UPDATE September 29, 2017: Upon doing further research on causes for mooring pendant failure I learned some interesting things about failure in nylon line. The Coast Guard has conducted extensive experiments with regard to the longevity, strength, and failure modes of line in order to evaluate its use for mooring buoys. The bottom line on the testing was that there is degradation in strength with time (duh!), and that failure can be sudden and catastrophic when the load on nylon increases rapidly from zero to more than 30% of the line's rated breaking strain. Translating that to my broken 3/8" snubber line. If the ABYC guidelines are close by a factor of two for the strain on a boat like mine in a 42-knot gale (2400 lbs), it is not surprising that a line rated for 3,000 pounds might suddenly snap, especially considering its vintage. Loads well above 30% of rated capacity may have been experienced.
I've anchored in Cuttyhunk hundreds of times over many different seasons. I like to joke that I just drop my hook in one of my old holes and I know all will be well no matter what. I've ridden out one full hurricane, Bob, and numerous close brushes by other hurricanes, tropical storms, nor'easters, etc.
Though some rate the holding as iffy there, I know that if you can get your anchor well set in a muddy spot there is plenty of holding power for anything. Knowing a lot about this harbor, its bottom characteristics, and what it has meant for various generations of my own anchoring gear this is a nearly ideal testing ground for new (to me) anchoring equipment.
A few years ago I acquired a Mantus 45-pound anchor for my 38-foot motorsailor, and I have been gradually testing it during my cruises in Southeast New England. I have been impressed with its nearly instant setting, and its ability to reset when the wind shifts. It holds well on shorter scope, is reasonably easy to handle on deck due to the hoop that forms a nice handle, and it is easy to break out once you get right over the anchor.
Of course, the ultimate test of an anchor is how it performs in a blow. The test came over the Columbus Day weekend in October 2016. We were the only boat at anchor in the north part of the pond, though there were a few boats on moorings downwind of us. One advantage of Cuttyhunk as an anchor testing ground is that the nearby Buzzards Bay tower provides an accurate report of wind speeds. The Columbus Day gale showed peak speeds reached into the low 40s (knots)--not a survival storm, but a good test of a main anchor.
I like to know my main anchor and typical anchoring setup is easily capable of holding my boat in a real gale of wind, without the need to resort to special storm techniques. Having this capability covers 95% of the nights at anchor an average cruiser will experience, and provides a good base to build upon when you find yourself in a more serious situation. Even in a comfortable anchorage in the summer, with good shelter, there is always the possibility of a thunderstorm popping up, and with the ability to hold into the 40-knot range you will usually be fine.
Backing up the well dug in Mantus was 100 feet of 5/16" HT chain, then another 200 feet of 5/8" three-strand nylon rode. I have found that 100 feet of chain means that I am nearly always on an all-chain rode in the shallow anchoring typical along the East Coast of the USA. We eventually had out most of the chain in only about 10 feet of water, so scope was not an issue. I have various snubbers available, but for the night we started out with a 3/8" three-strand nylon line, tied to the chain with my own version of the rolling hitch, and leading back to a bow eye just above our waterline. This takes the load off any deck equipment, provides plenty of bounce to prevent snatch loads, and also lowers the angle to the anchor. In this case, we had more than enough scope out for maximum holding.
I have used a similar arrangement for decades with various other anchors, so I know what to expect. Fortress aluminum anchors, genuine CQR plow anchors, Danforth steel anchors, and a Bulwagga have all held us securely in similar conditions, backed up by similar equipment. The 3/8" nylon snubber would be considered undersized by many, but I have found it provides the right combination of elasticity, strength, ease of deployment, and knot security--I have tested one so much I know it will work. A similar rig held firm in winds up to around 100 mph in Hurricane Bob.
In the October gale the Manta did fine. There was no perceptible movement and when we hauled the anchor up it was so deeply embedded in the bottom that something would have had to break for us to move. As it turns out, something did break--the anchor snubber!
Of course, the snubber snapped in the middle of the night (which is often when anchoring snafus happen), alerting me by the sound of the anchor chain working hard on the bow roller and the boat jerking back a bit on the bar-taut chain. Working by flashlight on deck I quickly deployed another snubber using a chain hook, let out a bit more chain, and we were back to riding comfortably. Using a boat hook I fished over the side to pull up the broken end of the snubber that was still attached to the bow eye of our boat, and I discovered that the line had snapped in the middle. I was surprised by that, since I assumed that if the line were ever to break it would do so at the knot on the anchor chain or where it was spliced onto the bow eye. Nope, the line just exploded in the middle!
I can't ever recall that happening before, indicating this was a pretty strong blow. The line was not the best to begin with, having been purchased on clearance at a bargain store. And, it had lived on the bow, in the sun, for several years, but it had also survived numerous lesser blows and even several pretty intense thunderstorms of unknown strength. My guess is that there must have been a tiny nick in the line of some sort that lead to the failure at that point, though if you look at the photo it seems to indicate a pretty general failure.
The breaking strength of 3/8" quality three-strand nylon is north of 3,000 pounds, but I suspect my crummy rope was much lower. My guess is the strain might have been in the nature of 1,500 pounds or so. A decent pull, but not a survival storm. I do find it interesting to be able to get some idea of the loads involved, even if the measurement is quite crude. I know the max load that line should be able to hold is up around 3,000 pounds, setting the upper boundary, and I suspect the lower limit would be about 50% of the line strength due to the knot holding the line at one end, the splice holding the line at the other end, and the age and condition of the line. Also, a 1,500-pound load is reasonably close to the old ABYC calculations for a 40-foot sailboat in a 42-knot gale (2,400 pounds).
Some have reported that rolling hitches are prone to slippage under high strain. My destructive test proved that to not be the case, though my rolling hitch is not typical. Mine is sort of a cross between the icicle hitch and a rolling hitch. I take multiple wraps around the chain, then multiple half hitches to secure the knot. Using traditional three-strand nylon this type of knot has been slip-proof for me.
This gale taught me a few things. First, I was very happy with the holding provided by the Mantus anchor--no muss, no fuss, no dragging. Did its job.
Second, my old standby 3/8" nylon snubber proved once again that it is plenty for a 38-foot motorsailor up to gale conditions, but it would be better to use quality line in good condition. I am convinced if the line had been of a better quality nothing would have happened. No-name line purchased at a bargain store, used for many days at anchor in all conditions, and left in the sun for several seasons is not the best!
Third, tieing on a snubber line works well, even in high winds, if you use the right knot. My modified rolling hitch once again performed well. Yes, chain hooks can be more convenient and would probably work well in most conditions--I frequently use one myself--but when things get bad I prefer the proven reliability of a knot that will hold the snubber on the chain no matter what without damaging the chain. Using a knot eliminates several points of failure, and also means it is easy to come up with snubbers of various lengths, strengths, etc. It is easy to tie on another during the worst of the storm, if necessary.
Fourth, loads experienced during a gale can be quite significant, though I believe they are somewhat lower than are predicted by the ABYC guidelines.
Fifth, once again I learned that having multiple backup snubbers is critical, along with the means to deploy them. I now have rigged up a very heavy duty snubber that would have more than twice the breaking strain and should be good for more than gale conditions, but I still like using that lighter line off the bow eye for typical anchoring. If it does break for some reason, it is relatively easy to tie on another from deck level then let out some more chain until the strain comes on the line.