WHY BIGGER ANCHORS AREN’T ALWAYS BETTER
It’s a common misconception that a bigger anchor will keep your boat safer, but the evidence shows that this isn’t the case. Vyv Cox explains
Reading the advice on what size anchor is best for your boat, the forums are full of advice that an anchor one or even two sizes bigger than the manufacturer’s recommendation should be carried for greater anchoring security. On the assumption that the manufacturers have tested their products in a wide variety of conditions, I began to wonder why people would believe that they need to go bigger, with all the expense and handling difficulties that this may entail.
Our yacht cruising experience began around 35 years ago in Anglesey, North Wales, from where we cruised widely to Scotland, Ireland, the Scilly Isles and Brittany, almost always anchoring. Throughout this period we used Delta anchors, a 10kg version on our first boat and 16kg when we moved up. We then lived in the Netherlands for seven years, weekend and holiday cruising south as far as Brittany, North to the Baltic Sea and west to the UK. Again, mostly anchoring overnight.
We began cruising 17 years ago on a 35ft boat for six months each year, sailing down the Atlantic coast, into the Mediterranean to Spain, the Balearics, France, Corsica, Sardinia, Italy and Greece. Again, more nights were spent at anchor than in ports. Once in Greece we experienced our one and only problem with the Delta, dragging in 50-knot winds on a soft mud seabed. So we replaced the Delta with a 15kg Rocna, the recommended size, although we still carry the Delta. About 20 years ago we bought a Fortress FX16, the recommended size for our boat, and use it as a kedge.
As you can imagine, during this long history we have seen our fair share of strong winds. In Sardinia, still on the Delta, we sat out 50 knots for nearly a week, not far from the Bonifacio Strait. In Greece, at the height of the Meltemi,
we have been at anchor in gale force winds many times, and a few times over 40 knots. We have anchored on the Fortress on a rope warp in winds just under 40 knots.
So back to the question: why go bigger? Taken to the logical conclusion, an enormous anchor would sit on the bottom without digging in at all – effectively a mooring. Bearing in mind that the pull exerted by the boat is constant for a given wind strength, it seems likely that an anchor larger than needed would behave somewhere between the perfect setting of the recommended size and the completely non-set of the enormous one; that is, there would be insufficient force for the anchor to fully set. Assuming the angle of the point of small and large anchors from the same manufacturer to be the same, the large one would set pretty much to exactly the same depth as the small one, leaving the back end of the flukes above the seabed, doing nothing.
So far what I have written is all very well but it could be argued that we have been lucky, or maybe the winds were not as strong as we thought. So I have carried out a little research, made a few calculations and taken some photos, with case histories, to prove the point.
Firstly, what load does my boat exert on the anchor in different wind conditions? I use the Knox formula (below): Some authorities suggest that the Knox formula predicts lower values than other expressions, although the values were verified in practice by Professor Knox using load cells. My own subjective measurements, by holding the chain in various wind speeds, suggest that these numbers are not far off. Direct measurements by naval architect Robert Smith, published in his book, Anchors: Selection and Use, give the loads for a 30ft boat as 87kgf (kilograms force) at 30 knots and 170 kgf at 42 knots, confirming that actual loads may be considerably less than those higher values predicted by some authorities.
So what do these loads look like when applied to anchors? In addition to all the charms of Greek cruising, the clarity of the water allows underwater photography and the study of anchors in all conditions. It is very common for skippers to dive on their anchors to ensure they are well set, but I also photograph mine quite frequently, especially in wild conditions.
My first photograph shows my Rocna on the seabed following a night of NW winds at around Force 5, 17-21 knots. Prior to this the wind had been about Force 2, 4-6 knots, from the opposite direction (See images above).
Having looked at the underwater performance of anchors in fairly testing conditions, how can we determine whether they are on their holding limit, and thus whether a larger anchor might be beneficial? In fact, for boats that berth stern-to in the Mediterranean, this can be demonstrated fairly easily. Our normal practice, having taken warps to the wall, is to haul in the anchor chain until there is absolutely no movement and the chain is bar tight. Giving up pulling while the anchor is still moving is a sure-fire route to dragging when conditions worsen. Our windlass is a Maxwell RC8 with a 1000-watt motor that develops a maximum pull of 600 kgf. Reference to the table above shows this to be equivalent to around 50 knots of wind for our 35ft boat.
Although the maximum wind in which I have photographed anchors shown here is around 30 knots or a little more, I have dived on the anchor without a camera in considerably more. In winds up to nearly 50 knots our anchor does not bury noticeably deeper. The holding power of modern New Generation anchors is immense and provided that the holding of the seabed is adequately strong, a well set anchor is unlikely to drag. Comparison of anchor performance when freeanchored and stern-to demonstrates that even in the most extreme conditions of 50 knots-plus the recommended anchor for the size of boat is more than adequate.
It should be emphasised that we always set a snubber that provides elasticity and shock absorption to the rode. In stronger winds, with yawing and possibly significant waves, this simple device pays dividends.
I conclude that there is very little justification for carrying an oversize anchor. By using an oversize anchor for which you will not be able to generate sufficient force to set fully, you are in fact increasing the likelihood of the anchor dragging should the conditions change. Furthermore, you are going to increase the loads on your chain, linkages, bow gear, windlass and in turn on your electrics and batteries, or on your arms if you don’t have a windlass.
The only caveat to that is that in very difficult seabeds such as pebbles, boulder fields, coral and possibly heavy weed, an oversize anchor just might be better if it helps to dig the tip, although I remain to be convinced, and would need to see compelling evidence to be so.