The COMPLETE ANCHORING HANDBOOK

Stay Put on Any Bottom in Any Weather

By Alain Poiraud, Achim Ginsberg-Klemmt, Erika Ginsberg-Klemmt

The McGraw-Hill Companies, Inc.

Copyright © 2008 Alain Poiraud, Achim Ginsberg-Klemmt, and Erika Ginsberg-Klemmt
All rights reserved.
ISBN: 978-0-07-147508-2

Contents

Foreword by Jimmy CornellAcknowledgmentsIntroduction by Erika Ginsberg-KlemmtCHAPTER 1 Seabed CharacteristicsCHAPTER 2 The Forces on an Anchor.CHAPTER 3 Anchor Selection.CHAPTER 4 Anchor RodeCHAPTER 5 Deck Equipment and LayoutCHAPTER 6 Anchoring TechniquesCHAPTER 7 Advanced Anchoring TechniquesCHAPTER 8 MooringsAppendix 1: A Theoretical Study in Rode Behavior by Alain FraysseAppendix 2: Unconventional Rode SolutionsConversion TableManufacturers and DistributorsAbout the AuthorsIndex

Excerpt

CHAPTER 1

Seabed Characteristics

Most discussions on anchoring begin with the anchor itself, but we believe itmakes more sense to look first at the seabed in which you want your anchor tohold, and then look at the forces that can make your anchor drag or dislodge.This is how we have approached the discussion in this book, and we hope thefirst two chapters provide a solid context for the discussion in Chapter3 on anchor selection.

The seafloor is one of the most overlooked aspects of anchoring. No responsibleskipper disregards visible problems, such as a loose cleat or a worn link in ananchor chain, but what remains unseen is often ignored. What you can’t see,however, can of course hurt you, so it’s essential to pay attention to thenature of the bottom in which you hope to set your anchor.

Unfortunately, not all anchorages offer large areas with excellent holdingground in dense clay or fine sand. Seafloor characteristics can vary greatlywithin a few feet. Two boats side by side in an anchorage may easily have theiranchors in different sediments, and even when the sediment itself ishomogeneous, you may encounter a slippery forest of dense weed just a few feetfrom a very good-holding sand patch. Asking your neighbor how the holding is ina particular anchorage may contribute to your decision making, but you can neverreally know the quality of any given anchor ground with absolute certaintyunless you retrieve a sample or dive down and look for yourself!

Sometimes crystal-clear water will offer a view of what you’re sinking into,even in deep water. Most seasoned mariners learn to read the chiaroscuro mosaicbelow, aiming for the lighter patches of sand between the dark boulders,seaweed, or other impenetrable spots.

But your eyes will only help so much. A good view of hard, compact sand may lookthe same as the loosely packed spot a few feet away. What’s more, even with anexcellent view of your anchoring field, that’s only the upper layer. Once ananchor has pierced the bottom surface, it should dig itself into the subsurfacelayer. A sandy surface may only be a few inches deep, hiding an impenetrablerock plate below.

If you can see your anchor safely tucked in the seabed, you may feel safe inyour bunk bed, too. But be aware that even a completely buried anchor is noguarantee; soft mud, seashells, and pebbles offer only precarious holding.

Nautical charts, cruising guides, and pilots often give helpful information onseabed characteristics in a particular area, so you should always check yourchart when preparing to anchor to see what it says about the type of bottom; seeTable 1-1 for a list of abbreviations. This information, however, may beinaccurate or may not represent your exact anchoring spot. So if you don’t wantto get out your mask and snorkel, do what the ancient mariners did: heave asounding line.

A sounding line or lead line is a length of rope with a leadweight at the lowered end. Used to measure depth, this handy device also allowsyou to check seafloor characteristics. Put tallow, wax, or grease on the bottomof the lead weight to pick up traces of mud, sand, or shingle from the seabed.If you don’t have a lead line, put some grease on your anchor, lower that, thenbring it back up to see what’s sticking to the flukes. This will work, but alead line is much easier!

