We have a screw type
backstay adjuster, operated with a
winch handle, that was standard issue on our mast
head rig (42' yacht). Tightening the rig will definitely take the leeward sag out of the headstay in strong winds, flattening the
jib. To some extent that will bend the middle of the mast forward somewhat, also flattening the main, but it is the
jib that is most effected and makes the most difference going to
weather. Running off, we slacken the backstay, allowing the jib to "belly out" somewhat, giving the sail more
power. In heavy winds, we also want the headstay quite tight when
furling the sail although we free the jib
halyard a few inches to take the strain off the
furling gear. By doing so, I can furl the jib, even in heavy
weather, by hand rather than needing a winch.
On a fractional rig, 7/8ths etc, the backstay adjuster has more to do with the shape of the main than the jib. However, on a
Catalina 30 (depending upon the year) with forward and aft lower shrouds, the effect of the backstay adjuster is limited in this respect as the lowers "lock" the mid-level of the mast in place. (Aside from the fact that I've never seen a fractionally rigged Cat 30.)
FWIW we had a backsay adjuster on our
Cal 2-29 when we had that boat (1976 to 2001) and have on our
current yacht (Bene First 42). Used to advantage, they can make a significant difference in the performance of the yacht. (Half a
knot doesn't sound like much but over a 24 hour run can give one a 12 mile advantage).
Again, FWIW...