Bowdash wants me to capture Billington -- help!

[gwek]

I'm trying to recruit Ofrill, and he sent me to talk to Bowdash. Bowdash has asked me to capture Captain Billington from a Navy Galleon or Frigate.

I've sunk at least half a dozen light galleons/frigates, and 1 or 2 'regular' ones, but there's no updating of the quest.

What am I doing wrong? Is this a bug? Do I need to board a flagship? What gives?

Any help would be greatly appreciated.