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| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Practical cruising Will my anchor hold? - Part 2 | | | | | | | | | | precise, that under the peak loading it'll drag or plough at three centimetres per second, which could be regarded as a maximum for safety. For this situation, we can then replace 'Peak Cable Tension' from Formula 3 with 'Maximum Anchor Holding': | | | | | | THE FINAL TEST | | | | | | | | | | Holding power and speed of drag | | | | | | | | | | FORMULA 4 Maximum Anchor Holding = (1/500) x (LOA)' x | | | | | | Speed)' | | | | | | | | | | What this implies is that if we want to keep below the Maximum Anchor Holding, the wind speed can't be above some critical value which can be found from Formula 4. So, we should now re-label 'Wind Speed' as 'Maximum Safe Wind Speed'. We can also incorporate the manufacturers' recommendation from Formula 2 and replace '(LOA)2' by '(9 x Anchor weight)'. Making these two changes, we now get: FORMULA 5 Maximum Anchor Holding = (1/500) x (9 x Anchor Weight) x (Maximum Safe Wind Speed)2 Therefore, we reach the conclusion that, if we agree a common Maximum Safe Wind Speed for all yachts, the maximum holding of an anchor, as recommended by manufacturers, is proportional to its weight. This is precisely what I found by direct experiment and have expressed in Formula 1. So, we can finally compare the manufacturers' recomendations, as expressed in Formula 5, with the experimental data, expressed in Formula 1, by replacing 'Maximum Anchor Holding' in Formula 5 with '20 x Anchor Weight' from Formula 1. This gives: FORMULA 6 20 x (Anchor Weight) = (1/500) x (9 x Anchor Weight) x (Maximum Safe Wind Speed)1 Anchor Weight is present on both sides of Formula 6 and cancels as it must. After re-grouping, the numbers we finally get are: FORMULA 7 Maximum Safe Wind Speed = (20 x 500/9)1/2 = 33 knots This is interesting because it implies that the recommended anchors for yachts of different LOA will be adequate up to Force 7 or 8 when they're buried in medium-hard sand, such as that used for my experiments. This seems an entirely reasonable conclusion, although I've not seen it expressed in just this way before. I'll call this the 'Force 8 Rule'. Of course, this Force 8 Rule applies to a specific seabed material, and has been deduced from data for two average anchors, the Delta and the claw, at a Scope of five. If you expect a Force 8 or more, the rule suggests that you should seek a seabed with superior holding, an anchor with superior holding, or that you should deploy two or more anchors. The SPADE, for example, can hold 40 or 50% more than the HiBlade, Delta, or claw. Since the safe wind speed goes up as the square root of the safe holding (Eormula 5), the SPADE would be adequate to roughly 20% higher wind speed. Likewise, if you deployed two anchors and if both developed their full hold, the wind speed you could expect to resist would be about 40% more than with a single anchor. | | | | | | | | | | ■ Before I started my experiments, I had little idea of how the hold of an anchor would depend upon the speed it was dragged through the seabed. Indeed, had I been asked, I would probably have answered that, while an anchor will hold in sand if it doesn't move, it would very likely come out if pulled so hard that it was forced to drag or plough its way through. Most yachtsmen would define 'dragging' as the situation which occurs when a yacht's anchor disengages itself completely from the bottom, and the yacht canters off downwind with possibly catastrophic consequences. The first thing my experiments showed was that there are in fact two quite distinct types of 'dragging'. PLOUGHING The first is what I've just described. The second kind of dragging occurs when an anchor is forced to drag or plough through the material of the seabed while still remaining buried. This is the kind of 'dragging' that I studied in my experiments. Ploughing is probably a better word to use. As I've shown, an anchor which drags or ploughs through the seabed while still buried will hold just as well as, indeed better than, an anchor which remains static. Ploughing is dangerous only when the anchor rolls out, or is forced out by some obstruction, such as weed or rock, as it slowly moves through the seabed material. Typical safe ploughing rates can be up to a few centimetres per second, or say a tenth of a knot (5 cm/sec). My experiments give no information on what happens when this rate is exceeded. The question I became interested in as a result of my initial experiments was: how does the holding force of an anchor depend upon the rate at which it ploughs through the seabed? | | To answer that, I pulled the anchors at a constant force and measured the speed at which they ploughed. Throughout these experiments I used a scope of ten. For the 5-7 kg anchors, there was no movement up to a force of 50 to 80 kg. This maximum force which an anchor can withstand without moving, I call its 'Static Holding Force', or SHF. When the pulling force was increased above the SHF for any anchor, it started to plough slowly through the sand at a constant rate. But as soon as the force was reduced to below the SHF, the anchor would stop moving. The force required to make an anchor plough through the seabed, I call its 'Dynamic Holding Force', or DHF. This force is a function of the ploughing rate, and is equal to the SHF when the anchor is just on the point of moving. Figure 3 shows the results for the 5-7 kg anchors. It can be seen that the DHF increased more or less linearly with the rate of ploughing. The HiBlade, Delta and claw behaved very similarly, while the SPADE gave somewhat higher values of both the static and dynamic holds. Figure 4 shows the data for the 15 kg Bruce and Delta anchors. The linearity is less good as the range of ploughing speed was rather small. Table 3 summarises the numerical results. WIND SPEED The broad conclusion is that, in the sand of Longniddry beach, the SHF of a modern anchor at scope ten averages around 11 times its weight, while the DHF at three centimetres per second ploughing rate averages around 27 times the anchor weight. However, there's quite a variation from one anchor to another. The SPADE gives the best performance, while the HiBlade, Delta and claw aren't far behind, in agreement with the conclusions reached in my first article. | | | | | | | | | | | | | | Correction | | | | | | | | | | ■ In the first feature of this series, published in PB0 427, while a genuine 15kg Bruce anchor was one of those tested, the picture on page 79 shows a 5kg Claw anchor, rather than the genuine Bruce. With the exception of the 15kg Bruce anchor tested, any reference to a Bruce anchor in the first article should read Claw anchor. Similarly, the anchor referred to as a CQR in the first article was, in fact, a copy of the CQR pattern and should be referred to as a plough. | | | | | | | | | | Practical Boat Owner 428 August 2002 | | | | | | 102 | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
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