In our discussion on anchoring, let's compare apples to apples, not pears to kumquats. The usual figure of merit in anchoring 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 to forces 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 it would have to be too heavy for most recreational boats to carry. 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 In a 20 kt. wind, a boat with an area of 100 sq. ft. at right angles to the wind with a drag coefficient of 1.0 will have 135.6 lbs. of wind pressure on its surface. 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 nylon rodes should only be loaded to 1/2 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. 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 it would have to be too heavy for most recreational boats to carry. Stretch is the enemy in most tasks involving rope. This includes all lifting and tying tasks. Also situations where the rope is to prevent some structure from moving. Shrouds and stays on a mast come to mind. But 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. Climbing ropes are specifically constructed with a woven abrasion resistant outer sheath and an inner core of loosely laid fibers, usually nylon, which can stretch up to 50% before breaking. In a long fall, the stretch is so great that the rope is permanently deformed and should not be used for climbing again. Boat anchoring also profits by rope stretch. The elastic limit of new laid nylon rope is about 25%. This means that it will stretch to 125% of its original length and return to its original dimensions when the load is removed. This will occur when the rope is loaded to about half its breaking strength. Stretch it much further and the fibers will permanently deform or break. Repeated stretching to this point will permanently 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 minimizing 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 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. The safety factor is generally determined by lawyers not engineers. In anchoring situations, where we want stretch to the attenuate transient shock of wind gusts and waves, a safety factor of 2 or 3 might be appropriate. Larry Z