I think base value of stats is a factor also. Even though naval battles are a bit biased towards the A.I. even on norm diff, when I put a bireme with lots of exp against a quingreme it will more likely lose but a quingreme versus a highly experienced bireme usually wins.
Also biremes take heavy casualties even if they do win against higher class ships. So all in all the higher the class of ship the longer it will last.
Biremes with 80 men may last on average 4 turns. Quingremes on average seem at times to last forever.
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