Musing on the Strength of Anchor Chain During an Arctic Gale

August 23, 2019
Cambridge Bay, Nunavut
69 06N 105 09W

Sitting out a gale at anchor is not the same as riding out a gale at sea. In the former case, one is moored in a secure harbor as the storm rolls over, and in the latter, he is being swept along with the forces of nature like a butterfly on an afternoon breeze. But which is safer is an open question.

The adage that contains “any port in a storm” would suggest that given his druthers, a sailor would choose safe haven over running off every time. But this cannot be so. If Mo had encountered this blow well offshore, a blow that at its height has seen 40 knot winds, she would have had a couple of screaming 180 mile days with a deeply reefed headsail, and everyone would have been happy, if somewhat sleep deprived.

What made me so ardently seek the shelter of Cambridge Bay for this blow was not the forecast winds but our proximity to land when those winds arrived. Being in the Arctic archipelago means one cannot simply run off. Here there are rocks, islands, and even ice to threaten one’s way. Everything is lee. 

But anchoring is not all bliss, either. Now, instead of betting on the seaworthiness of the boat, you are betting on the strength of the ground tackle. The boat has stopped being a storm riding machine; in harbor, she is simply a hulk tied to a length of chain, upon which all depends. 

Our gale came on during the night as foretold. By morning, winds were 30 from the W and the sky, low and menacing. By noon, we’d been overtaken by a cold, driving rain that hardened the wind. Now Mo heeled well over in the gusts and threatened to spill my coffee. Even with shore but a quarter mile to windward, a three foot chop developed. The small whitecaps had their tops blown off and trailed spume. Mo threw spray back on deck. The whole boat vibrated.

During this I could not help but contemplate the soundness of my ground tackle choices. Here is where they would pay or not.

Mo’s anchor is the SPADE type, a worthy design, but the chain that connects it to the boat does not immediately fill one with confidence. At 8mm (5/16ths), it is a full size smaller than what I expected to find when I did the initial purchase survey, and it took some time to figure out why the previous owners had made such a choice.

All chain is not created equal, and in this case, the chain that filled the anchor locker was of the highest tensile strength available, known as G7. Using this smaller diameter but higher strength chain (rather than 10mm of the more common and less expensive G4 chain) allows Mo to carry more length of chain (in this case, over 400 feet) for the same weight in the bow. More chain equals more deep water anchoring options. A smart move. 

The working load limit of this chain is 4,700 pounds. Is that enough to keep a 35,000 pound Moli secured in a gale? 

It is a question of paramount importance, and yet finding any data on this topic (as opposed to advice, of which there is plenty) is a challenge. Only one reference have I found, an offhand remark by Steve Dashew in his Encyclopedia: “Our fifty foot boat might generate 2,000 pounds of load in 50 or 60 knots of wind.” How he got to this number is not explained.

Today in email I received an article on this topic written by Don Jordan, the designer of the Jordan Series Drogue.* The article’s focus is the safety of mooring a boat by the stern rather than the bow in extreme weather, but it contains the following illumination: 

“Fortunately, very complete data on the aerodynamic drag of all reasonable objects are available from testing in wind tunnels and other facilities. Under these conditions [60 – 75mph winds], the drag of the hull [of a modern 40 foot boat]…will be about 300 to 400 pounds, and the drag of the mast and rigging will be 700 to 800 pounds. We might use a conservative estimate of 2,000 pounds for design purposes. [By comparison,] the breaking strength of a 3/4-inch nylon mooring line is 16,000 pounds.”

Data! What a comfort. And amazingly, it correlates with Dashew’s remark. So it appears that, baring shock loads, (accommodated for in this gale by 30 feet of nylon line), Mo’s ground tackle is more than sufficient to the task of keeping boat and skipper safe. 

6pm. The height of the blow is past. Rain has transitioned to puffy cumulus clouds and the barometer pushes above 1000mb. Tomorrow we fuel in town; change oil and filters, and then we are back on the Northwest Passage course. 

Fewer than 4,000 miles to home.


7 Comments on “Musing on the Strength of Anchor Chain During an Arctic Gale

  1. Whew! So glad to know you and MO and MON-TAY out did the Gale!!! I can imagine what a sleepless time you must have had. I remember so vividly the howl and scream of the rigging in a Hurricane we were in, at anchor, and we kept saying to ourselves, ‘Hold baby Hold!” We had to wear our diving masks to breath and see, when crawling on deck to check for chafe on the snubber, I guess it worked!! We didn’t budge but we had one heck of time breaking out that anchor from the sea bed when the sun came out again!!!

  2. Wow, 4000 miles. Make those miles young man, make those miles. Enjoy the arctic ocean.

  3. Your chain is good as it can support 2,800 pounds (< 4,700 lbs) , but IMO the calculations are potentially flawed and therefore dangerous. Having lost a few boats at mooring as a kid, I have researched the physics 🙂 (the real world testing numbers in the practical sailor article submitted by Ernest Vogelsinger are better) . LOTS of chain is good. 7:1 scope, etc. There are several components to this load, and wind drag is potentially the smallest! (although it works for drogue calcs) The pure academic load in a 70MPH wind force can actually be over 6,000 pounds for this example, if Mo was tied to a SOLID piling with no elasticity 10 meters from the bow. In a storm, a boat can have its line go relatively slack and then taught. The kinetic energy of Mo at 3MPH is over 14,000 Joules, so the stopping "load" is 308 pounds for 10 meters and 4,000 for 2 meters. Happily the long chain is a shock absorber, and the gravitational force of lifting the chain "aborbs" shock. I get the drag force at ~ 200 pounds for the hull bow first, doghouse, rigging. At anchor the water below will be moving as the second component of drag. To continue with this grossly simplified example, the water moving below would likely put an additional 500 pounds of load on the line, and a large wave might do more. The chain's angle might equal the rode's angle and the joint, so the sine of the anchor rode or chain at the bow might be useful, as this also increases load. Where I have also been burned is chafe at the chain to mooring/anchor point (however the energy is "hottest" at the bow and lowest at this point). But I like that article that explained the large discrepancy between the drag calcs and the real world testing, which yielded 4,000 pounds for a permanent mooring and 2,800 pounds for a storm anchor.

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