The Offshore Voyaging Reference Site

Spade Anchor Failure, Update and Summary

spade failure.jpg


A couple of weeks ago, I wrote an article looking at the possible causes of a Spade anchor bending spectacularly in use and then dragging.

Luckily, the boat was not damaged and no one was hurt, but a failure like this could have nasty outcomes, so it was worth digging into and writing about.

The article also spawned one of the most interesting and useful comment streams we have ever hosted here at AAC.

To refresh our memories, my original ranking on probable causes, most likely at the top, was [edited for brevity]:

  1. The anchor got jammed in a hidden seabed feature and so the stock was subjected to off-axis forces that no modern anchor could have withstood. This was exacerbated by:
    • The very short rode.
    • The snubber issues [see original article], particularly if it was polyester (Dacron), as seems likely, rather than nylon.
  2. The anchor was either previously damaged or deteriorated in use and that combined with #1 (above) to cause the failure.
  3. The anchor had a manufacturing defect that did not show up until an off-axis load was put on it.

I have now changed my ranking and some of the details of each possible cause, primarily based on some of the excellent comments to the last article, and I will also share my thinking on continuing to use Spade anchors.

But first, let’s look at why I changed my thinking—conclusions without process are useless.

This is not the Part 2 article I promised on general anchoring lessons I have learned from this incident. That will come later.

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Todd Edger

Having just received a new S160 for my Corbin 39, I was a bit put off by the quality of the welds and finishing of the shank. This was before I even read the first article on the failure. While still happy with my purchase choise, I can see there might be a quality control problem with the shank. For the money I spent I expected a much better looking shank then I received.

Brian Russell

If you’re referring to the fact that welds are not “invisible” , I.e. ground down completely, one must understand that on relatively thin material like the Spade shank, the weld bead is part of the strength of the weld and cannot be fully removed without sacrificing the strength. The Spade is not about cosmetics, but function and in my opinion, after having used a Rocna for several years and hundreds of nights and several severe dragging events after wind shift, and then switching to a Spade for use in Scotland, Norway and Newfoundland, I have had none of the wind-shift dragging issues. Does it set first time, every time? No, of course not. But when It sets, it’s set.

Iain Dell

My cruising grounds mean that I usually anchor in depths of about 10 metres for which the ‘standard’ ratios of 5:1, 7-1 etc have always worked well. I really had no idea, and I’m guessing that Gavin didn’t either, that these ratios just cannot be applied at much shallower depths. Doing my own research I was shocked to discover how little force is needed to lift the shank at a 5:1 ratio in 3m compared to the same ratio at 10m. This is something that I cannot recall being covered on any training course or in any sailing publication; normally they just go on about scope and even then its inconsistent.

I’m sorry the topic caused some angst but I’m really grateful to Gavin for raising the subject and to you and the other experts for such detailed analyses. You ‘don’t know what you don’t know’ and I’m thankful I’ve found out the easy way.

Carl Linley

Alain Poiraud’s book, The Complete Anchoring Handbook goes into great mathematical detail about the effects of shallow water anchoring. He is mister Spade!

Richard Ritchie

In this frenzy of analysis I suggest that there is a key fact that is being forgotten:
Despite serious damage, it is my understanding that the anchor held and did its job.
This anchor did not “fail”. It did its job. Unfortunately it has reached end of life. That happens.

Drew Frye

In fact, the anchor dragged as soon, or soon after, it bent. This is stated further up the thread.

“In these sorts of conditions we have never dragged. But this time we did….”

William Murdoch

There is a photo of another twisted and broken Spade anchor shank in the Practical Sailor article in the sidebar titled Bent or Twisted Shanks.

Drew Frye

This may have been an isolated QC failure. It’s hard to know how isolated, since most anchors are never seriously tested in use.

Rock climbing and industrial climbing equipment, for example, bases minimum strength ratings on 6 sigma testing. A representative number, pulled randomly from production, are tested to failure, and the strength rating must be 6 standard deviations less than the average strength. The greater the consistency during testing, the lower the required safety factor. This means that no more than 3.4 units per million will fail at less than the rated strength, and in practice, fewer than one in a million will fail in use. For anchors, that means there should be no failures unless the unit is deteriorated (since I doubt there are a million of any new generation anchor out there) or strained beyond the rating (stuck under a rock). Curiously, there is no standard for side loading (there is for climbing carabiners).

So the questions are, does the manufacture (or any anchor manufacturer) test production samples, do they establish and use a 6 sigma rating based on testing variability, and is there any manufacturer strength requirement for side loading? The answer to the first one might be yes, but I’m pretty sure the answers to the second 2 are no.

Rene Blei

Hi John,
You already mentioned, the problem of not enough rode.
Especially when the owner purchased a much heavier anchor than “required” and consequently got away with by using not enough anchor chain, until Murphy’s law kicked in.

