I remember reading something about this ages ago. I'm sure when the chain attached to the anchor is vertical, the anchor can be easily pulled free (via a winch) from the sea-bed. It's only when it's being pulled at an angle that it lodges itself.
That is probably very wrong...
so you have to drive the boat 'til it's above the anchor? that kinda mades sense but what if it's hooked under something. I was trying to work it out by thinking about a grappling hook i reverse. you couldn't pull that free by making the rope vertical (and wouldn't want to 'cos it'd fall on your head)