An anchor is an object that is used to attach a ship or boat to a specific point at the bottom of a body of water. The anchor prevents the vessel from drifting away by the action of winds and waves.

block-2500-lbs-1There are two primary classes of anchors: temporary and permanent. A permanent anchor, often called a mooring, is one that is rarely moved. It consists of a large mass, such as a rock, resting on the seabed, and it is usually not possible to hoist the permanent anchor aboard a vessel. By contrast, a temporary anchor is stowed aboard the vessel until it needs to be used. Almost all temporary anchors have metal flukes that hook onto rocks at the bottom or bury themselves in a soft seabed. A vessel may carry one or more temporary anchors, which may be of different designs and weights.

The following is the minimum deadweight mooring weight.

Vessel length:

10 – 17 1000 lbs.
18 – 26 2500 lbs.
27 – 35 3000 lbs.
36 – 45 4000 lbs.
45 – 55 5000 lbs.

block-2500-lbs-3The earliest anchors were probably rocks, as many rock anchors have been found dating from at least the Bronze Age. A sea anchor (drift anchor, drift sock), often made of canvas and shaped like a cone or parachute, is not attached to the seafloor but floats just beneath the water’s surface and acts as a source of drag in the water. It pulls large amounts of water along as the boat moves, and it is used to counter the effects of high winds.

Terminology

The anchor is attached to the vessel by means of what is called a rode, which may be a chain, cable, rope, or a combination of these. The hole in the hull through which the anchor rode passes is called a hawsepipe, because thick mooring lines are called hawsers.

An interesting element of anchor jargon is the term aweigh, which describes the anchor when it is hanging on the rope or cable, not resting on the bottom. This term is linked to the phrase to weigh anchor, meaning to lift the anchor from the seabed, allowing the ship or boat to move. An anchor is described as aweigh when it has been detached from the bottom and is being hauled up to be stowed.

The term aweigh should not be confused with under way, which describes a vessel that is not moored to a dock or anchored, whether or not it is moving through the water. Thus, a vessel can be described as under way (or underway) even when it is not moving.

block-2500-lbs-2A mooring refers to any permanent structure to which a vessel may be secured. Examples include quays, wharfs, jetties, piers, anchor buoys, and mooring buoys. A ship is secured to a mooring to forestall free movement of the ship on the water. An anchor mooring fixes a vessel’s position relative to a point on the bottom of a waterway without connecting the vessel to shore. As a verb, mooring refers to the act of attaching a vessel to a mooring. These moorings are used instead of temporary anchors because they have considerably more holding power, cause less damage to the marine environment, and are convenient. They are also occasionally used to hold floating docks in place.

Designs of permanent anchors aka Moorings

These are used where the vessel is permanently sited, for example in the case of light vessels or channel marker buoys. The anchor needs to hold the vessel in all weathers, including the most severe storm, but only occasionally, or never, needs to be lifted, only for example if the vessel is to be towed into port for maintenance. An alternative to using an anchor under these circumstances may be to use a pile driven into the seabed.

Permanent anchors aka moorings come in a wide range of types and have no standard form. A slab of rock with an iron staple in it to attach a chain to would serve the purpose, as would any dense object of appropriate weight (e.g., an engine block). Modern moorings may be anchored by sand screws which look and act very much like over-sized screws drilled into the seabed, or by barbed metal beams pounded in (or even driven in with explosives) like pilings, or a variety of other non-mass means of getting a grip on the bottom. One method of building a mooring is to use three or more temporary anchors laid out with short lengths of chain attached to a swivel, so no matter which direction the vessel moves one or more anchors will be aligned to resist the force.

Mushroom Anchor on the Lightship Portsmouth in Virginia.

The mushroom anchor is suitable where the seabed is composed of silt or fine sand. It was invented by Robert Stevenson, for use by an 82 ton converted fishing boat, Pharos, which was used as a light vessel between 1807 and 1810 near to Bell Rock whilst the lighthouse was being constructed. It was equipped with a 1.5 ton example.

It is shaped like an inverted mushroom, the head becoming buried in the silt. A counterweight is often provided at the other end of the shank to lay it down before it becomes buried.

A mushroom anchor will normally sink in the silt to the point where it has displaced its own weight in bottom material. These anchors are only suitable for a silt or mud bottom, since they rely upon suction and cohesion of the bottom material, which rocky or coarse sand bottoms lack. The holding power of this anchor is at best about twice its weight unless it becomes buried, when it can be as much as ten times its weight. They are available in sizes from about ten pounds up to several tons.

