I've minotored this over several campaigns and as far as I can see its pure chance.
I've had fleets escape multiple attacks unscathed, and others that sink on the first attmpt to decline a battle. Its just another example of a broken game mechanism.
In my opinion ships should always be able to evade battle as long as their speed allows them to outpace the fastest ship in the enemy fleet (assuming of course that ships are first modelled correctly on their historic counterparts). Likewise ships that escape a battle afloat should always remain aflot after they have escaped. It's up to the attacker to ensure that it has the ships necessary to catch its prey.
Plus, All evading units land and naval should be retreated in a direction chosen by their owning faction.
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