SPADE  Further Information

Why do anchors break free ?

If you, like me, enjoy anchoring in those small, remote coves, you are sure to have dragged your anchor at some time : no matter what anchor you use, poor holding or strong winds will have made sure of that.

I've certainly had my share of this kind of drama, and I have many memories of long nights of darkness and fear, checking my anchors and my position. And praying! .... Will I drag? Or am I safe enough to go to bed?

But such problems, rather than putting me off sailing, made me think seriously about anchors. I began my study of how anchors work - and of why they sometimes fail to work. Are all anchors equal, dragging with the same load on the same bottom? Will they drag with a constant resistance ? And if they do let go, will they dig in again? In a word, how do anchors behave?

I also studied my fellow sailors, each with his own relationship to his chosen, favourite anchor. How well did they understand their anchors? Were they really wise old "sea dogs", or were they simply acting with blind faith, blind also to the faults of their partner in anchoring? I soon learned that surprisingly few sailors ever really understand what is going on, on the sea floor under their boats.

I investigated the logic of anchor design, and began the extensive series of tests and experiments that led me to create the entirely new SPADE anchor. It is, I am totally confident, the best anchor that has ever existed.

This is the anchor that we can now offer you ! 

Testing method

The interplay of boat, sea and weather is already a complex dance. Attach that boat to the sea floor, itself an enormously variable material which is out of sight below your boat, and you have an interaction of variables that is very difficult to predict.

For a scientific study to be useful, its testing conditions must be reproducible. In order to obtain constant testing conditions, and to allow sensible comparisons between tests, we have deliberately avoided testing in real situations, from a boat.

We had several reasons for doing this :

although we had selected the smallest available sizes of anchors to test, the pulling effort required to drag some of them would have called for a boat's engine of more than 150 HP We didn't have such a boat nor the adapted equipment, heavy rope and shackles, strain gauges...

we wished to be able to observe the anchor's behaviour during the tests, and preferably in very shallow water, in which a boat of sufficient power could not operate. To reduce the number of variables we needed to pull the anchors at constant speed, which a boat under full power cannot do.

although we would always try to launch the anchor in the same place, the natural variation of any sea bottom gave unacceptably large variations, even for repetitions of tests with the same anchor. We have assumed that the large flat surface of sand uniformly subject to the action of waves ensure that sea floor is of fairly constant form along a long beach.

Note  : In Press Reports you can find the results of numerous tests carried out in real situations. The fact that many of these tests contradict each other totally can be put down to this difficulty alone - without even considering the pressures put on the testers by sponsors, or advertisers.

To better understand anchor behaviour, we sought to reduce the variables, and yet remain in a real marine situation. We decided to carry the anchors out by hand, into water about 50 cm deep (knee depth) off a beach. We chose a nearly horizontal bottom of good uniform sand, with a shore area of sufficient space to allow us to use an agricultural tractor to apply the load. Our beach was long, and uniform, long enough so that each test could be carried out on undisturbed bottom.

The mooring rode was made of 8 metres of 8 mm chain attached to the anchor; and a long rope that attached this to the tractor through a strain gauge, with input to a laptop computer. We could thus measure, graph, and most importantly, record the load profiles of each test.

An enormous advantage of these shallow water tests was that an observer could walk beside the anchor as it moved, observing through the water exactly how its behaviour related to graphs produced by the strain gauge. While this would be possible in deeper water, using scuba gear, it would be much less convenient.

Our test set-up also allowed us to closely observe anchors under conditions which are the most critical for a sailor, and yet which the sailor can never observe - under extreme load. Such conditions happen only rarely, perhaps once a year, and when they do arrive the sailor is too busy trying to save his boat to observe how the anchor is behaving. Likewise with tests from a boat, these conditions are hard to produce, or to observe. Our tests required two observers, one walking in the water beside the anchor, the other walking beside the tractor, with direct comparisons being made between actual behaviour and measured load-graphs.

Not surprisingly this study allowed us to confirm the well-known behaviour of some models of anchors, but we also observed some rather surprising behaviour. We found that the results of our tests allowed us to classify the various anchors into two main categories :

Unstable anchors

Those anchors that, once set, remained buried under increasing load, but only up to a certain load. They then broke free from the bottom, and dragged across the bottom surface. Some were able to re-set, repeating a cyclical, or cork-screwing pattern of digging in and holding, then releasing, and so on, as they moved across the bottom. Others were unable to re-set at all, and dragged along the surface with minimal resistance.

Stable anchors

Those anchors that, once set, remained buried under increasing load. Once loads became too great for the resistance of the sea floor, these anchors moved through the bottom but did not break out of it.

