Warning
Hello,
Dyform is definitely non-indicated for this backstay, at least with a hydraulic adjuster. This is because both core and cover are laid up in the same direction (it's part of how Dyform can have such a large cross-sectional mass), so it unwinds more under load, and has little inclination to rewind once the load is released. Apparently there have been some exciting backstay failures due to this. So if you have a Dyform backstay, be sure you have a mechanical adjuster. Navtec might have more info on this.
Meanwhile, 1x19 is definitely more elastic than either rod or Dyform, but I'd check the geometry and scantlings before going up a size; if the backstay angle is sufficiently greater than the jibstay angle to the mast, and especially if the old rod backstay is oversized to begin with (it happens a lot), then the same size 1x19 might be the right size.
There are places, by the way, where mixing rod and wire can make sense. Lowers of wire, for instance.
Fair leads,
Brion Toss
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