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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.
Most discussions on anchoring begin with the anchor itself, but we believe it makes more sense to look first at the seabed in which you want your anchor to hold, 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 the first two chapters provide a solid context for the discussion in Chapter 3 on anchor selection.
The seafloor is one of the most overlooked aspects of anchoring. No responsible skipper disregards visible problems, such as a loose cleat or a worn link in an anchor 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 the nature of the bottom in which you hope to set your anchor.
Unfortunately, not all anchorages offer large areas with excellent holding ground in dense clay or fine sand. Seafloor characteristics can vary greatly within a few feet. Two boats side by side in an anchorage may easily have their anchors in different sediments, and even when the sediment itself is homogeneous, you may encounter a slippery forest of dense weed just a few feet from a very good-holding sand patch. Asking your neighbor how the holding is in a particular anchorage may contribute to your decision making, but you can never really know the quality of any given anchor ground with absolute certainty unless 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 mosaic below, 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 look the same as the loosely packed spot a few feet away. What's more, even with an excellent view of your anchoring field, that's only the upper layer. Once an anchor has pierced the bottom surface, it should dig itself into the subsurface layer. A sandy surface may only be a few inches deep, hiding an impenetrable rock plate below.
If you can see your anchor safely tucked in the seabed, you may feel safe in your bunk bed, too. But be aware that even a completely buried anchor is no guarantee; soft mud, seashells, and pebbles offer only precarious holding.
Nautical charts, cruising guides, and pilots often give helpful information on seabed characteristics in a particular area, so you should always check your chart when preparing to anchor to see what it says about the type of bottom; see Table 1-1 for a list of abbreviations. This information, however, may be inaccurate or may not represent your exact anchoring spot. So if you don't want to get out your mask and snorkel, do what the ancient mariners did: heave a sounding line.
A sounding line or lead line is a length of rope with a lead weight at the lowered end. Used to measure depth, this handy device also allows you to check seafloor characteristics. Put tallow, wax, or grease on the bottom of 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, then bring it back up to see what's sticking to the flukes. This will work, but a lead line is much easier!
TYPES OF SEAFLOORS
Thanks to the meticulous work of cartographers, surveyors, and marine geologists, we have access to fairly accurate data pertaining to the seafloor sediments near the coastlines of the world. Geologists have classified the seabed into four major categories by particle size: muds, sands, gravels, and rocks. Keep in mind that these classifications are complicated by the fact that seafloors rarely have a homogeneous surface. Sand can be muddy, covered in algae, or contain a greater or lesser proportion of shell or coral fragments. The various types of seafloors differ in their penetrability and their capacity for holding anchors.
Geologists define mud as consisting of particles smaller than 62.5 microns in diameter. We can't see a particle this small with the naked eye, since it takes 1,000 microns to make 1 millimeter and 25.4 millimeters to equal 1 inch. Mud is subdivided 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 pure clay, and less sticky. The softer and soupier mud is, the weaker its holding capacity will be.
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 be further subclassified as fine sand (0.06 mm to 0.2 mm), medium sand (0.2 mm to 0.6 mm), and coarse sand (0.6 mm to 2 mm). A mixture of fine and medium sand is considered dense sand for purposes of holding (see Table 1-2). Note that even 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 sand is created not only by wave action but also by bio-erosion. For example, parrot fish bite off pieces of coral, digest the living tissue, and excrete the inorganic component as silt and sand. Coral sand is one of the better materials in which to sink your anchor. Long live the parrot fish!
Gravel and Rocks
The next coarser sediment class above sand is gravel, with particles ranging from 2 mm to 60 mm (i.e., from 1/10 inch to roughly 2.5 inches). We can subclassify gravel into granules (2 mm to 6 mm), pebbles (6 mm to 20 mm), and stones (20 mm to 60 mm). Since it has little cohesion, gravel is one of the worst materials in which to anchor. It can also prevent an anchor from setting even when it covers a more desirable sediment such as fine sand. You can only hope that if your anchor digs itself in deeply enough, the sheer weight of the gravel will keep it in place.
Above 60 mm (2.5 inches) in diameter, we have rocks, then boulders. (In the United Kingdom, cobbles are considered larger than pebbles and smaller than boulders, in the size range of stones and rocks.) Rock bottoms offer no holding power at all, unless you get lucky enough to hook a fluke under a boulder or in a crevice, in which case you will need equal luck to unhook the chain or anchor when the time comes to leave. A trip line for retrieval is recommended when dealing with a rocky bottom (see Chapter 7).
Table 1-2 compares the holding power of various bottom sediments relative to dense sand. For example, based on the holding coefficients listed, an anchor that would normally provide a holding capacity of 1,000 pounds when properly set in dense sand would hold 1,500 pounds in dense clay, but only 400 pounds in coarse sand. Dense clay is the most secure of all sediments, but only if 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 we might therefore be inclined to select an undersized one. But although a small anchor might function well in excellent holding grounds, it may fail in poor conditions. From Table 1-2 we can estimate that to achieve the same holding power in soft mud as in dense sand, we would need an anchor with more than double the holding power.
Knowing as much as possible about your chosen seafloor will give you an edge for sinking your anchor in and staying put. When an anchor slips, many are quick to find fault with their tackle or tactics, but ignoring the characteristics of the seabed is tantamount to "plug and pray." Even the best anchors may offer poor holding on a hard, compact seafloor or soft mud. No matter how "ideal" the anchor, rode, and tactics might be, one type of anchor will stab at or slide along the top of a hard surface, while another will rake through an ultrasoft soup of ooze and weeds. We will illuminate why in the next chapters.
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 of the moon and the sun on the tides. For another, Newton's three laws of motion describe the relationship between the motion of an object and the forces acting on it. We turn to the first two laws to help describe the effects of various forces on anchor gear.
