Anchor Rodes In Theory And Practice

ANCHOR RODES IN THEORY AND PRACTICE By Lawrence Zeitlin Which is better, the Spade or the Bullwagga, the Danforth or the CQR, the Bruce or a rock tied to the end of a rope? There are about as many philosophies of anchoring as there are boaters. At risk of sounding even more professorial than I usually do, here is a short tutorial on anchoring rodes which I liberally cribbed from my class notes for a course I gave to budding naval architects a long time ago. These guys were planning on making a career designing supertankers and few had any idea that boats smaller than the Staten Island ferry had to be anchored too. Soon after the course was given, the tanker market collapsed and I suspect that many of the students either ended behind the counter at MacDonalds or started internet dot coms. Most of those are now at MacDonalds too. The moral is "don't take anything too seriously", even the advice I give below. Not too many of us recognize that the nature of the anchor rode is as important to anchoring success as the shape and weight of the anchor. Many of the differences in anchor performance published in the boating press can be attributed to the different rodes used. Don't be put off by the equations. The math in the first part of the essay is used to justify the conclusions reached in the latter part. Of course if you like this sort of stuff, you can work through the equations for your own boat. There will be no exam. An anchor doesn't hold the boat by itself. It must be attached to the vessel by the rode, a rope, a chain, or a wire which transfers the forces of the elements to the anchor. The anchor resists those forces by burying itself in the sea bottom. The load on the rode is determined by the horizontal forces of wind and current pushing on the boat, the transient forces of wave action, the depth of the water, and the weight of the anchor rode itself. To find the required strength and scope of an anchor rode, we must calculate these forces individually, then sum them to get the total load. We will start with an all chain rode, then progress to elastic chain and nylon rodes. (Many of the following calculation formulas come from from Robert Ogg, "Anchors and Anchoring - 8th edition", a pamphlet published by the Danforth Anchor Division of the Eastern Company. Apart from being an undisguised sales tract for Danforth anchors, the pamphlet is the best short resouce for pleasure boat anchoring information available.)

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WIND DRAG in lbs = .00339 * Cd * V * V * A where: Cd = drag coefficient, a dimensionless number determined by wind tunnel tests. For most pleasure boats this number is approximately .80. V = wind speed in Knots. A = total frontal area in sq. feet. This includes hull, deck house, mast and rigging. Thus for an 80 sq. ft. area pleasure boat in a 60 kt. wind, the total wind drag = 781 lbs. Current drag is equal to the thrust necessary to move the boat ahead in still water at the same speed as the current. It can be found from propulsion data by this equation: CURRENT DRAG in lbs. = 325.5 * (P * RPM - 1216 * V) * H V * P * RPM where: P = propeller pitch in inches RPM = propellor revolutions per minute V = velocity in knots H = engine HP delivered at the desired speed. If you know this, great. If not, it can be estimated by Keith's formula which comes next. Thus a 40 ft. yacht, estimated to require 20 HP to go 5 kt. with a prop pitch of 8" at 1000 shaft RPM would have a drag of 312.5 lbs. in a 5 kt. current. Keith's formula for (roughly) estimating required HP is: HP = Displacement in lbs. * cube of (Knots/(12 * sq.rt. of LWL)) Wave action loads are difficult to estimate since they depend both on the length of the boat and the period of the waves. Basically wave action imposes severe loads when the boat is pitching in resonance with the waves. Fortunately in strong winds wave lengths quickly grow to the point where resonance is unlikely for modestly sized pleasure boats. In most cases, the boat can be reasonable well insulated from wave action if the weight of an anchor chain is supported by a buoy and a pennant led to the boat. Nylon rodes are easier on the boat because of their lightness and elasticity. The horizontal anchoring load is determined by the sum of wind drag and current drag, with occasional shock loads imposed by wave action. I usually increase my estimate of the horizontal load by 20% to provide a safety margin for the unpredictability of wave and other transient loads. We can calculate

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the horizontal anchor rode requirements using the following equations: RODE TENSION (in lbs.) FOR CHAIN T = Th + (w * d) where: T = maximum tension in line in lbs. Th = horizontal load in lbs. (i.e. the sum of wind and current and wave drags) w = underwater weight of rode in lbs/ft. d = depth of water in feet (including max. tides and wave heights) VERTICAL LOAD (in lbs.) FOR CHAIN Tv = sq. rt. of ((T * T) - (Th * Th)) where: Tv = vertical load in lbs. LENGTH OF RODE (in ft.) FOR CHAIN Length = Tv/w Let's see what it takes to anchor our hypothetical 80 sq. ft. area boat in a 60kt. wind. The underwater weight of steel chain is .87 of its weight in air. If we assume a horizontal load of 1000 lbs., 30 ft. of water and 5/16" chain (which weighs 1.0 lbs./ft. underwater) we find that: Horizontal load = T = 1000 + (30 * 1.0) = 1030 lbs. Vertical load = Tv = 247 lbs. Rode length = 247 ft. Scope = Length to depth ratio = 8.23 Now if we increase the chain diameter to 1/2" (underwater weight is 2.57 lbs./ft.), we get: Horizontal load = T = 1000 + (30 * 2.57) = 1077 lbs. Vertical load = Tv = 400 lbs. Rode length = 156 ft. Scope = Length/d = 5.2 Increasing the chain diameter lets us cut the length of the rode by 91 ft., about a third, however it almost doubles the vertical weight on the bow and would materially decrease its ability to lift over waves. A effect similar to using heavier chain can be achieved by fixing a weight roughly equivalent to the difference in weights of the heavier over the lighter chain near the

