Harbors reportedly protected by chains!

Probably all of you have at one time or another read about famous cities on the coast, which reportedly had long chains stretched across their entrances, designed it seems, to keep enemy ships from entering the harbor.

Constantinople, is one of those cities, which reportedly had a chain anchored on one side at or within the Tower of Galata /Pera, and the other end somewhere within or near the walls of Constantinople or Byzantium itself. Within the pages of this very site, there is an article which states that this chain, which is mentioned in the Crusader attack on Byzantium, stretched for 1,500 feet.

If we have any mechanical engineers or metalurgists on this site, I just wanted to ask; Just how reasonable it would be to anchor a chain, strong enough to deter a large galley? Just how possible would it be to make one that would not sag or sink under the water level, if designed to prevent the galleys from going over it, or how would they keep either end low enough to keep vessels from going under it? Just how much strength would the chain have to have to do either? And by what mechanical mechanism would be able to stretch the chain which, realistically must have been lowered into the water to a depth by which friendly ships might safely pass? Other questions might well be entertained?

Could it be we have all been hoodwinked?

Thanks,

Edited by opuslola - 05-Oct-2009 at 19:49