Deadweight

This is an anchor that relies solely on being a heavy weight. It is usually just a large block of concrete or stone at the end of the chain. Its holding power is defined by its weight underwater (i.e. taking its buoyancy into account) regardless of the type of seabed, although suction can increase this if it becomes buried. Consequently deadweight anchors are used where mushroom anchors are unsuitable, for example in rock, gravel or coarse sand. An advantage of a deadweight anchor over a mushroom is that if it does become dragged, then it continues to provide its original holding force. The disadvantage of using deadweight anchors in conditions where a mushroom anchor could be used is that it needs to be around ten times the weight of the equivalent mushroom anchor.

Deadweight anchors are the best choice for rock, gravel or coarse and sandy bottoms.

Concrete is popular because it is inexpensive but it becomes ~45% lighter underwater, so large blocks are needed for larger vessels. Conversely, granite loses 36%, iron loses 14%, and steel 13% of their weight when submerged (PADI, 2005). The reported breakout force for concrete blocks ranging in size from 680–3,624 kg [e.g.0.75 m x 0.75 m x 0.75 m – 1.25 m x 1.25 m x 0.95 m or 2.26 ft x 2.46 ft x 2.46 ft – 4.10 ft x 4.10 ft x 3.11 ft] (2500 lbs–8,000lbs) range from 362–1,812 kg (800–4,000 lbs).

Screw

Screw anchors can be used to anchor permanent moorings, floating docks, fish farms, and so forth.

These anchors must be screwed into the seabed with the use of a tool, so require access to the bottom, either at low tide or by use of a diver.

Weight for weight, screw anchors have a higher holding than other permanent designs, and so can be cheap and relatively easily installed, although may not be ideal in extremely soft mud.

Anchoring gear

The elements of anchoring gear include the anchor, the cable (also called a rode), the method of attaching the two together, the method of attaching the cable to the ship, charts, and a method of learning the depth of the water.

Charts are vital to good anchoring. Knowing the location of potential dangers, as well as being useful in estimating the effects of weather and tide in the anchorage, is essential in choosing a good place to drop the hook. One can get by without referring to charts, but they are an important tool and a part of good anchoring gear, and a skilled mariner would not choose to anchor without them.

The depth of water is necessary for determining scope, which is the ratio of length of cable to the depth measured from the highest point (usually the anchor roller or bow chock) to the seabed. For example, if the water is 25 ft (8 m) deep, and the anchor roller is 3 ft (1 m) above the water, the scope is the ratio between the amount of cable let out and 28 ft (9 m). For this reason it is important to have a reliable and accurate method of measuring the depth of water.

Anchoring techniques

Anchor winch on research vessel POLARSTERN.

block-1500-lbs-3Colored plastic inserts on a modern anchor chain show the operator how much chain has been paid out. This knowledge is very important in all anchoring methods, the basic anchoring consists of determining the location, dropping the anchor, laying out the scope, setting the hook, and assessing where the vessel ends up. After using the chart to determine a desirable location, the crew needs to actually see what the situation is like; there may be other boats whose crew thought that would be a good spot, or weather conditions may be different from those expected, or even additional hazards not noted on the chart may make a planned location undesirable.

If the location is good, the location to drop the anchor should be approached from downwind or down current, whichever is stronger. As the chosen spot is approached, the vessel should be stopped or even beginning to drift back. The anchor should be lowered quickly but under control until it is on the bottom. The vessel should continue to drift back, and the cable should be veered out under control so it will be relatively straight.

Once the desired scope is laid out (a minimum of 8:1 for setting the anchor, and 5:1 for holding, though the preferred ratio is 10:1 for both setting, and holding power), the vessel should be gently forced astern, usually using the auxiliary motor but possibly by backing a sail. A hand on the anchor line may telegraph a series of jerks and jolts, indicating the anchor is dragging, or a smooth tension indicative of digging in. As the anchor begins to dig in and resist backward force, the engine may be throttled up to get a thorough set. If the anchor continues to drag, or sets after having dragged too far, it should be retrieved and moved back to the desired position (or another location chosen.)

With the anchor set in the correct location, everything should be reconsidered. Is the location protected, now and for the forecast weather? Is the bottom a suitable holding ground, and is the anchor the right one for this type of bottom? Is there enough depth, both now and at low tide? Especially at low tide but also at all tide states, is there enough room for the boat to swing? Will another vessel swing into us, or will we swing into another vessel, when the tide or wind changes?

