Anchor of a Ship

I ended up reading a good deal about the Normandy invasion last night. Seems like the designer, engineer who made the Mulberry docks and bridges also invented an anchor thats proper use prevented destruction of the dock in a severe storm.

This particular anchor is called a Kite anchor. Two dock bridge assemblies were assembled on the beaches. The one the Americans put together was the one that was both improperly assembled, and improperly anchored.

Anchors dont stop boats moving like the brakes and a car, but restricts the movement.

in fact you could you could use thrusters to hold a ship in position.

imagine a ship in a fast flowing river, if the river flows at say 10 mph and you get the thrusters to push your boat against the flow at say 10.2 mph you would for all intents and purposes remain stationary or anchored in one spot, So great weight isnt needed.

http://www.google.co.uk/search?q=weight+of+anchor+needed+to+stop+ships+moving&sourceid=ie7&rls=com.microsoft:en-gb:IE-SearchBox&ie=&oe=&rlz=1I7DKUK_en-GB&redir_esc=&ei=J-ILTLqXE6H80wT8uLxo

The earliest known anchors were made of stone (Bronze Age). The one hole anchor mainly depended on weight. I guess if it didn't hold well, you'd get a bigger rock... They also made three hole anchors - holes in the bottom end were used to run sticks through to make the flukes or 'tines'.

Here is a guide for boats: http://www.cncphotoalbum.com/technical/anchorguide.htm

I got it by Googling on "anchor sizing"; there were other hits that might cover larger boats and ships.