Catalogues - tests-and-comparisons-of-anchors-part-2

Catalogue Tests and comparisons of anchors part 2

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sand whenever the wind is above about Force 6, and, more significantly, when the wind is up at Force 8. Of course, the ploughing won't generally be continuous, as the force on an anchor cable fluctuates wildly and randomly. It's only the peak forces which will be large enough to cause ploughing. So, an average ploughing rate, in say a Force 8, may still be quite modest, perhaps one centimetre per second. But even this means nearly 30 metres per hour which is hardly insignificant, especially if you've chosen a small patch of sand in a weedy patch for dropping anchor.

My observations may well explain why dragging incidents often seem to occur when an anchor has apparently been holding well under stress for a long time, and then jumped out suddenly and unexpectedly. We've certainly experienced this worrying phenomenon once or twice. What seems likely to have happened is that the anchor has initially buried itself perfectly well, but has then slowly walked downwind in small steps each time its SHF has been exceeded. If the anchor was a plough or Brittany, it may have simply rolled out after ploughing a certain distance or veere-ing, then failed to re-engage. Even with a roll-stable anchor, it could've ploughed into weed or deep water where the scope is too small for strong holding.



ETTÎT^I HOLDING FORCE AS A FUNCTION OF PLOUGHING SPEED AT SCOPE 10
✓		is* #	* M		#f
HiBlade	5 kg (4.7 kq)	75 kg	16	22 kg per cm/sec	140	30
Delta	6 kg (6.8 kq)	70 kg	10	21 kq per cm/sec	135	20
Delta	15 kq (16.3 kq)	150 kq	CO	155 kq per cm/sec	600 kg	28
Claw	5 kq (4.9 kg)	55 kg	11	26 kq per cm/sec	135 kg	27
Bruce	15 kq (16.2 kq)	105 kq	6.5	90 kg per cm/sec	375 kg	18
SPADE	6.5 kg (5.1 kq)	80 kq	16	38 kq per cm/sec	190 kg	37
MEAN			11			27

We can now calculate the wind speed at which anchors, at scope ten, will just begin to move when buried in medium-hard sand - when the SHF will be exceeded. This is simply done by by replacing the number 20 in Formula 7 by the number 11. This gives 24 knots, or about Force 6, as the the wind speed at which a recommended anchor will, on average, just start to plough in medium-hard sand. Applying the same argument to the DHF gives 40 knots, or Force 8 to 9, as the wind speed at which recommended anchors will plough at three centimetres per second. It now seems clear that we should accept that, even at large scope, our anchors will plough slowly through a seabed of medium-hard

HOLD vs PLOUGHING SPEED

Della Z = 100

y=21x + 171

HiBlade Z = 0

y=22x + 77

0 1 2 3 4 5

Ploughing Speed, cm/sec

FIGURE 3 Dependence ol hold upon ploughing speed for 5-7 kg anchors at Scope 10.

0 0.2 04 0.6 0.6 1 1.2

Ploughing Speed, cnVsec FIGURE 4 Dependence of hold upon ploughing speed for 15 kg anchors at Scope 10.

TO SUM UP..

Here, Prof Knox talks about what happens when an anchor is pulled or made to 'plough' through seabed. There are two relevant factors, first a buried anchor can exert without moving -static hold - and, second, the force it exerts it's made to plough - the dynamic hold. What found was that although anchors give a substantial hold when stationary, they give a greater hold when they are forced to plough the faster they plough, the greater the hold. The SPADE was best on both counts - static and dynamic - although the HiBlade, Delta and claw came close behind. Prof Knox also concludes that anchors will start to plough slowly around Force 7 when buried in medium-hard sand-

Practical Boat Owner 428 August 2002

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