TYPES OF SEAFLOORS

Thanks to the meticulous work of cartographers, surveyors, and marinegeologists, we have access to fairly accurate data pertaining to the seafloorsediments near the coastlines of the world. Geologists have classified theseabed into four major categories by particle size: muds, sands, gravels, androcks. Keep in mind that these classifications are complicated by the fact thatseafloors rarely have a homogeneous surface. Sand can be muddy, covered inalgae, or contain a greater or lesser proportion of shell or coral fragments.The various types of seafloors differ in their penetrability and their capacityfor holding anchors.

Mud

Geologists define mud as consisting of particles smaller than 62.5 microns indiameter. We can’t see a particle this small with the naked eye, since it takes1,000 microns to make 1 millimeter and 25.4 millimeters to equal 1 inch. Mud issubdivided into clay particles, which are smaller than 4 microns in diameter,and silt particles, which are 4 to 62.5 microns.

A semiliquid mixture of water and sediment, mud is soupier and lighter than pureclay, and less sticky. The softer and soupier mud is, the weaker its holdingcapacity will be.

Sand

Sand is comprised of rock that has been abraded by wave action into particles,or granules, ranging in size from 0.063 mm (63 microns) to 2 mm. Sand can befurther subclassified as fine sand (0.06 mm to 0.2 mm), medium sand (0.2 mm to0.6 mm), and coarse sand (0.6 mm to 2 mm). A mixture of fine and medium sand isconsidered dense sand for purposes of holding (see Table 1-2). Note thateven the coarsest grain of sand is no more than a tenth of an inch in diameter.

Some sands are made up of the skeletal material of marine organisms. Coral sandis created not only by wave action but also by bio-erosion. For example, parrotfish bite off pieces of coral, digest the living tissue, and excrete theinorganic component as silt and sand. Coral sand is one of the better materialsin which to sink your anchor. Long live the parrot fish!

Gravel and Rocks

The next coarser sediment class above sand is gravel, with particles rangingfrom 2 mm to 60 mm (i.e., from 1/10 inch to roughly 2.5 inches). We cansubclassify gravel into granules (2 mm to 6 mm), pebbles (6 mm to 20 mm), andstones (20 mm to 60 mm). Since it has little cohesion, gravel is one of theworst materials in which to anchor. It can also prevent an anchor from settingeven when it covers a more desirable sediment such as fine sand. You can onlyhope that if your anchor digs itself in deeply enough, the sheer weight of thegravel will keep it in place.

Above 60 mm (2.5 inches) in diameter, we have rocks, then boulders. (In theUnited Kingdom, cobbles are considered larger than pebbles and smaller thanboulders, in the size range of stones and rocks.) Rock bottoms offer no holdingpower at all, unless you get lucky enough to hook a fluke under a boulder or ina crevice, in which case you will need equal luck to unhook the chain or anchorwhen the time comes to leave. A trip line for retrieval is recommendedwhen dealing with a rocky bottom (see Chapter 7).

Table 1-2 compares the holding power of various bottom sedimentsrelative to dense sand. For example, based on the holding coefficients listed,an anchor that would normally provide a holding capacity of 1,000 pounds whenproperly set in dense sand would hold 1,500 pounds in dense clay, but only 400pounds in coarse sand. Dense clay is the most secure of all sediments, but onlyif your anchor will set properly. If your anchor will set in sand but not clay,you’re better off anchoring in sand—a subject we’ll return to.

Modern anchors are endowed with high holding power relative to weight, and wemight therefore be inclined to select an undersized one. But although a smallanchor might function well in excellent holding grounds, it may fail in poorconditions. From Table 1-2 we can estimate that to achieve the sameholding power in soft mud as in dense sand, we would need an anchor with morethan double the holding power.

Knowing as much as possible about your chosen seafloor will give you an edge forsinking your anchor in and staying put. When an anchor slips, many are quick tofind fault with their tackle or tactics, but ignoring the characteristics of theseabed is tantamount to “plug and pray.” Even the best anchors may offer poorholding on a hard, compact seafloor or soft mud. No matter how “ideal” theanchor, rode, and tactics might be, one type of anchor will stab at or slidealong the top of a hard surface, while another will rake through an ultrasoftsoup of ooze and weeds. We will illuminate why in the next chapters.

