I spent most of yesterday to read me up on submarine warfare. It looks like most of the subs were sunk by land based fighters and medium bombers using rockets. So its obvious from a historical point of view that planes should be able to sink subs by themselves, without a present destroyer.
I think the A&A Europe 1999 edition was the game that modelled the subs and convoys in the best way, too bad this mechanics were abandoned. Germany should start with lots of subs, and UK should start with lots of unprotected convoy zones. Then UK buy plenty of aircrafts and destroyers that sink the subs, faster than Germany can launch them. That is historical correct. That would work in a game starting in 1942, but not a global game starting in 1940, or 39.
Still not satisfied with the HR suggestions so far, and the rationale being what Toblerone77 says, the people I play with are not experts, and do not easily understand rules that are too complex. The way some of you wright rules is pedantic and great if you are a layer or judge, but not to my beer drinking friends. So I need a better HR than the present suggestions.
My suggestions so far, and with Raid rules where a damage marker are placed on the convoy box during the combat phase, and not during the owners collect income phase.
Sub cost 8, move 2, A2, D2, roll a preemptive first strike and may submerge after any finished round of combat.
Destroyer cost 8, move 3, A2, D2 and is immune to a subs first strike, so it rolls even if taken as casualty. Prevent a matching sub from submerging on a 1 to 1 basis
Tranny cost 8, move 2, D1 against air only. So it still needs escort protection against subs and warships.
Aircrafts and surface warships can attack and defend against subs even if no allied destroyer is present, but they cant prevent a sub from submerging.
Ex
1. Subs can decide to target a convoy box or join a naval combat.
A convoy raid is a one time roll, and the eyes on each dice decide how many damage tokens you put on the convoy box.
In this HR, a defending destroyer can only deny a preemptive shot against himself on a 1 to 1, not all preemptive shots against the whole fleet or convoy box.
History shows that no matter how many escorts and warships protecting the convoy, the sub would always get a hit, so this is historical correct. But since a sub in this HR cost 8, and can only inflict a max of 6 IPC damage, since the dice only got 6 eyes, it would not be considered very clever to sacrifice an 8 IPC sub in trade for an average 3 IPC damage, when the convoy box is protected by a fleet. A clever player would probably not attack. But it should be an option.
So in this case the sub choose to join the naval battle. Lets say 8 attacking subs roll dice, and this should be 8 preemptive first strike rolls. That means, the enemy warships that get taken as casualties, should not be allowed to return fire. But if you got 2 destroyers in that battle, you can choose to take them as casualties, and in that case they will return fire. But if you save them for next round, they can not deny the preemptive roll for the other ships. Now, if your 2 destroyers sank, and you only got planes left, then the planes can hit the subs without a destroyer, but the planes can not deny the subs to submerge after a finished round of combat.
Another example. A lone sub sit in seazone 118. You can attack that sub with an aircraft, and you don’t need a present destroyer. But after one finished round of combat, that aircraft can not deny the sub to submerge. Only destroyers can deny subs to submerge, on a 1 to 1 basis
Since trannies cost 8 and defend against air on a D1, they can now be taken as casualty any time. History has plenty of examples on trannies being used as blockade runners, thrown into minefields or coastal guns before the fleet comes, just to save the expansive capital ships. It should be the players choice what ship is fodder, not the rules