John Hansen

We love our Spade and love telling our Rocna friends how great it is. Sad to hear of a failure like this on such a great piece of gear.
Just noticed a similar failure on a boat a couple of slips down from us here in Hawaii. I don’t know the story on it and have not seen the owner but I’ll see if I can find out what happened and report back.

Pete Running Bear

Interesting stuff and as thorough as ever John, although I’ve got anchor fatigue now 🙂
As the snubber length and material was in question I’ll mention that to add a bit of variety here… Some info from the Rocna website. I’m paraphrasing here, not quoting their copy.
For a snubber they recommend a minimum of 10m (30′) or half the LOA, whichever is greater.
For a rode they recommend polyester over nylon for larger boats, but for a snubber they recommend nylon. The inference here is that too much nylon isn’t a good idea.
Incidentally I ordered some nylon anchorplait from Jimmy Green a while ago and even they confuse it with octoplait (which is made from polyester), see email snapshot.

Eric Klem

Hi John,

I would love to hear a review of your polyester rode once you have built up enough data on it. I was very tempted to go that route when buying our spare rode but didn’t know anyone doing it and wasn’t sure exactly what to get. The theory of a more moderate amount of stretch, less strength loss due to being wet and far superior fatigue resistance was quite appealing.


Marc Dacey

Have not read or commented in some time as we’ve been from Twillingate, NL to Charleston, SC where in our quest for endless summer we’ve decided to take the weekend off and actually hang out. We’ve been using our Spade 140 (66 lbs./30 kgs.) with 80 metres of 3/8″ BBB for every anchorage our 16 tonne pilothouse cutter has dropped the hook for some months and its performance has been as consistent as ever in that we put out more chain in mostly shallow (3 to 4 metres) depths and non-severe winds and, to judges by our anchor watch software, it is the chain that keeps us largely in place.

That said, breaking out the Spade invariably brings up a great wodge of the bottom, kilos of which must be boat hooked or sluiced off with buckets if we want to get the shank in place. I am resigned to installing a deck wash, I think.

We had about one actual test recently in an exposed anchorage in Wrightsville NC. A 20-25 knot Northeast wind found us in the morning and we found we were probably 180 degrees to the anchor set, making breaking out tricky. But we managed that with only the traditional profanities and the Spade still looks good. We have a slotted SS plate for the chain and nylon snubbing lines, but only use that rig when more wind is forecast. I will, as connectivity allows, follow this thread with interest. Our secondary anchor is a FX-37 Fortress, which we will likely use a little more in the Caribbean.

Dick Stevenson

Hi Marc,
I believe you will find a deck wash a very valuable companion to have on the foredeck. I have a simple domestic water pump for my washdown pump, which works fine, but is sometimes slower than I would wish. A fire hose would be nice, but short of that, perhaps one of the dedicated deck washes would be better than my set-up.
And a powerful deck wash with perhaps a bit longer hose allows for a good salt water wash of the decks and cockpit when anticipating a good rainfall to wash the salt away.
Any suggestions out there?  
And, I am a firm believer in the chain going into the locker clean of all mud and sea grass, weed etc. I believe that habit has contributed to the surprisingly long life I have experienced with my chain. This is only really possible with a deck wash.
My best, Dick Stevenson, s/v Alchemy

Marc Dacey

Hello, Dick, nice to hear from you and I am still receiving your occasional email reports. Yes, bucketing aside, we have had to remove the large gardening trug in which we flake out chain for a power wash and the obvious course is to swab down on deck. We have 20 amps VAC handy in the forepeak workshop, which means we can repurpose the inline pump we bought to fill our NS trailer water tank to merrily spray down the chain and frequently besmirched anchor well. Hose in, hose out. We even have one of those wands that sends out a powerful stream. And the best part is that there are no fresh holes in the boat. So I will try that next anchorage and see the price in amps. Sandy ground and a bucket are fine, but mud is tenacious.

Dan Tisoskey

Never to good to hear of any story about anchor dragging especially since the anchor being used was sized and rigged properly. Sometimes we just have bad luck – either the sea floor did not allow the anchor to pivot or defect in the weld as mentioned. Perhaps we should look at Don Jordans article on mooring and anchoring from the stern? Especially catamarans which would be more stable with the wind astern vs wind coming at the bow.

Don makes some interesting points in this article:

Drew Frye

I’ve done some testing (and more would be welcomed) that quantified how the rode tension can be increased 30-150% by yawing. The only times I ever dragged uncontrolled yawing was a factor. Reduction of uncontrolled yawing is probably more important than any other factor, including scope, anchor type and weight (within reasonable limits, of course).

Catamarans with a proper bridle and chain rode should not yaw more than monos, and typically less. If they dow, it is the result of a short bridle and windage problems.