CHAPTER 2

The Forces on an Anchor

Sir Isaac Newton contributed to the science of anchoring in more ways than one.For one, his work on gravity provided the basis for understanding the effects ofthe moon and the sun on the tides. For another, Newton’s three laws of motiondescribe the relationship between the motion of an object and the forces actingon it. We turn to the first two laws to help describe the effects of variousforces on anchor gear.

Newton’s first law of motion states that a body at rest will remain at restunless acted upon by an external force. Thus, a vessel floating in calmwaters—completely unaffected by wind and current—would stay put withno anchor at all. In practice, of course, this is never the case for long.

The second law describes how the velocity of an object changes when it issubjected to an external force. It states that the acceleration of an object isdirectly proportional to the magnitude of the net force acting on the object andinversely proportional to its mass.

Thus the equation:

F (force)= m(mass) × a(acceleration)

Put another way, force is defined as a change in momentum (mass × velocity) perunit of time. This law gave rise to the newton (N), the unit of forcerequired to accelerate a mass of 1 kilogram by 1 meter per second per second. Wewill use the decanewton (daN; i.e., 10 newtons) to quantify the forceexerted by wind on a vessel and thus on its anchor (1 daN = 1.02 kg or 2.25 lb.of force).

For a boat at anchor, the force in question is the load exerted on the anchor bywind, wave, or current acting on the boat—or by a combination of these.The load due to the pressure of wind on the boat is relatively easy toapproximate. It is much more difficult, however, to determine the intermittentloads on anchor gear that result from wave action. Even in a midsize vessel, theforces involved can reach several thousand pounds, which explains things likebroken ground tackle connectors and bent anchor shanks.

Wave action causes a boat at anchor to pitch and roll. Gusts of wind cause it tosheer back and forth on its rode, falling off first one way and then the other.The bow is blown off until the rode comes taut, snubbing the bow back into thewind. Then the boat surges forward, responding to the weight and elasticity ofthe anchor rode, until the next gust blows the bow off once more. An importantfactor in this horsing tendency is the location of the boat’s center ofeffort (CE)—the geometrical center of its exposed wind-surface areawithout sails relative to its center of lateral resistance (CLR) belowthe waterline.

The schooner in the left illustration shows a CE that is aft of the CLR. A windgust at anchor will thus tend to turn this boat’s bow into the wind,counteracting the undesired swaying motion of the vessel.

On the other hand, a catboat with stowed sails has its center of effort forwardof the center of lateral resistance. Wind gusts at anchor will tend to turn thebow of this boat away from the wind, amplifying the swaying motion and exposinga larger area of the hull and cabin to the wind. This horsing behavior putsadditional strain on the anchor gear.

When the CE is aft of the CLR, a wind gust at anchor will tend to turn thisschooner’s bow into the wind, resulting in an uncomfortable horsing movement.

By setting a small supporting sail—known as a riding sail—atthe stern of a vessel, or a reefed mizzensail on a ketch, sheeted amidships, askipper can move the CE farther aft to counteract the swaying of the boat andinduce it to lie more quietly to its anchor.

Another way to minimize the swaying of a boat is to form a bridle forthe anchor rode. When you’ve paid out most of the scope you think you need (seethe Importance of Scope section in Chapter 6), attach a secondary lineto the anchor rode with a rolling hitch. Thenpay out the last of your needed scope so that the rolling hitch is a boat lengthor so from the bow roller. Take the other end of the secondary lineaft—say to the primary cockpit winch—and put some tension on it. Theresult is an asymmetrical bridle, and the more tension you place on thesecondary line, the more you will misalign your boat’s keel to the winddirection. Keeping your vessel slightly misaligned with the wind can tone downits swaying motion substantially.

WIND FORCES

The force of the wind on an anchored boat—and thus the wind-induced loadon ground tackle—depends on two factors: the wind speed and the exposedsurface area of the boat. While wind speed is easily measured, exposed surfacearea is more difficult to discern. From the boat’s length, beam, and heightabove the waterline, we can derive a first-order estimate, but design and gearplay a large role as well. A sailboat equipped with roller furling, a largepilothouse, or a bimini—or a power cruiser with a canvas-enclosed flyingbridge or a tuna tower—will clearly present more surface area to the windthan similar boats without such appurtenances. A powerboat will generally havemore windage than a sailboat of equal length due to its higher freeboard,greater beam, and larger house structures.

