Tuesday, July 9, 2013

Shackles and Angles

I've been working on another article--this one about anchoring bridles--which always draws me into the details. I've posted about energy absorption and the risk of not having any. This time the question is how to form the apex of the bridle.

A commercially-manufactured bridle was submitted to me for testing, a solid piece of work. It was made of 3-strand with the apex formed by making one leg with an eye-splice on each end, and then adding a side leg with a spiced eye, attached to the first with a side-splice (like an eye splice with the eye cut open). Certainly strong enough at shallow angles, but what about at wide angles, like when the bridle is shortened for use on a mooring ball? On cats it is normal to shorten the bridle so that the ball will not hit the hulls, and included angle of about 120 degrees being normal.

My current bridle consists of 2 x 25-foot legs of 1/2-inch 3-strand, each with a large eye on one end and a small eye on the other. The small eyes are captive in a 3/8-inch SS anchor shackle attached to a Mantus chain hook. There is chafing gear inside the eyes and over the splices. Because there are 2 legs, there is some outward pull on the shackle; what affect does this have on the strength? 

Looking at the below chart, at any angle less than 90 degrees I have 70% of rated strength... but the loads are higher because of the angle too. With 25-foot legs and a 14-foot bow cleat spacing my angle is only about 32 degrees and the strain increase due to angle only about 5% I'm probably about 80% of rated strength, or about 0.8 x 2400 pounds  = 1920 pounds Working Load Limit (WWL). 1/4-inch grade 40 chain is a little stronger than that (2600 pounds WWL). But really, with 25 feet of shock absorber, I don't see myself getting there. If I were designing from scratch I could chose a 7/16-inch shackle. But what will the rope hold? If the load is equally spread, (7500x2)/(105%)=14,000 pounds breaking strength or about 1400 WWL. If all of the load is on one leg the WWL is only 750 pounds, so we see the 3/8-inch shackle is a fair match after all. Why not design to match the chain strength? because I'm more concerned about keeping my anchor in the mud and like the softer stretch. It will take a pretty good blow to keep it over 1400 pounds for long.

But what about tying off short to a ball? If the included angle goes to 120 degrees the load rating drops to 60% and the strain doubles. In effect, each leg is carrying 100% of the load all of the time. Still, we remain within the same math; the rope is limiting. This, in a round about way, explains Crosby's advice to NEVER rig beyond and included angle of 120 degrees and generally less than 9o degrees.


(borrowed from Crosby Shackles)

Side Loading Reduction Chart
For Screw Pin and Bolt Type Shackles Only
Angle of Side Load from Vertical  Adjusted Working Load Limit (WLL)
0°   (In-Line)                                       100% of Rated WLL
45°  (90 degree included angle)          70% of Rated WLL
90°  (180 degree included angle)         50% of Rated WLL


But what of the side-splice at extreme angles? 

I made up a side-splice in a bit of 3-strand I had lying about and proceded to pull it apart in a climbing gear test rack I had left over from other days. Though I would call my testing exhaustive, A few things became appartent:
  • Up to an included angle of 120 degrees there is little question it is a full-strength splice. The angles of the strands don't change apriciabley. While some of the turns become sharper, as in a rope-to-chain splice, the unlayd strands can better handle short radius turns than the whole rope.
  • Between 120 and 140 degrees it depends on the lay of the rope; soft lay ropes adjust, while harder lay ropes tend to place more of the strain on certain strands.
  • At 180 degrees the splice begins to behave like a knot. That is, inboard strands can no longer carry load. The splice efficiency rapidly drops to 50%, like a bowline. It will not slip but the strands will fail due to poor load distribution.

The moral of the story, or at least my opinions until they change?
  • Twin legs on a conservatively sized shackle makes a simple, safe bridle.
  • A side-splice bridle is a poor choice for tying short to a mooring ball. It is a fine construction for a standard bridle, but because all of the load is on a single strand, it must be made one step heavier than separate legs, reducing stretch. Not my choice.
  • If tying to a ball permanently make a very heavy single purpose bridle.
  • If tying to a ball over night or for a short term, use 2 docklines, doubled back to the cleats; strong and easy to release). I find carabiners can be a pain to release if single handed or if it's blowing hard.
  • Shackle angles over 120 degrees are a bad idea. So are splice angles over 120 degrees. Join such a bridle with a welded ring and shackle to that.