On a fractional rig, permanent backstay controls mast bend and the runner (attached at the hounds) controls and headstay sag. A fractional rig may also have a checkstay which controls lower mast bend much the way runners on a masthead rig work. Whatever your rig configuration, you need to make your main flatter as the wind increases. Choppier water requires a bit fuller sail for a given wind strength. You should also mark all adjustments so you can consistently reproduce fast settings.
Off the wind, when about half of full tension should be carried to depower the sail and support the rig.
Running back stays, or Runners are a pain in the behind, but they provide function in maintaining the headstay tension and limit mast bend on a fractional rig. The rotating spar swept back spreader design of the Mac eliminates the need for a backstay. I have always preferred the fractional for the power concentrated in its large mainsail and the Fractional Rig small headsail, benefiting shorthanded sailing)
These Photos show running back bungee restriction on a 27 boat with running backs. The 4 small Harken #417 single swivel blocks combine with 50 of 3/16 bungee using loops made with 8 Hog Rings. The Long bungee pulls the runner/runners forward out of the way when released, and stretch to allow full application of runner on the lower, closer to deck level.
It appears they might apply well to the Mac Lifeline Stanchion base spacing in the right dimensions. (See Photos for possible application) The upper running back can be restricted from going outside and forward of the spreader with a 30 of 3/16 bungee cord above the spreader far enough to allow the tensioned runner full extension. The Bungee is routed between the upper shroud and around the front of the mast, and back through between the opposite upper shrouds
Using bungee cord to pull the runner forward. The bungee wrapped around the mast above the spreader, when applied on the windward rail will pull tension to pull the leeward runner forward for booming out the main to leeward as necessary.
I cannot find fault in the Modern Sweptback Spreader Rig, especially with the Mac "M" rotating spar eliminating the backstay. Modern design has proven backstayless rigging a worthy cruiser and racer. As no one designs fits all demand, they are well engineered; theyve proven themselves with modern builders and have received positive evaluation by experts. I like the design, as does Perry on Design and most others.
We all recognize tension from the upper and lower shrouds in our venture to limit headstay sag to sail closer to the wind. The compression frictions, even with vigilant lubrication, from shroud tension maximized can somewhat impair mast rotation. The fact halyards drop to off mast turning blocks from a lofty height should not cloud the certainty of significant amplification, compounding mast compression with full tension of halyards and rigging terminated off the rotating mast. Compound the force from the upper and lower shrouds, halyards, with a backstay or runners, involve an increasing effort required in turning the mast.