The privateers have the ability to drain ships for fuel with their 
cloakers. This can be quite a problem especialy if you dont have an 
economy that is developed enough to make minefields everywhere. One way 
to reduce the impact of fueldraining from the privateers is to have 1 
unit of fuel on board ships orbiting planets and then load 1 unit of 
fuel every turn. The privateers can only get 1 unit of fuel evry turn 
this way.
   Another option is to set your battleships to intercept your frighters 
this way he can't tow frighters away and destroy them. He should always 
be kept in the dark as to wether your battleships have 1 unit of fuel or 
if they have enough fuel to do an intercept. He cant drain all your 
ships at the same time so these precautions are going to make life hard 
for him.