It is not hard to decide which is the best type of anchor to have! Even if your anchor does move under extreme load, if it remains buried, it will only move during the moments of most extreme load. Your boat will only move when the worst gusts hit, and then only a short distance - a few metres, perhaps. You will have plenty of time to consider moving to another anchorage, or setting further ground tackle. But if your anchor lets go completely, the boat comes side-on to the wind (creating much greater windage), and may accelerate to a speed where the anchor just cannot re-set. At best your boat may move tens of metres with each gust; and at worst ...... how far away were those rocks?

Let's look a little further at these two families of anchors, starting with the ones you don't want (but which, sadly, you may already own !).

Unstable anchors

That an anchor is unstable is not an accident : it's a result of its original design. At the time it was first designed, the detail of anchor behaviour was not well enough understood. Or perhaps the designer was forced to compromise between what would work in reality, and what would be convenient for the buyer : he was compromised by commercial pressures.

In our studies, the unstable anchors fell within two categories. They were either of the type we can call "flat anchors" (we don't wish to use particular brand names !), which are often called fluke anchors, or lightweight anchors ; or they were of the most common "Plough" type, where there is a hinge between the shank and the body.

Let's look a little further at these :

Instalility of flat anchors

These anchors all (and there are many different versions) have two flukes attached to a common axis which acts as a hinge through the shank, with a fluke each side of the shank. The angle that these flukes can turn, relative to the shank, is normally fixed at about 32 degrees, but some versions allow the user to change this angle, depending on the nature of the bottom expected.

There are two categories of these flat anchors. The first generation, common with French sailors, are as described above (i.e. they lack a stabilising strut). The second generation of flat anchors, more common with American sailors, have the hinge axis extended beyond the flukes on each side of the shank, this extension being a stabilising strut designed to prevent the incorrect behaviour described below.

During our tests we observed both of these types of flat anchors, and describe below how they behaved.

(a) - first generation flat anchors (without stabilising strut).

Contrary to what one might expect, by their symmetrical design these anchors are fundamentally unstable. Numerous studies have already shown this, notably that conducted by Beg Rohu, of the French Ecole Nationale de Voile (National Sailing School) in 1980.

Through these studies, and our own, we see that when a flat anchor sits on the bottom, only rarely will the two flukes dig in at the same moment. Depending on the density of the bottom, or on the angle between the load (the chain) and the shank, one of the flukes will almost always dig in before the other. Under load, this first fluke will be pulled down into the sea floor, causing the anchor to begin rotating around its shank, and thus causing the other fluke to move away from the bottom. Under sufficient load this process will continue until the first fluke breaks out of the bottom, on the other side from where it first penetrated. The anchor again lies flat on the surface. It will then drag some distance before the process begins again. Perhaps ! it may well become choked with weed or rubbish before it can dig in again.

These flat anchors, thus, "corkscrew" across the sea floor, alternately holding, then releasing, then holding again.

In our tests, we obtained many graphs of "load versus distance" that demonstrated this perfectly (please see Figure 1.) The anchor first slides a small distance, offering minimal resistance, before one fluke digs in. The load curve then rises to a maximum resistance of nearly 200 daN (deca-Newton, the metric unit of force), before dropping rapidly to nearly zero again. The process is repeated continuously, as the anchor is pulled across the bottom.

Figure 1: Holding curve of flat anchors

This is a very dangerous behaviour for an anchor. When you first anchor, and test the set with your engine in reverse, you get the illusion of good holding. Many sailors convince themselves (wishful thinking!) that their anchor is well set, and that it will settle in further over time (pure faith!). The sailor then relaxes into the pleasures he feels he deserves after a passage : time for a drink, a meal, a sleep .....

But remember that your motor, pulling in reverse, pulls with a traction equivalent only to about 20 knots of wind ! Testing with this kind of force gives a false sense of security : just when you've taken your corkscrew to your second bottle, your anchor may get the same idea. The winds may increase, and at 25 knots of wind, your anchor may begin to cork-screw ! Your dream can rapidly turn to a nightmare.....

Anchor manufacturers are fully aware of this problem, but - like you - they are forced to make compromises : they try to survive, and choose to produce a product they think will sell. Armand Colin, designer of one of the French types of flat anchors, tried to prevent this corkscrewing behaviour by incorporating, on both sides, folding stabilising struts. The manufacturer chose not to follow the design ! A second French manufacturer of flat anchors has added, on his aluminium models, a strut that aims to reduce this instability. Unfortunately, in a compromise between efficiency and size, the strut has been made so short as to be totally ineffective.

The French use another type of flat anchor, without a strut, and with a large heel between the two flukes. They flukes open to a larger angle than usual, about 45 degrees to the shank. In our tests, these anchors never dug in properly. The best they could do was to rise up on the tips of the flukes, and scratch the bottom, skidding across the sea floor with very low resistance. Such an anchor is to be avoided at all costs - or it will cost you all !