Newton's first law of motion states that a body at rest will remain at rest unless acted upon by an external force. Thus, a vessel floating in calm waters—completely unaffected by wind and current—would stay put with no 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 is subjected to an external force. It states that the acceleration of an object is directly proportional to the magnitude of the net force acting on the object and inversely 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) per unit of time. This law gave rise to the newton (N), the unit of force required to accelerate a mass of 1 kilogram by 1 meter per second per second. We will use the decanewton (daN; i.e., 10 newtons) to quantify the force exerted 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 by wind, 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 to approximate. It is much more difficult, however, to determine the intermittent loads on anchor gear that result from wave action. Even in a midsize vessel, the forces involved can reach several thousand pounds, which explains things like broken ground tackle connectors and bent anchor shanks.
Wave action causes a boat at anchor to pitch and roll. Gusts of wind cause it to sheer 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 the wind. Then the boat surges forward, responding to the weight and elasticity of the anchor rode, until the next gust blows the bow off once more. An important factor in this horsing tendency is the location of the boat's center of effort (CE)—the geometrical center of its exposed wind-surface area without sails relative to its center of lateral resistance (CLR) below the waterline.
The schooner in the left illustration shows a CE that is aft of the CLR. A wind gust 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 forward of the center of lateral resistance. Wind gusts at anchor will tend to turn the bow of this boat away from the wind, amplifying the swaying motion and exposing a larger area of the hull and cabin to the wind. This horsing behavior puts additional strain on the anchor gear.
When the CE is aft of the CLR, a wind gust at anchor will tend to turn this schooner's bow into the wind, resulting in an uncomfortable horsing movement.
By setting a small supporting sail—known as a riding sail—at the stern of a vessel, or a reefed mizzensail on a ketch, sheeted amidships, a skipper can move the CE farther aft to counteract the swaying of the boat and induce it to lie more quietly to its anchor.
Another way to minimize the swaying of a boat is to form a bridle for the anchor rode. When you've paid out most of the scope you think you need (see the Importance of Scope section in Chapter 6), attach a secondary line to the anchor rode with a rolling hitch. Then pay out the last of your needed scope so that the rolling hitch is a boat length or so from the bow roller. Take the other end of the secondary line aft—say to the primary cockpit winch—and put some tension on it. The result is an asymmetrical bridle, and the more tension you place on the secondary line, the more you will misalign your boat's keel to the wind direction. Keeping your vessel slightly misaligned with the wind can tone down its swaying motion substantially.
The force of the wind on an anchored boat—and thus the wind-induced load on ground tackle—depends on two factors: the wind speed and the exposed surface area of the boat. While wind speed is easily measured, exposed surface area is more difficult to discern. From the boat's length, beam, and height above the waterline, we can derive a first-order estimate, but design and gear play a large role as well. A sailboat equipped with roller furling, a large pilothouse, or a bimini—or a power cruiser with a canvas-enclosed flying bridge or a tuna tower—will clearly present more surface area to the wind than similar boats without such appurtenances. A powerboat will generally have more 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 derive one, would be insufficient for a precise calculation of wind forces on the anchored boat. We need to know the frictional drag induced by the boat's exposed surfaces, and that depends on the shapes of the surfaces and their orientations to the wind as well as their areas. In an effort to quantify this effect, aerodynamicists assign a drag coefficient (Cd)—a dimensionless measure of aerodynamic sleekness independent of size—to an object, usually after wind tunnel experiments. It would be very useful to know the drag coefficients 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 Cup budget to spend on aerodynamic research, we will try to approximate the actual Cd 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 square to the wind (picture a sheet of plywood) has a Cd of 1.98. For the hull and superstructure of an average sailing yacht with the wind blowing from ahead, we can assume a value of 0.7. For an average motor yacht, we can assume 0.8.
Where does this leave us? We can calculate forces exerted, or load, due to wind (Fw) on a given vessel by means of the formula:
Fw = ½ × ρ × Cd × A × V2
ρ = density of air (1.225 kg/m3)
Cd = 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 likely wind-induced loads on our boats from a table like Table 2-1, which was developed by the American Boat & Yacht Council (ABYC) and takes into account the surface area an anchored boat presents to the wind when it is sheering back and forth 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 wind rises. The load on an exposed surface increases by a factor of four if the wind speed 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 the corresponding values to get an idea of what kind of loadings your anchor and ground tackle should be able to cope with. If your boat's beam is greater than that 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 appropriate figures in the table to illustrate this). For a 30-knot breeze, the horizontal load on your anchor due to wind alone will be 1,600 pounds, or 730 daN. If the wind increases to 45 knots, the load will double.
Table 2-1 assumes that the water is flat. If the effects of wave surge are factored in, the intermittent loadings could be double or more. Still, independent calculations have shown that the values in the table are conservative enough to account for modest wave action.
Sheltered anchorages are usually protected from ground swell, but you cannot always avoid wind-induced waves of more local origin. What can happen when we are 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 from almost all directions. We have a heavy plow anchor deployed on an all-chain rode with 5:1 scope—a classic combination—and the weather looks quite good. We are enjoying a peaceful sun downer in the cockpit after a successful but grueling five-day passage to the Canary Islands from Gibraltar when—oops!—the weather forecast announces the expected arrival of a scirocco (a hot desert wind) during the night. After a moment of uncertainty we decide to put out more chain to increase our scope but not so much as to risk swinging into our neighbors, who seem to be making a similar decision. Sun and sundowners disappear while the clouds on the horizon come closer. The wind changes direction and slowly increases in speed. Then a few stronger gusts show up, but we are still confident that this thing will be over soon, and we are worn out from the passage.
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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