Destroyers are great all around units, but I’ve found them quite lacking in actual offensive and defensive abilities. You really need loaded carriers to have a solid defense, and submarines to have a tough punch. This applies less on the European side where land based aircraft are abundant and there’s less naval warfare.
In the Pacific area, I often find fleets consisting mostly or of just destroyers aren’t powerful enough to spar off with mixed fleets of carriers, destroyers, and submarines. They’re just cost inefficient in terms of defense points compared to carriers and attack points compared to submarines (I use the TripleA battle calculator):
288 IPCs of destroyers (36 destroyers, attacking) vs 288 IPCs of carriers, fighters, and destroyers (6 carriers, 12 fighters, and 9 destroyers, defending) = destroyers win 18% of the time, carriers and destroyers win 81.5% of the time. Obviously if the defending fleet was just destroyers both sides win 50% of the time, so having carriers is clearly superior. TUV swing of -65 to -70.
Meanwhile, 288 IPCs of submarines (48 submarines, attacking) vs 288 IPCs of carriers, fighters, and destroyers (6 carriers, 12 fighters, and 9 destroyers, defending) = while the attacker cannot fully destroy the defender, they on average end up with 25 to 26 submarines left (meaning they lost 23x6 = 138 IPCs) after all the defender’s ships are lost (6x16+9x8 = 168 IPCs) with a TUV swing of 32 to 36 IPCs. In other words, significantly better than the destroyers. If instead of carriers we replaced the defending units with destroyers of equivalent value the submarines win nearly 100% of the time and get a 155 to 159 TUV swing.
I hope I was able to demonstrate how, while destroyers are incredibly useful, they have to be complemented by carriers and submarines.
Also submarine defense points are really not all that bad. 1 cruiser attacking 2 defending submarines (12 IPCs on each side) is actually 50-50, while the submarine advantage attacking exceeds their distadvantage defending when compared to destroyers. As mentioned, while the cost efficient attack points of submarines absolutely decimate a destroyer fleet of the same cost 100% of the time, 36 destroyers attacking 48 submarines only win 72% of the time with a TUV swing of 43 to 44, while the likelihood of success for both sides decline at roughly equal rates when the costs of both fleets go down to more realistic levels (9 destroyers attacking 12 submarines, with both sides costing 36 IPCs, win only 63% of the time with a TUV swing of 10 to 14, while 12 submarine attacking 9 destroyers win 92% of the time with a TUV swing 33 to 35).
Cruisers and battleships are unfortunately unbuyable. I’m sure you can find some ideas to rebalance them on this forum.
