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Old 06-04-2007
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To Have and To Hold (anchoring)

Untitled Document

To have and to hold
The basic rules of anchoring -- and staying put
by Don Launer

Sponsored by Good Old Boat Magazine

Everyone who owns a boat will have occasion to anchor at one time or another, perhaps when cruising overnight, clamming, fishing, swimming, or visiting friends by water. There are many variables when anchoring: the type of anchor and ground tackle; the composition of the bottom; the type, size, and windage of the boat; the speed and direction of the tidal flow; and the wind speed and direction.

The wind

"...by late afternoon the sea breeze will have shifted around clockwise, and your anchorage may no longer be protected from wind and waves."

This last factor -- wind speed and direction -- makes anchoring a challenge along our coastal waters. On large bodies of water the wind usually follows a daily recurring pattern, which is termed diurnal. This pattern ordinarily consists of winds being very gentle in the morning. Then around 11 a.m., little cat's-paws wrinkle the surface as the sea breeze picks up. Wind speeds from this sea breeze increase until about 4 p.m. and begin to die down toward sunset. As any boater knows, these afternoon winds can sometimes become daunting, much greater than those that were forecast.

This sea breeze along the coast is a very local effect. It is created when the air over the mainland is heated by the sun and rises. Then cooler air from over the water blows toward the land to fill in the gap left by that rising air mass, creating the sea breeze. This sea breeze is also modified by the prevailing winds. Furthermore, the sea breeze will also change direction as the day progresses. In the Northern Hemisphere the earth's rotation causes this wind to shift clockwise during the day, as the wind tries to reach what is known as geostrophic equilibrium. (In the Southern Hemisphere this shift in wind direction is counter-clockwise as the day progresses.)

This makes anchoring a challenge. If you've picked a protected anchorage in the late morning or early afternoon, you can be reasonably sure that by late afternoon the sea breeze will have shifted around clockwise, and your anchorage may no longer be protected from wind and waves.

But have you ever wondered, when the wind was trying to blow the paint off your boat, exactly how many pounds of force the wind was exerting on your anchor line? There have been several attempts to characterize the loads on an anchor system. Some approaches use a purely theoretical calculation of drag caused by wind. Other researchers have simply made measurements in actual conditions.

The published results vary so widely that it is difficult to rationalize their differences. The American Boat and Yacht Council (ABYC) has a table in their standard H40. In the latest revision they call the table "Design Loads for Sizing Deck Hardware." For purposes of comparison, the design load for a 30-foot sailboat in 30 knots of wind (classed as a working anchor load) is 700 pounds. The same 30-foot boat is listed to have a design load of 1,400 pounds in 42 knots of wind (classed as a storm anchor load).

Direct measurements by naval architect Robert Smith, published in his book, Anchors, Selection and Use, give the loads for the same boat as 192 pounds at 30 knots and 375 pounds at 42 knots. This is a comparison between calculated loads and actual measured loads, but the ABYC table states that the loads are design loads, which certainly have a safety factor (although the ABYC does not say what the safety factor is), and the ABYC table claims to account for wind, current, and wave action. Robert Smith's tests were conducted in a level area of the Columbia River with a fine sand bottom and a fetch of four miles to windward.

Fortunately, retailers and anchor vendors offer advice on the selection of anchor gear. Also, there is an excellent and well-reasoned section in Nigel Calder's Cruising Handbook that goes into great depth about the selection of anchors and rodes.

So putting actual numbers to the matter may be difficult, but doing something sensible in the way of designing anchor points and selecting gear is still possible.

In any case, it is good to remember that the force exerted by the wind on your boat and the strain on the anchor line increases as the square of the wind speed. In other words, as the wind speed doubles, the force becomes four times greater. For sailboats (or powerboats with a fly-bridge and tuna tower), the wind resistance is dramatically higher because of the larger square-footage exposed to the wind.