Anchoring from the stern is cute and often helpful re. yawing, but try it in moderate conditions first. Rain will pour in the companionway. It will be cold in cool weather, even with the companionway closed. Waves will pound on the transom. Ventilation can be worse or better, depending on the layout. The boat will pitch more. Anchor handling and recovering the anchor are more difficult, probably even if the boat is set up for that. I have done this a number of times on several boats, and it was always worse. There are other ways to calm yawing.

Yaw reduction would be a great topic.

  • Chain rode or kellet (if rope rode).
  • Reduce windage forward. Remove reachers and dinghies.
  • Adding a drogue to the bow can help (it can help off the stern as well, but bow is better).
  • Reduce water drag aft. If you can lift the rudder it really helps.
  • Riding sails can help. Twin luff (V or Y) sails are FAR more effective than a single flat sail. They add some drag, but far less than the yawing they stop.
  • Twin anchors in a V. But this can be a pain to rig and can be rigged wrong.Spin an wrap to chain rodes together and you will be hating life. I would need additional reasons, such as a very soft bottom and very severe weather approaching … which is not that rare on the Chesapeake Bay. But 95% of the times I have set multiple anchors was to nail the boat down so that I could pull test other anchors off the stern.
Arne Mogstad

I don’t know how relevant this is, but I have dived on my anchor quite a lot of times in heavy weather. That is SCUBA dived to observe how the anchor and rode behaves over time (think, many hours in different conditions, I do deep dives and need to decompress for hours). Usually in somewhat heavy winds (30-50 knots) and sea state up to about maybe 0,7-0,8 meter in the anchorage. I almost always anchor in 8-12 meter depth, and around 60 meter rode.

What I see is that the entire rode is indeed lifted up from the seabed. Not all the time, but most of the time. And this in turn puts the load directly on the anchor that will get the entire job of pivoting to the pitching and yawing boat. This turns the seabed into a “gel-like” state around the anchor. Kind of when we were kids and tried to get our boots stuck in the mud (or at least I did). It was easy to wiggle them around when we did small quick movements, even with the mud over the ankles, but if we tried to lift them up, they were stuck.

Anyway, the anchor always moves quite a bit, especially in more muddy seabed. Much more than I would have thought. It does however stay completely still in the axis of pull (I have NEVER dragged my SPADE in more than 600 days on anchor in northern Norwegian conditions). So, to me, that tells me that this movement is quite important, and that hitting an obstruction like a rock, will hugely affect how the anchor behaves, and thereby the load it will experience.

Maybe this is common knowledge, but I didn’t know, and I found it an interesting observation. Makes me incredibly doubtful of anchoring in rock-gardens in heavy conditions!

Dick Stevenson

Hi Arne,
I doubt what you report is common knowledge and appreciate the field report. I have sailed the cold and wonderful Norwegian waters (and swum a couple of times) and I am admiring of your extended observation of your anchor on and in the seabed: even if well dressed.
For one thing, you add field reports supporting my, and others, observation that catenary in the chain means little in boisterous conditions: the chain gets lifted off the seabed, and, occasionally,  when wind, yaw and seas collude, will get the chain pretty straight from bow to anchor.
I would also be curious how often you found your anchor and stock completely buried in the seabed: I would have expected that to occur in the good Norwegian mud and the conditions you describe, but it sounds like you were able to observe the anchor’s behavior pretty regularly.
Thanks for your report.
My best, Dick Stevenson, s/v Alchemy

Arne Mogstad

Hi. So Dick, I think I’ve found the entire anchor to be buried about half the time (?). Very non-scientific number, but I am rarely able to bury the entire anchor just with the engine, unless it’s fairly soft. I find that the flat part that was painted yellow (fluke?) is usually buried, but I can still see the shaft. On a few occasions, I have had to use my hand to move away some sand/mud to see the shaft. The times I’ve dived on it in more than gale winds, it have been buried completely, but not very far. I have only anchored in fairly sandy bottoms in those conditions (because the places that give me the shelter I want, tends to be beaches in my area). In severe conditions, I have always found the anchor in a good orientation, even if there have been wind shifts. In benign conditions however, I often find that the anchor is slightly on its side.

And yes indeed John, it would be hard to believe that there would have never bean anything in the sand that will touch the anchor. What I do think however, is that since it is moving around in this “jelly” that is created around the anchor, I would guess that the anchor would be somewhat able to move away from obstructions or push the rocks away into the sand. And considering that conditions usually worsen over a bit of time, the anchor will often have a bit of time to either push the rock away in the sand, or to let the anchor move a bit. This is however purely guesswork from my part!

I too am feeling a little anchor fatigued and just want to sail and drop the anchor and not worry too much, but I do find it interesting and fascinating. Especially as forces ramp up and things no longer behave the way we expect it to do.

