Capture Luther aboard a navy ship
 

How do you do that? Every time I sink a navy ship it doesn't say anything. Do you have to board a flagship or something? This is part of the treasure unlock: the nine rogues quest.

It took me a while. Just stop trying and Luther will come!

Actually I just kept sinking ships till it told me I had him. Its gives u sailing EXP and helps your quests.

Simon

I actually found Luther by accidently shooting a sloop. I normally don't bother with them but it was being annoying so broadsided and lo and behold it came up I had Luther. Just keep trying like Simon says (sorry...no pun intended) it helps level up your sailing experience.
Kani