The anchor

Boatowners are very defensive when it comes to the type of anchor they use; they defend their choice of anchor with an almost religious vigor -- so I will refrain from entering the fray by not suggesting any specific anchor type. Generally, manufacturers' ratings for anchors, nylon lines, and chain are dependable. The type of bottom, however, has a profound effect on any anchor's ability to set and hold. Soft mud, for example, can reduce holding ability by as much as 85 percent. Also be aware that a "bargain" anchor may not be such a bargain when the wind pipes up.

One of the blessings of modern anchor design is the high holding power compared to anchor weight. This is because, rather than depending upon deadweight, modern anchors are designed to make use of the pull of the boat, together with the forces of wind and wave, to make the anchor bury deeper into the bottom.

The anchor rode

To do this, we need to let out enough anchor rode to allow the anchor to lie nearly flat on the bottom. Aside from the anchor itself, the angle of pull on the rode is most important. The length of this rode is expressed as scope. This is the ratio between two distances: the length of the anchor rode and the distance from the sea bed to the deck of the boat at the point of pull. If the distance from the deck of the boat to the sea bed is 10 feet and we have let out 50 feet of anchor rode, our scope ratio would be 5:1 (see illustration).

Most anchor manufacturers recommend minimum scope ratios between 5:1 and 7:1, but with high winds or a soft bottom, more scope is needed. Below 5:1, the holding power of an anchor falls off very rapidly, so many experts suggest at least a minimum of a 7:1 scope, if you have enough swinging room. Remember that in areas of high tides or when storm surges are expected, a respectable scope at low tide can turn into a poor scope when the tide or surge comes in.

Although we have been talking about anchor line, many boaters insist on using all chain for the anchor rode, since chain has the ultimate chafe-resistance. Their reasoning is also that the chain's weight causes a sag, called catenary, and as the boat surges, some of the catenary is taken up, absorbing the shock load.

Depending entirely on this catenary to absorb shock loads is not recommended. It takes very little force to make this sag disappear, leaving the boat at the mercy of what amounts to a solid bar of steel between the anchor and the bow of the boat. And when your bow is connected to the anchor with a solid bar of steel and starts pitching up and down in a chop, something has got to give. This often results in the anchor being yanked out of the bottom. With an all-chain rode you need a cushioning for those jarring shock loads. A nylon line, with a chain hook at one end acting as a snubber, is a simple way to take up these shocks. This snubber line should be of relatively small diameter to provide as much elasticity as possible, since the amount of stretch is inversely proportional to the square of the line's diameter.

Abrasion-resistant

An advantage of all-chain rode is that it cannot be cut by sharp rocks or coral or abraded at the bow chock. In addition, the chain's weight makes the pull on the anchor more horizontal, aiding in the setting process.
Additional factors to consider when using all-chain rode are that it is expensive, has a great propensity for collecting mud, and puts a large weight into the chain locker at the bow of the boat where it's not wanted. Also, with age some of the galvanizing will wear off and the chain will begin to rust.

It's in heavy blows that we need good shock absorption in our anchor system the most, an anchor rode of three-stranded nylon provides the best shock absorption. This is derived not from catenary but from the line stretching. However, an all-rope rode will be subject to chafe as it scrapes along the bottom, especially if some of that bottom is rocky. This all-nylon rode is also subject to abrasion where it passes over the bow chocks.

"Boatowners are very defensive when it comes to the type of anchor they use; they defend their choice of anchor with an almost religious vigor..."

Nevertheless, nylon has excellent strength, abrasion-resistance and, of all the fibers available, it has the best shock-absorbing ability. At its normal working load of 11 percent of breaking strength, three-stranded nylon has a 23 percent elongation rate. At 75 percent of breaking strength, a 100-foot section of nylon will become 142 feet long. Rigger Brion Toss, on the other hand, suggests considering Dacron instead of nylon. Although Dacron doesn't have as much stretch as nylon, it does stretch. Also, although it doesn't have the strength of nylon when nylon is dry, wet nylon loses 10 to 15 percent of its strength and becomes weaker than Dacron. Regardless of whether nylon or Dacron is used, the bow chock it passes through should have the smoothest, largest radius possible, and the line should preferably be protected by chafing gear at this point.

