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Many 7/8th rigs do use running backstays and/or checkstays. Jeff H can weigh in on the many advantages (from a sail trim/shape perspective) that a fractional rig offers - in fact do a search and you'll find many excellent articles on the topic already here. As Paul mentions, smaller more manageable headsails and spinnakers is another 'advantage'.
Whether or not a frac rig uses runners is really a combination: of the intended purpose of the boat (ie racing vs cruising), the flexibility and structural properties of the mast section used, the arrangement of the spreaders and shrouds (ie swept vs in-line) and the interest of the owner in pulling "extra strings".
Many of the current crop of production fracs use varying degrees of spreader sweep angles to aid in supporting the mast section and prebend - and provide some forestay tension without runners. Hunter is most extreme here, with severely swept spreaders and the B&R diamond stay pattern. Beneteau and others are using more moderate sweep angles.
A stiff enough, strong enough mast section will not need runners for structural integrity, but performance will benefit by using runners to tension the forestay. With a bendy top section above the hounds cranking on the backstay will induce mast bend without necessarily adding to forestay tension. Here is where runners serve a good purpose, directly applying forestay tension, and contributing to the precise shape of mast bend in combination with the backstay.
When cruising, however, runners need to be handled on each tack, and while gybing they can really be problematic if the end up getting snagged on the wrong side of the sail. Our boat, a 3/4 frac with a relatively robust section, is rigged for runners. We have chosen not to use them at the obvious cost of pointing ability, but the advantage of simplicity.
Production boats today are built to simplify sailing (eg.. roller mains - which don't really like mast bend much) and so most of their rigs are designed to function without runners... that doesn't mean they wouldn't benefit performance-wise from adding them.
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; 03-19-2011 at 03:33 PM.