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.

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(borrowed from Crosby Shackles)

Side Loading Reduction Chart

For Screw Pin and Bolt Type Shackles Only

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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

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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.

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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.

Eight Days in the Wilds of the Chesapeake

OK, only 5 nights were spent outside of cell and wifi range, so perhaps only those days count, but we had a good time anyway. After all, we wanted to reach civilization for the 4th of July.

The itenierary, which changed from the original plant but was good all the same. All tail winds, a rare occurance in itself.

• Tangier, VA (3 nights)
• Smith Island, MD (1 night)
• Solomons Island, MD (2 nights)
• Un-named Cove near Tilghman Island, MD (1 night)

I'm too tired to tell the story, so I'll just post some pictures, captions, and description where I feel so moved. Not literature. The office misses me and Practical Sailor Mag has me on deadline for several articles and I am thus distracted.

  This is what happens when you leave

 the head window open on a rough 

day; a minor flood of no consequence.

 
Day 1
Light winds, a lot of motoring... and then we hit a fall of 20-knots winds just north of Tangier, which made for some spirited weather work in big waves.  More importantly, we were nicely secured at Park's Marina before a major front charged through. There is really no practical anchoring in the area, and for a $1/foot with a nice bath house and rural quiet at the dock, who would want to? Parks is not the typical big city marina experience.

 Just in time

 

All settled in

Days 2-4, Tangier Island
I've got the activities all mixed up in my head and don't care which came first. Mostly we relaxed and un-hooked. You can get wifi at one lunch place, there are no cell phones, and what's the rush anyway? We swam, caught crabs, ate, explored, kayaked, read... and just hung out too.

 The fishing proved unproductive, but it seems Jessica can always round-up a bucket of crabs with a net and 30 minutes of effort. Crabs that have only been out of the water for 15 minutes are a whole other kind of fresh. 

 The "Uppards" are the norther island of the pair and are no longer inhabited due to erosion and general land subsidence. Nice kayak country.

 Over Tom's Hook

 

 East of Tom's Hook

 This is the main road, so small that a power outage blocks all traffic for the day. It's OK, though; there are smaller roads.

 A major industry is the shedding of pealer crabs into soft shells. The tanks have to be sorted every few hours, lest the soft shells turn into paperbacks any thus be released (pealers are generally too small to make legal hard shell crabs).

 A skiff on the beach in the Uppards. It holds water but just isn't worth enough to drag back the water, I guess. Soft in the bottom, perhaps.

Day 4, Smith Island
A roaring sail with a 20-knot tail wind, but all too short as the islands are within sight of each other. We kayaked, but didn't take cameras. We had the best crab cakes on the planet for lunch; Ruke's store is a hole in the wall, but they don't use filler and do everything right. The Smith Island 7-layer cakes are also obligatory.

Smith island is so quiet I would not recommend spending 2 nights unless it is simply a kayaking base camp.

Days 5-6
Another rollicking downwind sail, averaging over 8 knots for the passage, dock to dock. Chute up most of the time.

Solomons Island. Walked around town, ate, swam in the pool and watched the fireworks. Nice enough.

The Calvert Maritime Museum is well worth a visit, much different from its cousin in St. Michaels. More focused on life in the area (people and marine) than work boats, it is largly indoors, which is a blessing in mid-July. The screw pile light house is a twin to that at St. Michaels, but yet furnished very differently; the St' Michaels house was a spartan man cave for a pair of  watch keepers, whereas the Drum Point Light was virtually on-shore and was occupied by a family with children. The difference in furnishings (more bedrooms, less need to hoard supplies against the risk of issolation by storms) reflects the presence of a woman and is much more friendly.

  A fossil megaladon shark at the Calvert (Solomons Island) Maritime Museum. We wouldn't have mugged for the camera, but it REALLY looks like a comic book shark. My daughter found a fossil megaladon tooth 2 days later in Herring Bay. Big. She really has the eagle eye for fossil teeth and arrow heads.

  

We put miles on the kayaks, tested the Mantus chain hook a few times, ate, drank, watched movies, and stared at the heavens. Nice.

Day 7
Still down wind, but in a fading breeze, all spinnaker running.

The night was spent in an un-named cove off Harris Creek. A little shore line exploration and wading with my daughter. A nice picnic supper on the trampoline, facing into a cooling breeze, as temperatures fell and humidity fell. Our own private zen spot, not in guides, not visited by cruisers. We go there often.

Day 8
A brief stop at Dogwood Harbor on Tighlman Island to look at a pair of skipjacks. There is no anchorage so we tied to the town dock for 30 minutes, before the crabbers returned to off-load their catch in the early afternoon.

A nice slow reach across the Bay in a rising breeze to Herring Bay, where Jessica found the largest shark's tooth we have found to date (sand tiger, over 1-inch).  We anchored near shore for a time, packing up in the breeze rather than in the confines of the harbor.

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A nice family outing.