Chain adds weight

To protect an all-rope rode from bottom abrasion and to add more weight during the setting process, a section of chain is necessary. The length of this chain will vary for different regions. For lunch-hooks, as a general rule 6 to 12 feet of chain is more than enough chain in most anchorages, except where extensive outcroppings of rock or coral may be encountered, but the anchor rode should always include nylon or Dacron line . . . ideally, as much as possible. When anchoring overnight or in storm conditions, it is recommended that a chain length of one half to one full boatlength be used. In these conditions an all-chain rode could also be used, but it should have a nylon snubber.

When an anchor line is connected to chain, the chain should be at least half the diameter of the nylon line; that is, for a 1/2-inch nylon rode, you should use at least 1/4-inch chain. There are several methods for connecting these two components of the anchor rode. One is by using an eye-splice and thimble at the end of the nylon rode. A shackle is then used to connect the thimble at the end of the nylon section to the chain. This shackle should be one size larger than the chain. That is, if 1/4-inch chain is being used, use a 5⁄16-inch shackle to attach the nylon line to the chain.

One of the problems with using a shackle for this chain-to-rope connection is that, if the anchor hasn't broken loose when this connection reaches the bow chock, you have a problem. One trick is to join the rope and chain so there are no hang-ups as it passes through the bow chock. This can be done using a chain splice, which allows the attachment point to come through the bow chock so the chain can be led into the gypsy of the anchor windlass. However, this splice can be tricky for the novice. An excellent reference work for this job is The Complete Rigger's Apprentice, by Brion Toss.

Chain types

There are many options in selecting the chain part of your rode. In the section on windlasses, we'll discuss the requirements for the links of that chain.

Proof coil chain is made of low-carbon steel that is not heat-treated. It is usually designated as Grade-30, which describes its strength and will usually have "G3" cast into each link. Its name comes from subjecting the chain to tensile strengths until it breaks. This is its proof load. The chain's working load rating is usually 50 percent of the proof load. Although proof coil chain has comparable weight and strength to BBB chain, it is the least expensive of the three types, since there are fewer links per foot.

BBB chain (known as triple-B) is also a Grade-30 low-carbon steel that has not been heat-treated. It can be identified by the "3B" cast into each link. It has been the chain of choice for boaters for years since its added weight creates more of a catenary and its shorter links have less chance to deform.

High Test (HT) chain is made from high-carbon steel that has been heat-treated to increase its strength. It is Grade 43 and has a higher strength-to-weight ratio than either proof coil or BBB chain. It has "G43" or "G4" cast into each link. Its higher strength-to-weight ratio means that for a given strength there will be less weight in the chain locker. Although it has slightly longer links than BBB chain, many windlass manufacturers specify HT chain as the most compatible with their winches.

All these chains are galvanized by electroplating or hot-dipping. Hot-dipping is preferable because of the thicker zinc coating that is produced. Short sections of anchor chain can be purchased with a thin polymer coating that improves the chain's appearance and provides some deck protection. Vinyl coatings are also used. This makes the overall chain thicker but adds much more protection. Unfortunately, with coatings it's hard to tell what type of chain is being used, and the coatings have a limited lifespan.

Stainless-steel anchor chain is also available. This is 316 stainless. It is bright and shiny and has excellent corrosion-resistance. For any given size of chain, it is stronger than either of the low-carbon steel, galvanized counterparts, proof coil, or BBB, but not as strong as high-carbon HT galvanized chain. It is also considerably more expensive than any of the galvanized chains. It does, however, prevent rust stains on the foredeck and there is no galvanizing to wear off.

Windlasses

Large anchors firmly embedded in the bottom become almost impossible to break loose by hand, especially after a blow. Of course you can usually motor or sail them out, but that can be tricky if you're sailing solo. And you're usually left with an anchor very heavy with mud. For larger boats or for boats with older sailors an anchor windlass -- electrical, hydraulic, or manual (vertical or horizontal) -- is a great asset.