(b) - second generation flat anchors (with stabilising strut).

Tests of American fluke (flat) anchors, in steel or aluminium, have shown a slightly different behaviour. Their designers have incorporated an extension to the hinge axis, creating a long strut each side of the two flukes, designed to prevent the corkscrew behaviour. Sadly, the strut not only fails to prevent the fault, but it also forms a trap on which the chain can readily entangle during major wind or current shifts.

Even worse, when these anchors drag on their sides they tend to stand up on three points: the end of the shank, the point of the lower fluke, and the end of the strut. In this stable position, they will slide under load, with almost no holding resistance, and - in every case where we observed this behaviour - they will never dig in again. (Please see Figure 2.)

Figure 2 : Holding power of flat anchors with a stabilising strut

Note how the curve shows the anchor sliding on the bottom for some distance before digging in. The holding power then increases rapidly until, at a force of about 800 daN the anchor releases completely. The holding power drops suddenly to almost zero, and stays there as the anchor drags freely across the bottom surface. It does not dig in again, not even with the cork-screwing behaviour of the previous types.

This pattern of behaviour is, in my view, even more dangerous than that shown in Figure 1. I leave it for you to imagine how it would be, on your boat, some dark night when the wind picks up to 35 knots .... and there's a rocky shore 100 m downwind ...

Instability of plow anchors with hinged shanks

One might expect that the plow family of anchors would be found in the stable group, but to our surprise we found a pattern of behaviour quite similar to that of the flat anchors. We studied these, too, in detail. Figure 3 shows a typical holding power graph for a traditional plow type anchor, i.e. one with a hinge in the shank. 

Figure 3 : Holding power of a plow anchor with hinged shank

The curve shows how the anchor digs in readily, and presents an increasing holding resistance, up to about 600 daN. This holding might be adequate for most conditions, and you will come to feel secure with such an anchor. But as the load increases further - a squall comes through, for example - suddenly the resistance drops to about one third of the peak - 200 daN; and then the cycle is repeated. The anchor has released momentarily, and the boat will have moved - but at least it hasn't let go completely!

Unless, that is, the squall endures, or worsens : in that case the anchor may break out for a longer period, and the boat will rapidly accelerate, moving sideways and gaining momentum beyond the power of the anchor to control. Remember, in our tests the velocity of the tractor was constant : in reality, this is far from the case !

But how to explain this behaviour ? Please now consider Table 1, which shows how weight is expressed onto the bottom by a plough anchor at its various points of contact.

Table 1 : How weight is distributed on a typical plough anchor 

A surprising proportion - 47% - of the anchor's weight is on the hinge. Once the anchor has dug in, and a large load is applied, the hinge portion tends to go down into the bottom, and the anchor begins cork-screwing. Walking beside these anchors in the shallow water was very valuable in understanding their behaviour. We often saw them dragging through the bottom completely inverted, with the tip poking up through the surface - like a submarine's periscope ! Perhaps this is due to the hinge-weight relative to the point-weight, and you cannot avoid the laws of gravity!

What appears to happen with these hinged anchors is that once dug in and as the load comes on, the body of the anchor is thrown to one side of the shank, perhaps by areas of uneven density within the bottom. With the centre of resistance thus thrown off to one side, the body begins to act like a rudder hard over, causing the anchor to begin its rotation. Once in every full rotation it will break out, partially or totally, before digging deeper again. The holding resistance thus cycles abruptly from high to low as the body digs in, and breaks out - (see Figure 3.)

Many sailors have total trust in this type of anchor, and will tell you they "have never been let down" by their favourite plough.... most of us prefer to forget our own occasional lapses from perfect behaviour ! so, too, is the nature of sailors in relation to their anchors. If you dig deep enough, they will remember nights of terror when the anchor wouldn't hold, or when they juggled two anchors on one chain. But rather than choosing to learn from these experiences, the average sailor will often excuse the problems as aberrations. It is such moments, when loads are extreme, that our boats are most at risk, and when we need the very best anchor. Most sailors are traditionalists, and traditions resist change !

Remember how the famous solo navigator Moitessier lost his equally famous yacht, JOSHUA, off the coast of Mexico ? When the squall came through, his well tested plough broke free from the sand bottom, and the yacht was buried in the surf. My personal experience of this type of anchor is negative, often having found them very difficult to set in the hard sand, or weedy, bottoms of the Mediterranean. I also found their holding power to be unreliable

Strangely enough my bad experiences were fortunate : they set me to searching for the best possible anchor !

