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| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | SCOPE S/D AND ANGLE AT ANCHOR, 1 0 (S/D = 1/ SINO) | | | | | | | | | | | | | | Angle, 0, at Anchor 0° 2° 3° 5° | 7° | 9° | 11° | 14° | 20° | 24° 30° | | | | Scope, S/D infinite 30 20 12 | CO | 6 | 5 | 4 | 3 | TA 2 | | | | | | | | | | | | | | | | | | Basically, what this all comes down to is scope and angle. As the scope gets less and the angle becomes bigger, the less likely your anchor is to hold well. In Prof Knox's experiment, the SPADE gave the flattest dependence on scope, while the claw and HiBlade gave the steepest. | | | | | | | | | | | | | | angle at the anchor increased, and the scope lowered. The decrease in hold was more or less linear with the angle but wasn't the same for all anchors. Those which showed the least decrease as the angle steepened were the SPADE and Danforth. The Delta came next. The steepest decline of hold with angle was found on the claw and HiBlade; Table 2 summarises the data on the basis of scope. I was surprised at the large differences between anchors, with the Danforth coming out best in terms of how its hold decreased with scope. Of course, this anchor has disadvantages referred to in my first article; it was particularly difficult to get it to engage symmetrically - its stability is also suspect because it can roll out when | | veered. The SPADE is, therefore, the best anchor overall in terms of stability, and holding when dragged, which declines only slightly with the amount of scope. The hold of the Delta seemed to go through a flat range between ten and four, then quickly declined as the scope went down to TA. The ciaw and HiBlade showed the sharpest decline in hold with decreasing scope, so, with these anchors, it's particularly important to work with a large scope when conditions are bad. Nevertheless, a scope of five for the anchors tested ensures that not more than 30% of the hold is lost. As I've argued before, a scope of five should be regarded as the minimum for safe anchoring. | | | | | | HOLDING AND SCOPE WHEN PLOUGHING AT STANDARD RATE | | | | | | TABLE 2 | | | | | | | | | | | | | | | | | | | | | | | | | SPADE 6 kq | 250 | 85% | 210 | 70% | 180 | | | | Danforth 2 kg* | 170 | 90% | 150 | 80% | 135 | | | | Delta 6 kg | 180 | 80% | 145 | 60% | 110 | | | | HiBlade 10 lb | 190 | 75% | 140 | 53% | 100 | | | | Claw 5 kq | 170 | 70% | 120 | 45% | 75 | | | | | | | | | | | | | | | | | | | | | •Note. These values apply to the Danforth only when it is engaged symmetrically. | | | | | | | | | | | | | | | | | | weight would be in the range of 11 to 16 kg, and for an 18 metre yacht (60 ft), it would be 30 to 45 kg. The upper figures would apply to cruising yachts and the lower ones to racing vessels. Boat owners will recognise that these figures are much in line with normal practice. The question now is: can anything more be deduced from Formulae 1 and 2? Indeed, yes, but a third relationship is needed which connects peak cable tension with LOA and wind speed. As I and others have argued elsewhere, the wind resistance of a yacht is proportional to its frontal area which is roughly proportional to its LOA squared. It's also proportional to the wind speed squared. Of course, the actual cable tension experienced by a yacht at anchor varies widely as the yacht tacks and surges, trying to shake itself loose from its anchor. However, if the anchor isn't snubbing at its chain, the peak load experienced, in practice, is given to a reasonable approximation (see PBO 386) by Formula 3: | | | | | | The mean dependence of hold upon weight at scope 5 for the claw and Delta anchors, when ploughing at three centimetres per second, can be expressed as: FORMULA 1 Anchor Holding Force in kg = 20 x (Anchor Weight in kg) The question now is: what anchor weight should be used for a particular yacht? Manufacturers recommend anchor weights for yachts of different lengths, but they don't explain how they arrive at their recommendations, nor whether they base them on theory or simply experience. Having surveyed the recommendations for steel anchors by several manufacturers and vendors, I find they can be represented quite well by a very simple formula. There are exceptions, particularly the recommendations in the Plastimo catalogue for Brittany anchors (where somewhat heavier anchors are recommended for shorter yachts), but overall, recommendations follow Formula 2 within about ±20% for anchors between five and 50 kg. FORMULA 2 Anchor Weight in kg = (1/9) x (LOA in metres)' Thus, for a yacht of 11 metres (33 ft) LOA, the recommended | | | | | | | | | | FORMULA 3 Peak Cable Tension = (1/500) x (LOA)2 x (Wind Speed)' | | | | | | | | | | In this and subsequent formulae, any force, such as cable tension or anchor holding, is measured in kg, anchor weight in kg, LOA in metres, and wind speed in knots. Let's suppose that conditions are such that a yacht's recommended anchor is just holding, or rather, to be more | | | | | | | | | | 101 | | | | | | Practical Boat Owner 428 August 2002 | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
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