Everyday Moorings

By Beth A. Leonard

A quiet evolution over the past two decades has improved two key elements of the mooring system. Have you given your mooring a makeover yet?

Photo of damaged boat after Hurricane IreneBoatUS claim files show that dragging deadweight or mushroom anchors account for roughly one-third of mooring failures. (Photo: David Wiggin)

Mooring systems are a lot like that old song about the thigh bone connected to the knee bone. Most moorings consist of a dozen separate pieces including whatever's on the bottom, two or more swivels, a half-dozen shackles, and a couple of lengths of both chain and line. For your boat to stay where you left it, every part has to be up to the task, and the whole will only be as strong as the weakest link. While most of the losses of boats on moorings during hurricanes in the BoatUS Marine Insurance claims files have to do with failure at the pendant, the part of the mooring lying between the surface of the water and the boat, "everyday" losses — where the boat doesn't stay put in conditions up through gale force — are just about equally divided between dragged moorings, failed pendants, and problems somewhere between.

But the files show that mooring and pendant failures can be reduced — and maybe even eliminated — with new technologies and techniques that first began to find their way into public mooring fields in the wake of Hurricane Bob in 1991. These new technologies — and annual maintenance on all the pieces in between — make it far less likely your boat will go wandering when the wind kicks up.

On The Bottom

Until recently, mushrooms and deadweight mooring anchors — which can include anything from an engine block to rough-hewn slabs of granite a yard or more across — have been the only mooring options.

But both have serious issues. While a deadweight anchor may gain some advantage from suction in a mud bottom, in most cases its holding power is completely dependent on its weight, or, more exactly, on its submerged weight. Cast iron weighs about 12.5 percent less in the water than out; concrete loses nearly half of its weight underwater. The reduced weight and lack of shape to help the anchor dig in to the bottom mean that it takes a very large and heavy deadweight anchor to hold a boat in place in gale-force winds. As you would expect, in various tests deadweight concrete moorings had a holding power of right around half their dry weight (see Table 1 below).

Table 1: Comparison of breakout force for mooring anchorsClick image to enlarge.

Theoretically, mushroom anchors can provide substantial holding power — up to 10 times their submerged weight — if they are sufficiently buried in mud. In most harbors, though, a mushroom anchor doesn't sink more than a foot or so into the bottom, and it is most often oriented pointing away from the prevailing winds. A storm that swings the boat into a different direction will first pull the mushroom anchor upright, and it may not reset when it gets pulled back down in the new direction. In a storm with a couple of wind shifts, the chain can also wrap around the shank of the mushroom anchor, shortening effective scope and decreasing the chance of it resetting. Mushrooms tend to drag through silt and bump along without digging in to denser bottoms like clay or hard-packed sand. In tests, the holding power of mushroom moorings has varied from 2.4 to 5.7 times their weight.

In the last year, all of the incidents of moorings being dragged in the claim files involved either mushroom or deadweight anchors.

Weighted mushrooms are designed to overcome some of these issues. A weight at the top of the shank is meant to tip them back on their side when they come upright, and the sharper edge on the bell-shaped bottom helps them to dig in. We don't have enough data to evaluate their holding power, though.

In the last two decades, two alternatives have come into increasing use and the evidence has been mounting that these are substantially better at staying in one place than the traditional solutions. While all the other mooring anchor types rely on weight or weight and embedment for their holding power, helical screw anchors are screwed directly into the seabed. They have long, high-tensile steel shafts with large screw threads on the bottom and an attachment eye at the top. A barge-mounted hydraulic device is used to install them, and they work in most bottoms.

Comparing the holding power of a helix anchor to that of a traditional mushroom or deadweight anchor is like comparing a wood screw to a thumbtack or paperweight. As Table 1 shows, helical moorings offer an order of magnitude more holding power than any of the alternatives. They're also significantly less sensitive to scope to maintain their holding power, which means more boats can be fit into a smaller area. Helical screw anchors are not completely failure proof, though. They must be installed properly, and they need sufficient scope to allow for storm surge and waves.

Dor-Mor anchors look like upside down pyramids with a point at the bottom and a short shank on the flat top. The mooring lands point down and buries deeply in mud, clay, or sand bottoms. The high center of gravity tips the sharp edge down so that it digs in. When the boat swings, the mooring may shift, but it doesn't come upright like a mushroom. While nothing can compare to the holding power of a helix anchor, in tests, Dor-Mors had almost twice the holding power for their weight as the average of the mushrooms. Dor-Mors now account for 75 percent of the moorings at Woods Hole, an anchorage known for a difficult bottom and strong currents. The Coast Guard has been using Dor-Mors for Aids to Navigation in areas subject to currents. Dor-Mors are more expensive than mushrooms, but their installed cost is similar to helix anchors.

At The Boat

When the wind starts to kick up, in all but the most sheltered mooring fields, waves will soon follow. The cycling loads from waves on the mooring pendant can generate forces anywhere from two to 10 times the static load. If the wave period is four seconds, the mooring pendant will be subject to those shock loads over 20,000 times per day.

In these conditions, the mooring pendant provides elasticity to limit the shock loading that could destroy the boat's hardware or jerk the mooring anchor from the bottom. But that same elasticity results in chafe and may cause the pendant to part if the line passes over any hard spot as it lengthens and shortens. Normal chafe protection can allow heat to build up in the nylon strands, leading to a failure when the internal stresses cause the fibers to melt. There are many of these failures in the claim files. What's needed is a mooring pendant that provides a great deal of elasticity without chafe — a seemingly impossible combination.

After Irene, Seaworthy reported on a new mooring pendant designed to separate the pendant into two lines, one with high elasticity and one with, for all practical purposes, no elasticity and very high chafe resistance. Nantucket Moorings, in conjunction with MIT, developed Cyclone Mooring Pendants, a two-part pendant with the upper part made from New England Ropes STS-12 line coupled to a standard nylon double-braid lower. STS-12 is made from Dyneema fibers, which are incredibly strong, abrasion resistant, and UV resistant. Soft eyes are spliced into each of the two lines, and they're then connected together. A second, longer Dyneema line can be added to make a bridle and act as a backup if the first Dyneema line were to fail.

Photo of chafe resistant Dyneema penchantThis pendant uses chafe-resistant Dyneema on the boat, coupled with standard stretchy nylon line at the mooring.

Dennis Metcalfe, manager of Nantucket Moorings, told us that in the three years Nantucket Moorings had been using Dyneema, he had never seen any signs of chafe. You can buy a pre-made pendant or make your own.

What Lies Between

The anchor mooring and the pendant are only two pieces in the mooring system that makes up the mooring. Traditional systems use heavy chain on the mooring to ensure that the pull on the mooring remains at an appropriate angle even in strong winds, and lighter chain above to reduce the weight on the mooring pendant. Eye-to-eye swivels and shackles are used to connect the two chains. The entire system needs adequate scope for the type of mooring anchor. Failures between the traditional mooring and the pendant almost all have to do with corrosion. The key to preventing failures is a comprehensive maintenance program that includes an annual inspection of every element in the mooring system.

Unfortunately, it's not enough to make sure your mooring is perfect. Your boat's safety also depends on the condition of every mooring upwind of you. A professionally managed mooring field with an annual maintenance schedule and specific standards with regard to the mooring anchor, pendant, and everything in between will reduce the chances another boat will ruin your day. 

— Published: January 2014

Seaworthy, the damage-avoidance newsletter, is brought to you by the BoatUS Marine Insurance Program. For an insurance quote, please call 1-800-283-2883 or apply online at BoatUS.com.

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