Severe Weather Anchoring Tips
by: Craig Johnston
Comment about anchoring when expecting high winds: if you have all-chain rode, gusts can be accommodated by the changes in the rode’s catenary curve. Those with partial rode rodes can get some benefit in moderate conditions with a kellet, a weight that is let down on the anchor rode to about the mid-way point. But once you straighten the chain out, even small waves can add big peak loads to the sustained force of the wind. That’s when having an adequately stretchy and strong snubber is needed. Snubbers are subjected to repeated cyclic loading and will fail through heat build-up if not incorporating sufficient mass of nylon. (From experience, we know that a 15’ snubber of ½” nylon will not survive a night of 30 kn winds on a 36’ boat, whereas our dual snubbers of ¾” nylon, up to 50’ long, have never failed on the larger Sequoia.)
Did you know that the wind’s force on the hull and rigging goes up with the square of the wind speed? The average force of a 20kn steady breeze has been variously estimated from 250-500 lbs. for a 10-15m long boat (that includes most of us). But at 30 kn wind speed the load rises to 560-1125 lbs., and at 40 kn is a whopping 1000-2000 lbs. That will easily exceed the holding power of 50-60 lb. anchors for most types of seabed. At the high end, the strength of the entire system is tested: anchor, shackles, chain, rope, and deck fittings. For winds exceeding 30kn, Sequoia will have two anchors out, one of which is our largest (a Fortress FX-85). fittings.