If you are at all unsure as to how your anchors will behave, try taking them onto the beach, or into shallow water beside the shore. Try pulling with a rope : you may be very surprised by what you find with this anchor you have trusted your safety to so often !

Stable anchors

As before with the unstable anchors, these stable anchors also owe their behaviour to design, not to chance. But by behaviour, as by design, they are the opposites of the unstable anchors. In our tests we found three types of stable anchor : the new generation plough anchor, redesigned without the hinge in its shank ; a new German anchor ; and our newly designed SPADE anchor.

All three of these types shared some common design elements :

their effective resisting surfaces lead to a single point, which is in the same plane as the shank

their structures are rigid, without hinges, so that the symmetry of the effective resisting surfaces can not change through articulation

their effective, resisting surface areas lie around a plane set at a fixed angle of about 35 degrees from the line of the shank.

they all fall onto the sea floor on their sides, and commence digging in from that side once the load comes on

The resisting surface areas of all these anchors pull them into the sea floor, starting from the side on which they fall. The forces that pull them into the floor are created like those on a kite, or a wing, by the resistance of the medium - the bottom - they are moving through. As they bury into the floor, they settle to a position within the floor in which the plane through the shank and the point is at right angles to the floor. There is no asymmetry around this plane, and therefore there exists nothing to cause them to start cork-screwing. They remain buried, no matter what the load, even if they do start to move through the sea floor under extreme load. Because they remain buried, they must offer a fairly constant, and high, resistance to the loads applied. (Please see Figure 4 : Holding Power of a typical stable anchor.)

Figure 4 : Holding power of a typical stable anchor

Note how the first part of the curve is similar to that in Figure 3, both curves showing how the anchors dig in rapidly, and offer a holding resistance that increases quickly to about 600 daN. From this point on the difference between these two types is very important : while the unstable anchor in Figure 3 begins its cyclical behaviour, offering alternately high and low resistance, the stable anchor continues to offer roughly constant, and high resistance to load, even as it moves through the bottom.

Stable anchors do not break out of the bottom : they may move through it under high loads, but they remain deeply buried. They continue to offer a reliable, high and fairly constant resistance to the loads the vessel applies onto them through the rode. The level of that resistance depends, of course, on the size and type of anchor used, and on the nature of the bottom.

Compare again the graphs shown in Figures 3 and 4, and ask yourself which type of anchor you'd rather have when the wind's getting up, and the night is dark !

Comparisons of holding resistance of various anchors

We assembled the results of all our tests, and compared the various types of anchors tested. We choose not to name the types of anchors tested here because we prefer not attack the performance of other makes, but rather to show how performances vary. We are, however, proud to name the SPADE anchor in these results, as you will understand when you consider the following. Please see Table 2, which compares the holding resistance of various anchors tested.

Table 2 : Holding resistance of various anchors tested. 

Figure 6 - Holding comparison between unstable and auto stable anchors

Comparisons are usually made between the weights of anchors and the holding resistance they offer to load. We consider it is much more valid to compare anchors on the basis of their effective surface areas, because it is these surfaces that do the holding, not the weight. Some anchors, by unintelligent design carry a lot of useless weight. Others are made of different materials, such as the aluminium anchors which have about half the weight of steel anchors of the exact same dimensions. Comparisons based on effective surface areas are much more fair to all anchors.

In Table 2, the most important line (other than the first two) is the last line. The data here relates the holding power of each anchor tested divided by its effective surface area, in units of force/area : we have used the metric units, daN/cm²

Table 2 summarises our findings, and can be put into words that clearly show which anchors to choose for peaceful navigation.

Unstable anchors all

let go of the bottom under high load

fail to give constant holding resistance under load

even when holding, offer rather low holding resistance / unit surface area

 

Stable anchors all

never let go of the bottom under high load

offer fairly constant, and high, holding resistance under high load

offer a holding resistance/unit surface area at least double - and up to six times that of the unstable anchors.

SPADE anchors all

offer holding resistance/unit surface area much higher than that of other stable anchors ; and four to six times that of the unstable anchors.

Conclusion

Our tests have shown two families of anchors, based on behaviour under high load.

Unstable anchors give a cyclical pattern of holding resistance, and at best a relatively low average holding resistance. At worst, they may break out of the bottom and completely fail to engage again. These anchors are unreliable and will always put a vessel at risk when weather conditions apply high loads to ground tackle.

Stable anchors never break out of the sea floor, and even if forced through the sea floor under adverse conditions of high load, they continue to offer a high and fairly constant holding resistance. These anchors offer greatly increased safety to vessels using them, and are certain to become the standard type of anchor of the future.

Anchors are best compared on the basis of units of effective surface area, rather than on weight. Even within the family of stable anchors, average holding resistance per unit of surface area can vary by a factor up to 2.