Many posters on forums advise 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, my question is why would people believe that they need to go bigger, with all the expense and handling difficulties that this may entail?
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 of the same make 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 well 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 a variety of wind conditions? I normally use the Knox formula for this calculation, Force ( in kilograms force ) = [1/500] x [LOA (in metres squared)] x [wind speed (in knots squared)]. For a 35 ft boat the predicted loads are:
|Wind (knots)||Load (kg force)|
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 30 ft boat as 87 kgf 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 as 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.
Patmos, Ormos Livadhi. The anchor is fairly well buried but by no means fully covered. The rear end of the flukes is visible and almost all of the hoop is above the surface. Some yawing of the boat has taken place, resulting in chain dragging to an angle of around 15-20 degrees.
Note that the identifying float on about 1 metre of line has been pulled down to the seabed, perhaps when the chain dragged across the top of the anchor.
Lipsi, Ormos Kouloura
Next, the same anchor after a wild night of Force 7, 28 – 33 knots. Here my anchor is considerably more buried, only half of the hoop is exposed, none of the flukes and only a small part of the shank. Note how much yawing has been taking place, dragging the chain across the sand. At this wind speed the chain is often completely off the bottom and almost straight.
Kythnos, Ormos Fikiada
A different time and place after another night at Force 7. Again, the flukes are completely buried.
Kythnos, Ormos Fikiada
And just for comparison, a 33 kg anchor holding a 55 ft boat in the same location as the previous photo. Prior to the NW force 7 the wind had been S force 4, accounting for the evident rotation of the Manson anchor. Note the yawing angle, 30 degrees or more. Note also that this New Generation anchor has not dragged at all when resetting, the shank has simply rotated about the flukes. This feature is common to most concave types.
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 35 ft boat.
Berthed stern-to with the chain initially hauled in hard but photographed after a couple of days.
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 strength of the seabed is adequately strong a well set anchor is unlikely to drag. Comparison of anchor performance when free-anchored 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. The only rider 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, although I remain to be convinced.
A version of this article was originally published in Yachting Monthly magazine.