But even a precise calculation of exposed surface area, were we able to deriveone, would be insufficient for a precise calculation of wind forces on theanchored boat. We need to know the frictional drag induced by the boat’s exposedsurfaces, and that depends on the shapes of the surfaces and their orientationsto the wind as well as their areas. In an effort to quantify this effect,aerodynamicists assign a drag coefficient (C_d)—a dimensionlessmeasure of aerodynamic sleekness independent of size—to an object, usuallyafter wind tunnel experiments. It would be very useful to know the dragcoefficients of your boat with the wind blowing from ahead or at angles up to,say, 30° off the bow; but since most of us do not have an America’s Cupbudget to spend on aerodynamic research, we will try to approximate the actualC_d value for a given vessel with a “best possible” guess.

A sleek car has a drag coefficient of about 0.30; a flat surface erected squareto the wind (picture a sheet of plywood) has a C_d of 1.98. For thehull and superstructure of an average sailing yacht with the wind blowing fromahead, we can assume a value of 0.7. For anaverage motor yacht, we can assume 0.8.

Where does this leave us? We can calculate forces exerted, or load, due to wind(F_w) on a given vessel by means of the formula:

F_w = ½ × ρ × C_d × A × V²

Where

ρ = density of air (1.225 kg/m³)

C_d = drag coefficient

A = frontal surface exposed to the wind

V = wind speed in km/h

More practically speaking, we can take a conservative estimate of the likelywind-induced loads on our boats from a table like Table 2-1, which wasdeveloped by the American Boat & Yacht Council (ABYC) and takes into account thesurface area an anchored boat presents to the wind when it is sheering back andforth on its ground tackle at angles of up to 30° from the wind.

Table 2-1 shows the immense increase in anchor loading as the windrises. The load on an exposed surface increases by a factor of four if the windspeed doubles. If the wind speed triples, the load will be nine times higher.

To use the table, select your boat’s length and beam and read off thecorresponding values to get an idea of what kind of loadings your anchor andground tackle should be able to cope with. If your boat’s beam is greater thanthat indicated for its length overall (LOA), drop down to the next line.

For example, for a 40-foot (12 m) sailboat with a width of 13 feet (3.95 m),enter the table as if for a 50-foot sailboat (we have bolded the appropriatefigures in the table to illustrate this). For a 30-knot breeze, the horizontalload on your anchor due to wind alone will be 1,600 pounds, or 730 daN. If thewind increases to 45 knots, the load will double.

WAVE FORCES

Table 2-1 assumes that the water is flat. If the effects of wave surge arefactored in, the intermittent loadings could be double or more. Still,independent calculations have shown that the values in the table areconservative enough to account for modest wave action.

Sheltered anchorages are usually protected from ground swell, but you cannotalways avoid wind-induced waves of more local origin. What can happen when weare unable to prevent wave-induced shock loads on our anchor gear?

Let’s imagine lying-to the hook in a popular anchorage that is protected fromalmost all directions. We have a heavy plow anchor deployed on an all-chain rodewith 5:1 scope—a classic combination—and the weather looks quitegood. We are enjoying a peaceful sun downer in the cockpit after a successfulbut grueling five-day passage to the Canary Islands from Gibraltarwhen—oops!—the weather forecast announces the expected arrival of ascirocco (a hot desert wind) during the night. After a moment of uncertainty wedecide to put out more chain to increase our scope but not so much as to riskswinging into our neighbors, who seem to be making a similar decision. Sun andsundowners disappear while the clouds on the horizon come closer. The windchanges direction and slowly increases in speed. Then a few stronger gusts showup, but we are still confident that this thing will be over soon, and we areworn out from the passage.
(Continues…)Excerpted from The COMPLETE ANCHORING HANDBOOK by Alain Poiraud. Copyright © 2008 by Alain Poiraud, Achim Ginsberg-Klemmt, and Erika Ginsberg-Klemmt. Excerpted by permission of The McGraw-Hill Companies, Inc..
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