Do you really need to splice a thimble into that line? Maybe not so much.
Issue 143: March/April 2022
Traditional wire or metal thimbles are common gear on boats, usually spliced into a line where a rope bears on a shackle—for example, a rope anchor rode attached to a chain, or a halyard end. But when it comes to truly high loads—tow lines, drogues, and sea anchors—there is a growing understanding that traditional wire thimbles (the most common type) can do more mischief than good.
The problem with traditional thimbles is that nylon stretches under load. No matter how tight you form the eye and seize the throat, it becomes loose under high load. If subjected to pulling loads and being jerked from side to side, the metal thimble can shift from its proper position and abrade the rope, and its sharp edges can easily cut nylon that is wet and under load.
In other instances, the thimble damage is more annoying than life threatening. For instance, over-hoisting a halyard can cause the thimble to gouge the halyard sheave, which then begins chafing the halyard itself.
This boom end plate could chafe this Dyneema topping lift. A short bit of webbing stops that with less weight and bulk than a thimble.
In most cases you just don’t need a thimble; I haven’t used one in 20 years. Instead, I attach smooth hardware directly to the eye or connect drogue bridles and rode sections with a luggage tag/girth hitch (which is simply a way to attach a loop to another loop or object—like a shackle—by passing it through itself). Note that rope is best connected to chain by splicing the rope directly to the chain, eliminating both the shackle and the thimble. This is mandatory for rodes that pass over a windlass, but I like it better for hand-hauled rodes as well.
To best calculate the types of attachments to make, it’s useful to understand the D/d ratio—the ratio of the diameter around which a line is bent, divided by the body diameter of the line. This ratio helps determine how a line is weakened due to sharp changes in direction; a higher number means the rope is weakened
less by the turn. Single braids, like Amsteel, are still 100 percent strength at a D/d of 1 when used as an eye, but double-braid and three-strand are slightly weakened.
In fact, very few lines break because they are overloaded, because most are sized for stretch, not strength. They fail because of chafe, and most often that is not the apex of the eye. All bends weaken the line, but because there are two legs in an eye, as long as the line is not weakened more than 50 percent, the eye remains 100
percent strength.
While polished hardware, such as a halyard shackle, does not really abrade line, if it seems like there’s potential for chafe, if the D/d ratio is less than 1 for single-braid, or if the line is double-braid or three-strand, I slide a length of tubular webbing over the rope before splicing. This will increase the D/d ratio and provide considerable wear protection.
Specialized coatings, such as Yale Maxijacket, can also extend wear four to ten times (I tested this on a chafe test rig and in the field). They stiffen the rope some, so they’re excellent in jammers, but limit the coverage to areas that are not handled or on winches.
Movement between two lines can be prevented by luggage-tagging one around the other. But doesn’t a knot weaken the line? Not in this case. The luggage tag or girth hitch is unique in that both legs carry half of the load. The bend around the other line weakens the line about 50 percent, but each leg carries only half the
load, so it works out. However, when a rope runs around a pulley, the D/d ratio rules are different. Because both legs carry the full load and because there is friction between moving fibers, an 8-to-1 D/d ratio is recommended.
And of course, that rusty mooring shackle is a different case. The pendant should be well oversized to provide long wear and fatigue resistance. This conservative
sizing will also keep the eye from stretching. Without a thimble, the shackle would eat the rope and even chafe gear for lunch.
Even so, I recommend using a tube shackle for better stability, or at the very least, carefully polishing all sharp edges. Yes, this is impossible on galvanized thimbles, but they seem less prone to shifting than stainless, most likely due to greater friction.
Good Old Boat Technical Editor Drew Frye draws on his training as a chemical engineer and pastimes of climbing and sailing to solve boat problems. He cruises Chesapeake Bay and the mid-Atlantic coast in his Corsair F-24 trimaran, Fast and Furry-ous, using its shoal draft to venture into less-explored waters. He is most recently author of Rigging Modern Anchors (2018, Seaworthy Publications).
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