I have 2 roving stacks currently in my game (one had a 200-unit Kasuga, another 80-unit ship and 3 gunboats; the other has a 132-unit, an 80-unit and 3 gunboats). Both of these fleets are able to sink the 80-unit or so single ship stacks the AI likes to send to harass your ports and trading lines on autocalc while taking no damage. My generals were rank 4-5 already so I don't know if that affects it.
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