Hurricane season is upon us. It is the fifth anniversary of Hurricane Katrina. The east coast just escaped Hurricane Danielle but others are on the way. It is unfortunate that the peak of the hurricane season coincides with the best of the boating season, especially in the northeast. For those of us who anchor out, trusting to luck when the wind blows, here are some things than you might want to consider. The usual figure of merit for an anchor in a given type of bottom is the horizontal load that the anchor will withstand before breaking free divided by the physical weight of the anchor. This figure can then be compared to a weight resting on the surface that would provide the equivalent horizontal holding force. Lightweight anchors, the type most of us use are just that, relatively light shapes that gain their holding power by digging into the bottom and using the mass of mud or sand to add to their physical weight. Admittedly west coast conditions differ from east coast conditions, deep water from shallow water, rock bottoms from clay bottoms. Further the nature of the rode affects the performance of the anchor. Transient forces of wave and wind may momentarily exceed the holding power of the anchor. An effective rode will spread those forces out in time to reduce the transient forces to a horizontal pull to below the anchor's horizontal holding power. This attenuation is a function of the rode's elasticity gained either through stretch or catenary forces. It is quite evident that a heavy all chain rode permits a shorter scope than a rope, wire, or composite rode. But to equal the elasticity of a nylon rode a chain would have to be too heavy for most recreational boats to carry. Unless you are in a breaking sea, the main component of horizontal force is caused by the wind not by movement of the water. Wind drag on a boat is a function of the projected area at right angles to the wind, the square of the wind speed, the density of the air, and the dimensionless coefficient of drag which depends on the shape of the boat. Drag coefficients have been determined from wind tunnel tests. Some representative drag coefficients are: Open parachute (or efficient spinnaker) = 2.0 Hollow hemisphere, concave to wind = 1.7 Flat rectangular plate = 1.28 Wires, cylinders, masts = 1.0 Cargo ship, wind dead ahead = .95 Fishing trawler, wind dead ahead = .9 to 1.05, depending on superstructure, outriggers, etc. Streamlined passenger vessel = .70 Recreational trawler = .70 to 1.0, depending on superstructure, masts, outriggers, etc. Sphere = .47 Hollow hemisphere, convex to wind = .38 Modern automobile = .26 to .35 Airplane = .09 Using Area in sq. ft., wind Velocity in knots, and the U.S. Standard Atmosphere for air density, the equation for Drag in lbs. can be written as: Drag = .00339 x Coefficient of Drag x Knots^2 x Area A conservative way to estimate frontal area is to multiply the beam by the height of the superstructure. An even simpler way is to multiply the beam by 3/4 of the beam. By this calculation, a Willard 30' trawler has about 100 sq. ft. of area. A Nordhavn 40 has 160 sq. ft. of area. A Nordhavn 47 has about 195 sq. ft. of area. A Nordhavn 72 has 330 sq.ft. of area. The strain on the anchor rode in hurricane force winds is far greater than most boaters imagine. For each 100 sq. feet of area: 20 Kts = 136 Lbs. 40 Kts = 542 Lbs. 60 Kts = 1220 Lbs. 80 Kts = 2170 Lbs. 100 Kts = 3990 Lbs. 120 Kts = 4882 Lbs. 140 Kts = 6644 Lbs. 160 Kts = 8678 Lbs 180 Kts = 10984 Lbs. The breaking strength of proof coil 5/16 chain is 7600 lbs., of 3/8 chain is 10,600 lbs. The breaking strength of 1/2 nylon is 7500 lbs., of 5/8 nylon is 12,200 lbs., but even brand new nylon rodes should be loaded to no more than 1/4 breaking strength to assure adequate stretch with a margin of safety. At first glance it appears that a 3/8" chain, typical of many trawler yacht anchoring rodes, would be sufficient to hold a boat with 200 sq. ft. of area in a Category 2 hurricane but all is not what it seems. Long before the chain broke, it would be stretched bar tight. Any transient forces of waves or surge would be transferred directly to the anchor, which, in all probability would be jerked out of the bottom. Either than or the cleats would rip out of the deck. If you prefer rope as your anchor rode it is wise to remember that while stretch is the enemy in most tasks involving rope, rope stretch is an asset in climbing and anchoring. The rock climber wants a rope that will stretch if he/she falls to attenuate the shock. Boat anchoring also profits by rope stretch. The elastic limit of new laid nylon rope is about 25%. Stretch it much further and the fibers will permanently deform or break. Repeated stretching to this point will weaken nylon rope and it should be replaced as soon as possible. Nylon loses about 20% of its ultimate strength when wet. But wet nylon will handle transient shock loads even better than dry nylon. The water acts as a lubricant, permitting fibers to slide over each other and minimize internal heating and friction. It is important to remember that the safe working load of a rope is dependent on its use, not its ultimate strength. The SWL is the breaking strength of a rope divided by a safety factor, generally from 2 to 12, depending on use. Tasks that involve low stretch or overhead loads in working areas require the highest safety factors. A 3/8" nylon rope with a breaking strength of 3000 lbs, used in an overhead lifting situation, might have a SWL of 200 lb. Dacron rope, roughly equivalent in breaking strength to nylon, might have a SWL of 600 lb. in the same application because of its lower stretch. No matter what the rope manufacturers say, the safety factor is generally determined by lawyers not engineers. In anchoring situations, where we want rope stretch to attenuate the transient shock of wind gusts and waves, a safety factor of 3 or 4 might be appropriate. Thus for a boat with 100 sq. feet of projected area, an appropriate anchor rode for storm anchor sized to resist a 60 kt. wind would be 1/2" nylon at a minimum. A 5/8" rode might offer a greater safety margin but would offer less stretch in lighter wind conditions. Naturally all cleats must be sized and secured to handle the maximum shock loading expected. Because of gusts and wave action this may be two to three times the load listed in the wind force table.