The last time we were able to leave Vanish was 5 days ago. We'd returned from a shopping/restaurant outing on Monday to find the wind in the anchorage had increased to 25 - 30 knots and was building with the development of an offshore low. As it was nearing sunset, we decided to move Vanish a few hundred meters closer to an area called Shackleford Banks in the Cape Lookout anchorage providing us with flatter, more protected conditions. Maynard put out 65 meters of chain but in hindsight we should have, and could have put out 95 meters of chain. Better in the water than in the locker!

Whilst lying in bed listening to the wind howl at 2:30 am for the third night in a row at a very lonely Cape Lookout anchorage, one tends to try to keep their mind busy thinking about almost anything but where you are actually at. Inevitably, you begin to wonder how that anchor could possibly hold this big yacht as you feel a particularly large gust pound into the starboard side of the yacht forcing it to heel over and charge off in the opposite direction. The loads must be tremendous!

Maynard has a mathematical mind and being completely trapped on Vanish during the current East Coast low sitting off Cape Hatteras, decided to do a bit of research to quantify the forces involved in holding a boat the size of Vanish in 30 to 40 knots of gusting wind day after day. It turns out that there a number of real world empirical formulas which at least gives one an idea as to what the holding power of an anchor is and the forces acting on a yacht associated with wind. The following are his thoughts on the matter.

"Vanish is a Marlow Explorer 76LR, an 82 foot vessel whose primary anchor is a Lewmar CQR 185lb steel anchor. We have been very happy with it as it sets well and has held us in all sorts of conditions. I realize that there are more modern anchors which have received a lot of acclaim but at this stage we are sticking with the CQR.

The following formula has been derived to describe the maximum holding power of modern steel anchors:

F=K*M to the power of 1.4 where F= maximum holding force (assumes infinite scope)
K= holding coefficient
M= Anchor Mass or weight in pounds

K, the holding co-efficient is dependent on anchor type and the holding strength of the sea bed material. For hard bottoms such as a thin veneer of sand on coral, our CQR has K between 3 and 6. If we use a K of 5 then on hard bottoms with no or very little anchor penetration, our anchor would have an F of 5,400 pounds For soft low density muds, K is generally between 12 and 15. These are the sorts of bottoms that, when you back down on a set anchor, the anchor slides a little bit or feels spongy. Using a K of 12, our anchor would have an F of 13,000 pounds. Most recognized anchorages have good holding denser muds or sand combinations and have a K in the 20 - 30 range. For a K of 22, our anchor would have a maximum holding force of 23,000 pounds. It is interesting to note that the next size up CQR is 225 pounds. For this small 40 pound increase in weight, the maximum holding power increases by 30%. So size really does matter.

These values of K tend to work out pretty accurately based on our experience with our CQR anchor. Modern manufacturers of anchors such as Rocna argue that the holding coefficient for a given bottom type is 30 - 50% higher with their new improved designs. This may well be true but testing and confirmation is continuing.

Scope is another important consideration. For our CQR with a scope of 3 to 1 you need to multiply the maximum holding power F by 40%. At 6 to 1 multiply F by 70%. At a scope of 10 to 1 the maximum holding power should be multiplied by 95%.

As the wind was whistling overhead in the middle of the night I began to wonder why I had not put out more chain. We put out 65 meters of chain in 6 meters of water at low tide. The tides are about 2 meters and the bow height is also about 2 meters. So at 10 meters maximum effective depth with 65 meters of chain out we have a scope of 6.5 to 1 which gives us a holding strength of 16,000 pounds. I wonder how much force is being exerted on Vanish by the 40+ knot gusts?



Fortunately the American Boat and Yacht Council (ABYC) have worked up a simple nomogram that can be used to estimate the forces acting on your yacht type for a given wind speed. For Vanish, the ABYC nomogram yields a windage force of 1,000 pounds at 15 knots, 3,600 pounds at 30 knots, at 45 knots the force acting on Vanish is 8,500 pounds. In a hurricane with 70 knots of wind it would exert 20,000 pounds of force on Vanish. Definitely time to start the motors. These force values are pretty conservative and have been found to be remarkably accurate in real world situations. One word of caution. Vanish does tend to wander around a bit and sometimes the wind attack angle exceeds 25 degrees so that the wind is hitting the vessel more side on. At a 45 degree wind angle you can increase the wind force by up to 50% from the above numbers.

So let's recap Vanish's situation over the last few days. If we assume a maximum gust of 45 knots and a maximum angle of attack of 35 degrees, then Vanish would have had approximately 10,000 pounds of maximum force acting on it. At a scope of 6 to 1 Vanish's anchor had a holding power of at least 16,000 pounds assuming a K of 22. I expect that the holding in the firm Cape Lookout sand would have a K closer to 30 making my estimate of 16,000 pounds of holding power pretty conservative. Hence, one would expect not to have moved. In fact when you look at Vanish's anchor pattern throughout this gale, Vanish indeed did not move at all. Next time however, I will put out a scope of 10 to 1 so as to maximize my safety factor and hopefully improve my sleep!"

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