Question:   How heavy do the anchors have to be to keep a 100,000 ton cruise ship in place? How heavy are QM2's anchors?
(courtesy of Tom Anthony)

Answer: Its a common misconception that the anchors are what holds the ship in place. In actual fact, the anchor is merely to keep the end of the cable stationary and in order to weigh the cable to the sea bottom.

It is the weight of the cable and not the anchor that keeps the ship in place. Although QE2's anchors are 7.5 tonnes, each link of the cable is 60 kilos and the joining shackles are over 100 kilos each. The common rule of thumb is to use at least 4 times the depth of water as the length of cable and up to 7 times if required.

We used to use the stern anchor on QE2 in anchorages such as off St Thomas if the wind made the approaches choppy so that we could offer a little lee for the tenders. The problem is that the stern anchor on QE2 is over to the starboard side so if the cable led round to port, it was extremely difficult to recover the anchor upon departure.