Shoot the ship until it is weak enoguh and then 4 little green circles appear, once you have these switch to grapples and fire over 4, 1 for each circle and you will become bound to the other ship. Off the cannon, move to the edge of the gunwhale (edge of your ship, rail or what ever) and hit 'control' once you get teh prompt. it has recently begun to work.... HOWEVER !
Once aboard i usually end up sunk as either the ship I am on is sunk by others or my original ship goes down at the target ship contiues to fire...
I'll work it out yet
