We already tweaked this ship vs. convoy balancing several times, we are quite satisfied with the current balance. Most of the times ships can sink convoys while reaching a 3:1 or 4:1 cost efficiency in doing so (meaning it destroys 3x-4x more resources than you yourself lose). Which is totally enough and acceptable, especially when comparing these values with hard counters on land. In CoW1.0 this was very imbalanced on sea because you were able to sink convoys with an efficiency of 10:1 or so, which is just beyond sensible bounds. If you look at land or sea units you also will never reach these values even when using direct counter units (e.g. 1 tank destroyer cant beat 10 armored cars).
From a logical perspective convoys are not considered to be unarmed, instead they are considered to be armed and escorted by light ships.

Please compare the stats of the units (health & damage values) to figure out how many units of a type you need to destroy a convoy.

I quickly ran the calculation. Without taking into account any Doctrine bonuses 4 Cruisers lvl1 should handily win vs. 10 convoys lvl1. According to the calculation you will likely not even lose a unit and your army will only lose ~25% health.

If this was not the case in your game, the following reasons can apply:
- there was a hidden damage source from somewhere that you did not see
- your ships were already damaged
- the transport ships were not lvl1 anymore, but lvl2 or higher. 4 Cruisers lvl1 will still win (with ~30% health left) vs. 10 convoys lvl2 when both sides were undamaged before, but you will lose Cruisers in the process.

Also keep in mind that you always still have the option for range bombardment, not taking damage at all. So yes, overall ship vs. convoy balance is fine in 1.5.

According to the calculation 6 convoys lvl1 also cannot beat "a couple of subs" if both armies are still on lvl1 and without any external damage source. 2 lvl1 submarines are enough to beat 6 lvl1 convoys; with 1 submarine it would be almost a tie, with the convoys slightly edging out the sub. So an efficiency ratio between 3:1 and 6:1.

10 convoys lv1 can indeed beat 3 light tanks lvl1. Without taking terrain modifiers or doctrine modifiers into account, you would need 5-6 light tanks lvl1 to beat 10 convoys lvl1. Which is still an efficiency ratio of almost 2:1, so still ok. These values were considering the light tanks attacking the convoys while they disembark, making use of the superipor offensive stats of light tanks.

(yes I simplified the resource efficiency ratios a bit, it still depends on how much the units in the convoys have cost and one could argue that losing health lowers the efficiency, just for making calculations easier.)

If land units become "escorted" when they board, then it should cost something to build those escorts. Like combining destroyers into a convoy. That makes sense.

Of wait, we can already do that! We can stack destroyers and cruisers with transports, and they do fight to protect the convoy, and this *does* make sense.

It's adding "ghost destroyers" for free that does *not* make sense.

1.5 rewards "unintelligent" game play, and this is not an improvement.