Folks used to say that fastflight was a breaker of wooden bows because it stretched less than a Dacron or linen string.
Well, it does stretch a little less than Dacron, but it's real advantage performance-wise is that fastflight weighs less.
And to be honest the only bows that I have ever seen broken with a fastflight string on them were either radically overstressed or weak in construction.
They probably would have broken sooner or later whatever the string was made from.
Making a fastflight string overbuilt would in fact probably be safer inasmuch as the increased string mass would slow the bow down.
You might just as well say that shooting a heavier arrow will break the bow.
I have an old boo backed laminated target bow 56lb @ 30"(now honourably retired) that has shot a 15 strand fastflight string it's whole competitive life in NFAS, BLBS, EFAA and GNAS.
It has often been criticised by folks with fancier bows for it's 3 inches of unbraced string follow which arose from the boo overpowering the belly from day one, but these same folks have more than once then had the doubtful privilege of seeing this same bow finish in the medals whilst their "superior" bows were in the also ran category.
This bow has also survived the scary experience of a few days experimentally shooting flight arrows with a 6 strand fastflight string, something quite painful on the fingers and making a sound so strained and high pitched that it will not soon be forgotten.
The only reason I use 15 strands is that it gives me a perfect nock fit when served and I figure if I had one on a heavy bow, this and comfort in the draw might be the deciding factor for me in a choosing string thickness, unless I wanted to optimise the string mass by reducing it so as to maximise the cast.
I also use fastflight on a short osage recurve and have had no problems, although my drawlength with this short bow did cause the original Fullerplast varnish to check repeatededly in horizontal lines all along the whole length of the working limb on both back and belly.
This was made tidy by carefully sanding off the offending surface coat of varnish and rubbing the bow with emulsified beeswax.
The original horn overlays both popped off under the pressure of the string at different times, having been fixed in place with an unspecified superglue.
When reattached with Warn system epoxy no further problems were experienced.
What did surprise me was how well the fastflight held a properly made and waxed bowyers hitch, something I had been assured was not likely.
In conclusion, I guess I would have no worries about using fastflight on a sound well tillered bow, though I might make the string fat enough for comfort but light enough for durability on a heavier bow.
But I guess that I wouldn't feel right with fast flight on a single stave yew....
FWIW
Rod.