> Does anyone happen to have a saved copy of the great anchoring treatise > by > Capt. Wil Andrews from New Bern, NC; published many years ago. I wanted > to > pass it on to a friend but have lost my saved copy somewhere in > cyberspace. > > Thanks. > > Dan Stone > St. Petersburg, FL > > Capt. Wil's piece on anchoring is an excellent tutorial but I think that he tends to neglect the importance of the anchor rode. Here is a short compilation of pieces I posted on the TWL about five years ago. - - - - - - ANCHOR RODE TUTORIAL BASIC CONSIDERATIONS: 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.) 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/(1/2 * 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 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 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)/1/8000) Thus our hypothetical pleasure boat, anchored in a 60 kt. wind, with an 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. RODE ELASTICITY: No argument that a heavier boat needs more substantial ground tackle but the anchor line should not be expected to withstand the boat's inertia by strength alone. That's what the elasticity of the rode is for. At very low speeds, water is an essentially frictionless supporting medium and a small force exerted over a long period can move a very heavy mass. That's how a single mule could drag a 20 ton barge on the Erie Canal. What requires a strong line is the rapid change in velocity of the heavy mass. If the velocity change (acceleration) is very small, the force is equally small. An elastic line transforms a short quick change in velocity into a long slow change in velocity. Think of the rode as a horizontal bungee cord. If a bungee jumper lept from a high bridge attached to an unyielding steel cable (or a chain) the force his body would feel when he reached the end of the cable would be the same as if he hit the ground. It would certainly rip his legs off. Attach him to an elastic bungee cord which stretches as he falls, and the force is reduced to the point where it is tolerable and even fun. The energy is stored in the bungee cord and he rebounds almost to the starting height. The same with a mooring line. The inertial force at the end of a "sailing" arc or due to boat motion from a strong wave impact would be transmitted directly to the anchor if the rode did not have any elasticity. It is the elasticity of the line that stretches the force out in time and diminishes its pull on the anchor. The energy is stored in the line and released by slowly pulling the boat back into position. A nylon line stores the energy by stretch. When loaded to 50% of its breaking strength, a nylon line will stretch almost 25%. That's why Ogg preferred light nylon lines to get the maximum stretch. In a chain rode, the energy is stored by gravity, flattening out the catenary. Gravity pulls the boat back into position as the line resumes its normal curve. The heavier the chain, the more energy stored and the more elastic the rode. What seems to be the cause of the confusion is that big boats with all chain rodes, as is typical of most trawlers on the list, require a heavier chain to store enough energy to provide sufficient elasticity. Many trawler owners confound this with the idea of rode strength. In reality a relatively small diameter chain has enough breaking strength to anchor all but the largest trawlers. Wire rope, pound for pound, is much stronger than chain but is rarely used in trawler anchor rodes because it is too light to provide any shock attenuation at all. But a wire rope with a big kellet at its midpoint would be just as good as the equivalent weight of chain. I come from a sailing environment where weight is the enemy. I've gotten used to using nylon rodes and lightweight anchors and have been doing so for nearly half a century. Now I hedge my bets by using the "ideal" rode based on experience with oceanographic reseach vessels. That is, a boat's length of chain attached to the anchor and a well chafe protected nylon rode that is 50% loaded at the maximum wind force the boat is likely to encounter. For my Willard, I never use anything greater that a 1/2" line. Sometimes that seems too thick. One of my anchors has a 3/8" line that is adequate for most conditions. I use two independent 3/4" lines as a pennant to my permanent mushroom mooring chain. But plenty of 1/2" chain and a 50 lb kellet to the anchor. It has held the boat through two hurricanes in the last 10 years. OK, one hurricane was a near miss but we still had over 70 kt. winds and a storm surge. Remember we are talking only about anchoring where the boat is essentially motionless except for action of wind, waves and current. Towing is an entirely different matter. Don't drag! Larry Zeitlin