Two things
Hi there,
Actually, three things, the first being to let everyone else know that Mark is new to the site, and hadn't figured out how to start a new thread. Welcome to the site, Mark.
The second thing is that prespinning Dyform is a bad idea, as it simply puts untwisting load on swageless terminals, with no benefit.The problem with Dyform and hydraulics is that it straightens under load, but friction in the cylinder can keep it from resuming its original twist; as it tries to, it can loosen swageless fittings instead, in addition to losing strength and stiffness in the wire itself. Multiple load/release cycles can produce progressively more untwisting, if left unchecked, and the torque can be hellacious.
The third thing is that while 1/4" might be small for the uppers, 5/16" 1x19 or Dyform is probably not worth going to. Larger wire will stretch less, but the hull might move more, as tuning loads are higher. The fact that you have an extra-tall rig just adds to the issue. The first thing I would look at is the stoutness of the chainplate attachments for the upper shrouds. On the Searunners I've seen, these go through the housetop, right where a port pierces the house side. Any give here,or where the chainplates land, will produce slack, and thus falloff of the masthead.
Therefore I recommend going to 9/32" 1x19 or Dyform, not 5/16". You'll keep the same pin and turnbuckle size, instead of having to go up to 5/8".Stay with 1/4" for the backstay, as it sees relatively low loads now anyway. You might also get in touch with John Marples, who may or may not agree with me, but who always has good and interesting things to say about rigging.
Fair leads,
Brion Toss
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