I think its probably behavioral as in the AI just drops anchor as well and shoots at you rather than moving towards you when you stop your galleys and just fire at them. Only seems to work when fighting 1 or 2 enemy ships, not more (then they actually (almost) always move towards you).
So I effectively managed to stop Spain from fielding any sort of decent navy by just parking 2 light galleys on the west coast - it'd send a galleon (sometimes two) right at me whenever it finished buiilding them rather than field a larger naval force.
Galleys are simply great at long range because they are small targets and don't suffer the morale penalty of being next to the enemy ship.
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