In catan seafarers, does using a knight on a pirate ship give you double resources?
No, the pirate ship acts in the same way that the robber does, allowing you to steal 1 resource.
Playing a Knight card by itself does not allow you to steal a resource from a player. Playing a Knight card allows you to move the robber, and if you place the robber next to a player, then you get to steal a resource from that player. The pirate works exactly the same, placing the pirate next to a player allows you to steal a resource from the player.
From the Seafarer rules:


If you roll a &quot;7&quot; during your turn, you may elect to move the pirate INSTEAD of the robber. Like the robber, you place the pirate in the center of any hex you choose--except that you may only place it on a sea hex. You are then allowed to steal one random resource from any ONE player who has a ship adjacent to that hex! If a player has more than one ship adjacent to that sea hex, you are only allowed to steal one card from that player.




If you play a knight card, you now have the choice of moving either the robber or the pirate.


And from the base game rules:

If you play a knight card, you must immediately move the robber. See &quot;Rolling a '7' and Activating the Robber&quot; above and follow steps 1 and 2.

So the Knight card by itself does not allow you to steal a resource, it is only the robber, or the pirate in the case of Seafarers, that allows you to do so.