The latest hijacking of the VLCC Sirius Star introduces a new element in the pirate situation. The pirates can seize a vessel in two different ways:
One way is for one or more armed pirates to board an unsuspecting vessel, make their way to the wheel house and demand that the watch officer stop the vessel.
The second way is to bring the vessel under heavy attack with machine guns and RPGs while demanding the vessel stop.
The defense for the first method for tankers and freights alike is proactive lookouts, evasive maneuvers embarked security teams, and so forth. In the second case the defense is to have the crew retreat inside the house, call for help and run at full speed while making evasive maneuvers. With luck help arrives before the ship is forced to stop.
Now, in the case of a VLCC being forced to stop while being fired upon puts the master between a rock and a hard place. What if the master refuses to stop and the pirates hole the tanker?
The only viable solution in the case of these large tankers is escorted convoys.
Eagle Speak has a good discussion of the ongoing situation here at U.S. Defense Department on the Pirates of Somalia