In a marina, boats moor bow or stern to the dock. This leaves the other end free to float about and especially in and out, thus bonking it to bits on the dock. To prevent this a series of concrete blocks are laid into the seabed and chain run between them. this provides sufficient drag and weight to hold the boats off the dock. From this main chain, secondary, smaller, less weighty chains run towards each boat. to these is fastened the mooring line. The weight of this secondary chain pulls the individual vessel towards the main chain and blocks that carry the weight of the hundred-odd boats. Of course, the boats pull on opposite sides, so much weight is counterbalanced. These secondary chains are fastened to the main chain by shackles, these need checking regularly. When you cast of these mooring lines (stern or bow according to preference, ability at docking, nosy neighbours and difficult berthing conditions) the weight of the chain sinks the rope and prevents it tangling around the propellor and shaft thus causing you to dive down and hack it off with a bread knife. In many harbours this is a life-threatening activity and unpleasant. On re-entering your berth,you pick up this line at the dockside,where it's made fast with a preventer (what prevents 'er from sinking to the slimy depths) run it back (or forwards) to the outboard end of the boat, tug on it like billy oh and finally make it fast. If this, and several other little jobs have been done in a seaman-like unfussy and nonchalant fashion, you can now mix yourself the stiffest gin and tonic (in summer, or rum and lemon in winter) and sit back and think that, after all, life is wonderful…