Todd Edger
I love this app. It figures the load and angle of the road at the anchor shank. Accounts for windage, wind strength, snubber elasticity, and swell energy. Well worth a look.

Hugo Janse

Interesting Analysis John, I know of one friend with a big 72ft lightweight racing cat they use as a cruising boat for their family. They bend the shaft on their spade S160 if I’m correct, about 2 months ago. They were on anchor and had some winds in the 40+ knots which made the cat yaw and move behind the anchor a lot. So it seems like the added yawing a cat can have in high wind conditions can put too much force on the Spade shank maybe. Spade sent is sending them a new shaft under warranty and said their anchor was correctly sized.

Stein Varjord

Hi Hugo,

I also come from racing multihulls, but our current cat is properly in the cruising segment, albeit semi performance, as in quite light. That usually means it’s more prone to sailing on the anchor. The reason being more windage in the forward section, creating more unbalance.

Still, after decades of sailing many different multihulls and monohulls, my impression is that cats sail far less on the anchor. The core prerequisite for this is using a two part snubber, one from each bow and attached at least 15 meters (50 feet) down the rode. Even in really gusty winds, the boat just calmly reorients. No sudden jerks. (Some may be inspired to call me a jerk, 🙂 but that’s just teasing, I think.)

The reason for this behaviour is that if the boat changes its angle to the wind, the two snubbers will immediately get a significant difference in tension and pull direction, stopping the rotation quickly. If using just one central snubber, cats will sail like crazy on the anchor. I frequently see this done, and really can’t understand how it’s possible to go on with it.

Stein Varjord

Hi John,
I agree with everything you said, except for the tendency of a cat with a proper bridle to sail more on the anchor than a mono. Decades of experience on lots of different boats tell me this. For me it’s an observed fact, but for anyone else, doubt is the healthy attitude.

My understanding of physics of this seems quite simple and likely. On our present 12,2 metre (40 foot) cruising cat The bridle attachments on the boat is 6,6 meters (almost 22 feet) apart. Typically 10 times as much as on a similar monohull. That means a given change in boat orientation angle to the rode will get a corrective force at least 10 times as early, the windward bridle has no load. All the load is on the leeward bridle leg. This means cats use another balancing dynamic.

Cats are also vulnerable to windage in front of the lateral balance point, of course, but the efficiency and quick reaction of the bridle means it usually doesn’t come into action, unless there are strong catabatic winds with large directional changes in no time. I’ve tried this too, Lofoten, and it’s not ideal conditions (!), but my impression is still better.

Upsizing the anchor for a cruising cat goes without saying. I can understand why the mentioned racing oriented cat wanted to not do that. It can be compensated by always keeping anchor watch…

Hugo Janse

Hi John, I thought the 160 was also on the smaller side for that boat. But spade confirmed that it should be okay to them what I gathered from the owner, so I don’t know. I believe it was in fairly shallow waters, probably 5-10m deep but don’t know the scope that was out. He is a professional captain on 40+metre yachts so I would guess he had a sufficient amount out for the weather. But he did mention they were sailing behind the anchor a lot.

It was just a slight bend in the shaft, not as dramatic as in this article. But still noticeable that these cases are popping up now cats have become way more popular in the last few years.

Christine Erickson

I am so confused. It looks like the shank is bent almost 90 degrees to the side but still attached to the anchor. How is the cause of the bent shank (in the middle of the shank) a weld failure? If it was a weld failure the shank would be straight with the welded attachment point torn and twisted. Am I missing something?

Jacques Girouard

Dear John, first, I want to commend you and to thank you for your beautiful and very educational web site. I am a 70 year old mechanical engineer (and neophyte Island Packet 40 cruising sailor). I would like to focus your attention on the mode of failure (when bent) of a hollow structure (such as the Spade anchor shank) vs a solid structure (like most other anchors). This can be done in a ‘touchy feely’ manner by bending a plastic drinking straw and allowing your fingers to feel the resistance to bending of this thin wall structure. Initially, your fingers will feel that the drinking straw oppose the bending amazingly well (given the light weight of the straw)…and then ‘poof’, the tubular structure collapses, the two walls touch each other and form a hinge that allows you to bend the straw with no effort at all. A rectangular hollow structure is initially very stiff when bent but it fails catastrophically when the two walls collapse and touch each other (or when the corner welds fail and the rectangular ‘box’ geometry is lost …). In contrast, a solid wall structure will continue to substantially resist bending well beyond a 90 degree bend. The hollow shank of the spade will save weight and remain stiff for a while when deforming but it will always fail catastrophically ‘pouf’ past a certain point. You can experiment with bending plastic or metal pipes vs a solid rod of the same material to contrast the behavior of hollow vs solid structures… Sincerely, Jacques Girouard