonuses from
tech, i will assume 14.5% here
leader trait, 5% here
crew experience, 4.5% here
DC penalties from
organisation, if org is over 75% no penalty, if less then half of missing org below 75% is penalty, so, 74%=.5% penalty, 65%=5%, and so on, peaking at 37.5% for zero org. This is directly multiplying your chance to sink, NOT additive.
weather, stormy weather should impact things, assuming 15% check hourly
sea state, not necessarily linked to a storm, may just be windy, assume 10% for rough seas. check daily
calculate chance of sinking daily.(assume the abstraction as a communication delay, she sank 1hr after combat, but your just now being informed.)
we will have three ships
A)90% strength, 40/40 org
B)95% strength, 20/40 org(12.5% org penalty)
C)80% strength, 0/40 org(37.5% org penalty)
to simplify, the combined bonus is 20%
A)=(100*90% strength)+(20% DC bonuses)=110% chance to survive in calm seas, 100% chance/hour to survive in rough seas(0% chance to sink), 85% chance/day to survive in stormy weather(includes rough seas)(15% chance to sink)
B)=(100*95% strength)+(20% DC bonuses)=115% chance to survive in calm seas, 115%-(10*1.125)=103.75% chance/hour to survive in rough seas, 115%-(25*1.125)=86.875% chance/day to survive in stormy weather(includes rough seas)
C)=(100*80% strength)+(20% DC bonuses)=100% chance to survive in calm seas, 100%-(10*1.375)=86.25% chance/hour to survive in rough seas, 100%-(25*1.375)=65.625% chance/day to survive in stormy weather(includes rough seas)
now, org will recover and as it does, dmg control will improve removing that penalty, Ship C is in no danger of sinking in calm waters and 75% or better org, and ships A and B need only avoid storms and they won't sink either.
And yes, at those levels, a 100% strength/org ship has a 5% chance of sinking in a storm.
if you consider org as a bonus to damage control when above 75%, a 100% org ship gets an additional 2.5% DC(20*1.125=22.5), not enough to save a ship in a storm, but closer, and a little more tech/experience and your set.
on that note, if a storm causes org losses hourly/daily, this would increase the chances of sinking while in a storm, the longer your there the greater the chance of sinking.