The Delta rig was one of those flash in the pan ideas that showed up in the late 1970's or 80's. The idea was that jibs were more efficient than sails that were attached to a mast. The concept was that there is huge turbulence behind a mast and that a sail operating in that turbulance is inherrently inefficient. There was also a belief that the interaction between two sails made two sails less efficient that a single large sail.
With the advent of reliable roller furlers, the idea to deal with these two issues was to use one huge jib on a furler. The head of the forestay for that jib was attached to a bi-pod that was leaned aft. The headstay could be tensioned with an adjustable backstay that raked the bi-pod further aft.
Of course the problems were huge; lots of weight aloft, lots of windage, huge sheet winches to tack that guargantian jib, problems with reefing as the sail plan moved radically forward on the boat as the sail area decreased, and in the technology of that era, lots of sail stretch and headstay sag.
The rig found some popularity on Multihulls since it partially solved the mast compression on the main cross beam problem.
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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay and part-time purveyor of marine supplies