most people don't use the synthetic stays on beachcats because of the issue of having to retension them frequently (and it can take a while to get all the creep out). I think the synthetic stays do shrink a little bit over the winter when they are unloaded but it shouldn't be much. Franta at Colligo would have the answer as well. Most of the high tech line used (Dux or amsteel) will "creep" meaning it gets longer as the fibers "align" but then are pretty stable and but do, IMO, require some retensioning.