About this problem:
Does your HR have a real impact on the survivability of subs? Yes in some cases. However, I still have my original problem with a single DD and a bunch of planes attacking a stack of subs. Even with my house rule, the subs still have to survive the first shot of the attackers. If there is 1 DD and 5 bombers attacking a stack of 4 - 5 subs, it’s very likely all the subs will get wiped out.
I may have a way to limit the slaughter on the subs.
First, it is clear that neither your house rule (because, with enough planes (A3 and A4) and 1 single DD, it only takes 1 round to destroy a bunch of subs units) nor mine (because, in this situation, there is only subs left as eligible units, so every hits from plane can be allocated to defending subs) have a great impact on this problem.
Maybe you can add this unusual subs escape capacity:
Any hit by a sub under an air attack, which can not be allocated to DD or any other ship, negates 1 hit against a sub unit.
Said otherwise, any successful defensive hit @1, made by any sub, usually lost (because there is no available target) saves 1 sub unit which would normally be taken as casualty.
How to play it in game: is that any unused “1” diced make disappear a successful attacking dice.
Then just have to add up the remaining successful attacking dices to know how many subs are destroyed.
Example 1, 1 DD and 5 Bombers roll 5 hits against 5 Subs.
First case, subs defence is zero hit @1, then all subs are sunk.
Second case, subs defence result is only 1 hit @1, then all subs are sunk and the DD is sunk.
Third case, subs defence result is 2 hits or higher, up to 5 hits, then 1 up to 4 subs are considered saved and able to submerge on the second round (or the end of the first according to HR).
Example 2, 1 DD and 5 Bombers roll 1 hit against 5 Subs.
First case, subs defence is zero hit @1, then one sub is destroyed.
Second case, subs defence is only 1 hit, then one sub is destroyed but the DD is sunk.
Third case, subs defence result is 2 hits or higher, up to 5 hits, then no sub are destroyed.
Example 3, 1 DD and 5 Bombers roll 2 hits against 5 Subs.
First case, subs defence is zero hit @1, then 2 subs are destroyed.
Second case, subs defence is only 1 hit, then 2 subs are destroyed but the DD is sunk.
Third case, subs defence result is 2 hits (one still sunk the DD, the other negated 1 hit), then only 1 sub is destroyed.
Fourth case, subs defence result is 3 hits or higher, up to 5 hits, then 5 subs are considered saved and able to submerge on the second round (or the end of the first according to HR).
Another different way (less in favor of the sub) is to treat defensive overkill “1” rolled by subs against a few DDs and a lot of planes like this:
Each hit made by subs on defence which can not be allocated to 1 attacking unit (because there is only aircraft left) gives 1 immunity against destruction to 1 Sub unit.
So 1 sub overkill (of DD & other ship) means at least 1 sub unit will survive the first round (regardless of the attacker’s dices results).
2 subs overkill means 2 subs units get immunity, etc.
How to play it in game: just put back on the board as many subs units as there is overkill subs defensive rolls @1.
I think this second version is simpler to apply in game.
Do you think it can be the little twist to give a better chance of survival to subs and enhancing them just that much?