hijack

[micko1962]

If you hijack a plane, what is it called when you take over a ship ???

[Bangkok]

Piracy

[micko1962]

Is that not when the crew of a ship steal property from other ships ???

[kempie]

Hijack is not specific to aeroplanes; it can be applied to any form of transport in transit.

[Little Lady]

Hmmm lowjack........    ;0)

[tetherend]

What about mutiny?

[Bangkok]

The first three definitions from Encarta are:

1. shipping robbery on high seas: robbery on the high seas, especially the stealing of a ship�s cargo

2. transport robbery on any form of transportation: robbery committed on board any form of transportation, especially an aircraft
 
3. transport hijacking: the hijacking\~of an aircraft or another form of transportation
 

Robbery and hijacking seem to be interchangeable.  I based my answer on reports from the Straits of Malacca, the most pirate infected waters in the world, where any takeover is described as piracy, regardless of the outcome

[Grunty]

Like many words, piracy depends on who is using it.  In a naval museum in Spain, I saw a poster offering a large reward for the capture of the English pirate, Horatio Nelson.

I agree with Bangkok, but in normal speech it is usually mutiny by a member of the ship's crew and piracy by someone else.