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midpoint of the rode. In both cases the chain is far stronger than necessary to handle the load, the scope reduction attributed to the larger chain comes simply from its increased weight. The anchor, of course, has to be capable of holding the horizontal load. If the Danforth catalog is to be believed, a 5H anchor, just about key chain sized, holds 2700 lbs in hard sand. A muddy bottom would require a 20H to 35H anchor. My own approach is to use the biggest anchor my wife can lift and the longest nylon rode that I can let out in a given anchorage. Let me qualify some of the things I've just said. Both chain and nylon rodes, or any combination of the two, are special cases of a hypothetical general anchoring system which consists of a weightless, unstretchable line connecting the anchor and the boat with a single concentrated weight hung somewhere along its length. The weight serves two purposes. It keeps the anchor shank angle more nearly horizontal, reducing the chance of the anchor pulling out or dragging; and, it gives a degree of elasticity to the rode, requiring the weight to be lifted before becoming taut and jerking on the anchor. The best position for the weight depends on what you want to do. To keep the anchor shank as flat as possible, the weight should be attached to the lower end of the rode near the anchor. For best rode elasticity, the weight should be positioned at one water depth from the upper end of the rode. Maximum elasticity is achieved when the rode hangs straight down from the bow to a weight resting on the bottom, then goes horizontally to the anchor. Such an arrangement really hurts the bow's ability to lift over the waves. The Danforth booklet suggestion that the weight be hung halfway along the rode is a compromise between the two requirements. There are several basic problems when using a nylon rode. The first is to assure adequate elasticity. New, three strand standard laid nylon rope can stretch up to 50% before breaking. The stretch at lower tensions is almost directly proportional to the strain. An elastic nylon rode should be designed to stretch about 15% to 20% between maximum and no load conditions. If the rope is too thick, it will not stretch sufficiently and you have the equivalent of a chain rode without the catenary effect of chain. The best way of determining the optimum diameter for the nylon rope is to calculate the maximum expected tension on the line, double it, then consult a table of nylon rope strengths to determine the required diameter. This assumes, of course, that the rope is comparatively new, has no knots or abrasions, and has a well formed and thimbled eye splice at the anchor. If you don't have a table of nylon ropes available, the following equation works pretty well for determining rope diameter: Diameter = sq. rt. of ((3.1416 * Tension)/18000) Thus our hypothetical pleasure boat, anchored in a 60 kt. wind, with an

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anchor rode tension of 1030 lbs. would require a nylon anchor rode .424 inches in diameter. In this case I would use a 1/2" rope. In a 40 kt. wind, a 3/8" rope would be more than sufficient. Most modern lightweight anchors are designed to work with the pull on the shank being no more than 8 degrees above the horizontal. This includes Danforths, Fortresses, Ploughs, Deltas, and their variants. The sine of 8 degrees = .139. To achieve the required angle with an unweighted line, the line must be (DEPTH/.139) long or about 7.2 times the depth of the water. This is where the famous 7:1 scope requirement comes from. Any weight placed near the shank lowers this requirement. So a length of chain placed next to the shank of a lightweight anchor satisfies two requirements at once. It lessens the slope of the line and protects the vulnerable nylon from abrasion. Since the anchor rode is only as strong as its weakest link, the chain should have at least the proof strength of the breaking strength of the line. This condition is usually satisfied by chain one size smaller than the line size. Thus a 1/2" line with a breaking strength of 7100 lbs. should have no less than a 7/16 chain with a proof strength of 7200 lbs. A 3/8" line can use a 5/16" chain, etc. Calculations for the optimum length of chain on a combined nylon chain rode are complex. However William van Dorn in "Oceanography and Seamanship"; Dodd, Mead (1974), presents a graph based on calculations for anchoring oceanographic vessels in storm conditions. It suggests that the optimum chain/nylon combination for anchoring vessels < 50ft. in 30 ft. of water under storm conditions is a 20% chain, 80% nylon rode with an overall scope of 6:1. Assuming that the boat's bow chock is 6 feet above the water and that the waves are 4 feet (8 feet peak to trough) this works out to a 240 foot total rode comprised of 48 feet of chain and 192 feet of nylon. Clearly these are extreme conditions. In shallower water the rode could be reduced proportionately. However, the length of chain required approximates one boat length and a good working rule for a combined rode is a boat length of chain plus whatever nylon is required to give a 6:1 scope. In shallower water, the scope should be increased, within swing limitations, to 7:1 to permit the bow to lift more easily to the choppy waves near the shore. In summary, at a fixed anchoring depth, the longer the rode, the less chain required. The shortest rodes are achieved with all chain, the heavier the better, but the penalty is increased weight and handling difficulty, and the slightly increased possibility of catastrophic failure if the chain stretches taut. Don't drag! Larry Zeitlin

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