This is called a baby stay, and is usually there to prevent the mast pumping. Most likely you have single lower diagonals which lack the support of typical fore & aft lowers, esp with in-line spreaders.
Some rigs use an adjustable babystay to induce mast bend. Adjustment can take the form of a multipart tackle at the base or an adjustable track on the foredeck.
The stay is probably not crucial to rig integrity, but if you remove if you may find the mast is less stable under some conditions, esp upwind. It's probably not necessary when running deep.
btw if it is causing you some grief when you're tacking (hanging up the jib sheets) disconnect it and thread it through a piece of 1 1/2 " pvc or other plastic pipe. (Black is least visible) the sail and sheets will slide by the larger diameter no problem. Make the pipe tall enough that it's in the area that the sheets typically hit on the way by.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
Last edited by Faster; 07-04-2008 at 10:23 AM.