Most anchor windlasses have a drum, or capstan, for hauling up the line portion of the rode and a chain gypsy to haul up the chain portion. This chain gypsy has cutouts that must match the links of the chain being used. It's necessary, when buying anchor chain or a new windlass, to make sure the two are compatible.

When anchoring, many boaters use an anchor buoy, which floats above the anchor and allows the anchor to be retrieved vertically, enormously reducing the pull required to break the anchor out. However, one of the problems when using an anchor buoy for an overnight anchorage is that when there is a tide or wind change during the night, the boat can override the buoy's rode, which can become tangled in the propeller or rudder.

Dragging anchor

In most waters, 180-degree wind shifts are not unexpected. Added to this is the reversal of tidal currents along the coasts. So anchoring for an extended period can cause drastic boat swings while at anchor. But won't that anchor reset itself automatically following a wind or tide shift? Of course it may, but you're playing Russian roulette if you count on it. The only way to increase your odds substantially is to set two anchors.

The most effective anchor plan using two anchors is known as Bahamian mooring. With this system the anchored boat can swing through 360 degrees while affecting the direction of pull on either anchor very little.

With two anchors deployed in a Bahamian moor, the risk of changing the direction of pull on the anchor is greatly reduced when there is a tide or wind change, and the boat's swinging circle is also reduced dramatically. The disadvantages of this anchoring system are that your swinging circle may not correspond to other boats anchored nearby.

It's also possible that one of the anchor rodes can become tangled in your prop or rudder. In addition, the two rodes can become twisted around each other as the boat swings around. These possibilities could make leaving in a hurry difficult if conditions in your anchorage deteriorate.

Grassy bottoms

There's one further consideration when anchoring in coastal waters: grassy bottoms. Most of us condemn this grass when it fouls the props of our outboards and for its noxious smell when tossed up on the beach or drawn into the head.

But bottom grass occupies an important place in the ecology of our waters, providing breeding grounds for essential parts of the food chain. Since grass requires light to prosper, it will not be found in deeper water, but deeper water is not usually where boats are anchored.

The problem when anchoring in grass is that the anchor may lodge itself firmly in a clump of grass and seemingly be well-set . . . until the wind picks up and the grass clump is pulled out by the roots. Then the anchor, with its flukes firmly embedded in the clump of grass, will go dragging along the bottom and be incapable of resetting until it is pulled up and disengaged from its lump of sea bottom. There's no simple solution for this problem other than trying to make a visual inspection of your anchoring area and picking a bare spot with no grass. This not only preserves the grass, but it also allows for greater holding power.

Every time you anchor, the tide, wind, waves, and bottom conditions will be different and challenging. Although anchoring may seem to be a vexing problem, the rewards of boating include its multiple facets, one of which is the challenge of being faced with a problem and solving it based on your skill and experience. As knowledge and confidence in your boating skills increase, the pleasure of being on the water is also heightened. Safe anchoring is one of those skills.

Resources

The Robert Smith book, Anchors, Selection and Use, 3rd edition
Send $11.95 to: Isabelle Smith
1825 NE Fremont St., Portland, OR  97212
Anchor tests run by the Safety at Sea Committee of the Sailing Foundation
http://www.vicmaui.org/2000/main.htm (Click: Resource. Scroll down to: Articles. Choose: Safety.)

For further reading . . .
The Complete Book of Anchoring and Mooring, by Earl Hinz; Staying Put, by Brian Fagan; and Nigel Calder's Cruising Handbook, by Nigel Calder. All are available at http://www.goodoldboat.com/bookshelf.html or by calling 763-420-8923.



Author's bio

Don Launer

Don Launer is a Good Old Boat contributing editor. He is a USCG-licensed captain and author of the book, A Cruising Guide to New Jersey Waters. He sails his two-masted schooner, Delphinus, out of Forked River, New Jersey.

Sponsored by Good Old Boat Magazine

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