Thimbles, probably yes
Hello, A thimble or other radius-increasing device is a way to distribute load to more fibers. The idea is that a too-tight radius — think knife edge — will cause the rope to fail, because it puts too much load on too few fibers. And a large radius, like a thimble, will ease the rope's turn around a corner, so it won't be weakened much, if at all. All-Dacron ropes are relatively comfortable with very tight radiuses, partly because they can flatten out as they go around the shackle bail, thus increasing the relative radius, partly because the fibers are relatively weak, so the loads that come on the rope should be much lower than the loads on a high-modulus (HM) rope of the same diameter, and partly because the fiber itself isn't as sensitive to tight radii as the fibers of HM ropes are. If the "built-in-thimble" you used was on one of Wichard's shackles, you will note that the width of the slot is greater than the diameter of an HM rope of about the same strength as the shackle. This means that the HM rope will be able to flatten out a bit under load, increasing its relative diameter. In addition, the radius of the formed part of the shackle is such that you will get a shackle bail/rope core diameter ratio of as much as 2:1. Both of these factors are extremely important for HM rope, as the loads that come onto it are expected to be much higher than Dacron of the same diameter, and because HM rope starts experiencing severe loss of strength when you get down around a 1:1 relative diameter. This is part of why HM ropes are so drastically weakened by knots. Therefore, if your rope is the right diameter for the job (and that's a big if), it really, really needs the extra radius that the built-in-thimble provides. The bail on your jib halyard shackle is almost certainly skinnier than the core of the rope; you might easily have a 100% efficient splice pulling on a 50% or less efficient eye. There are some ways to work around this. Depending on the circumference of the eye, you might be able to wedge a modified thimble into it. Another approach is to take a close look at the strength of the rope relative to the load. It is very common for people to replace a Dacron halyard with the same diameter HM halyard, even though the latter is probably twice as strong. This is a great way to spend extra money on your boat for no reason. But if you did it, the relative load on the line will be trivial, and you can probably get away with a tight turn aroundtthe shackle bail. If the rope is the right size, the sensible thing to do is to cut off the splice and start over. You can either get another thimble shackle, or you might be able to Ring Hitch a large eye to the current shackle. This is the commonest way to deal with HM's tight-turn aversion, as the extra area on the bail offsets the weakening effect of the knot. It also makes for a shackle you can replace if it gets damaged, without cutting off the splice. Again. Any time you replace a halyard is a great time to assess elasticity, lead, configuration, cost, and material. And not just of the rope, but of all the components: thimble; shackle; masthead sheave; stopper; winch; pendants; tack shackle; even the sail itself. You'll often find that, contrary to usual experience with yacht gear, you can improve sailing efficiency and safety, and still save money. Amazing. Fair leads, Brion Toss
