Add a new rule to the dynasty rules entitled “Looting”, with the following text:

As a weekly action, a Captain who possesses a ship may attack another Captain who has a ship.  The strength of each Captain’s ship is equal to DICEX, where X is the size of that captain’s army.  A ship’s strength is determined at the beginning of every attack as well as every time it comes under attack.
The captain who has the highest strength is declared the winner of the attack (even if that captain is not the attacker).  The winner of the attack may take from the loser an amount of gold up to 100 times his rank (i.e. a rank 3 Captain can plunder up to $300) by rolling DICEY where Y is the total amount of gold the losing captain currently has according to his GNDT statistic.
A Captain who attacks a ship with an army size of ‘0’ automatically wins the attack, but can only take half of DICEY as his winnings.

I was originally thinking of just having the attack fail if the rolled strength wasn’t high enough, but then I decided it added more strategy if the defending Captain could attack back.