Some other techniques have been developed to reduce swing, or to deal with heavy weather:

  • Using an anchor weight, kellet or sentinel.
  • Forked moor.
  • Bow and Stern.
  • Bahamian moor.
  • Backing an anchor.

Protection

A good anchorage offers protection from the current weather conditions, and will also offer protection from the expected weather. The anchorage should also be suitable for other purposes; for example, proximity to shore is beneficial if the crew plans to land.

Seabed

Charts should indicate the type of bottom, and a sounding lead may be used to collect a sample from the bottom for analysis. Generally speaking, most anchors will hold well in sandy mud, mud and clay, or firm sand. Loose sand and soft mud are not desirable bottoms, especially soft mud which should be avoided if at all possible. Rock, coral, and shale prevent anchors from digging in, although some anchors are designed to hook into such a bottom. Grassy bottoms may be good holding, but only if the anchor can penetrate the foliage.

Depth and tides

block-2500-lbs-5If the anchorage is affected by tide, tide ranges, as well as the times of high and low water, should be known. Enough depth is needed so that low tide does not present obstacles to where the vessel might swing. This is also important when determining scope, which should be figured for high tide and not the current tide state.

Swing range

If the anchorage is affected by tide, one should keep in mind that the swing range will be larger at low tide than at high tide. However, no matter where the vessel is anchored, the largest possible swing range should be considered, as well as what obstacles and hazards might be within that range. Other vessels’ swing ranges may overlap, presenting a further variable. Boats on permanent moorings, or shorter scope, may not swing as far as expected, or may swing either more rapidly or more slowly (all-chain cables tend to swing more slowly than all-rope or chain-and-rope cables.)

There are techniques of anchoring to limit the swing of a vessel if the anchorage has limited room.

Using an anchor weight, kellet or sentinel

Lowering a concentrated, heavy weight down the anchor line – rope or chain – directly in front of the bow to the seabed, behaves like a heavy chain rode and lowers the angle of pull on the anchor. If the weight is suspended off the seabed it acts as a spring or shock absorber to dampen the sudden actions that are normally transmitted to the anchor and can cause it to dislodge and drag. In light conditions, a kellet will reduce the swing of the vessel considerably. In heavier conditions these effects disappear as the rode becomes straightened and the weight ineffective.

Forked moor

Using two anchors set approximately 45° apart, or wider angles up to 90°, from the bow is a strong mooring for facing into strong winds. To set anchors in this way, first one anchor is set in the normal fashion. Then, taking in on the first cable as the boat is motored into the wind and letting slack while drifting back, a second anchor is set approximately a half-scope away from the first on a line perpendicular to the wind. After this second anchor is set, the scope on the first is taken up until the vessel is lying between the two anchors and the load is taken equally on each cable.

This moor also to some degree limits the range of a vessel’s swing to a narrower oval. Care should be taken that other vessels will not swing down on the boat due to the limited swing range.

Bow and stern

Not to be mistaken with the Bahamian moor, below.

In the Bow and Stern technique, an anchor is set off each the bow and the stern, which can severely limit a vessel’s swing range and also align it to steady wind, current or wave conditions. One method of accomplishing this moor is to set a bow anchor normally, then drop back to the limit of the bow cable (or to double the desired scope, e.g. 8:1 if the eventual scope should be 4:1, 10:1 if the eventual scope should be 5:1, etc.) to lower a stern anchor. By taking up on the bow cable the stern anchor can be set. After both anchors are set, tension is taken up on both cables to limit the swing or to align the vessel.

Bahamian moor

Similar to the above, a Bahamian moor is used to sharply limit the swing range of a vessel, but allows it to swing to a current. One of the primary characteristics of this technique is the use of a swivel as follows: the first anchor is set normally, and the vessel drops back to the limit of anchor cable. A second anchor is attached to the end of the anchor cable, and is dropped and set. A swivel is attached to the middle of the anchor cable, and the vessel connected to that.

The vessel will now swing in the middle of two anchors, which is acceptable in strong reversing currents but a wind perpendicular to the current may break out the anchors as they are not aligned for this load.

Mooring systems are a lot like that old song about the thigh bone connected to the knee bone. Most moorings consist of a dozen separate pieces including whatever’s on the bottom, two or more swivels, a half-dozen shackles, and a couple of lengths of both chain and line. For your boat to stay where you left it, every part has to be up to the task, and the whole will only be as strong as the weakest link. While most of the losses of boats on moorings during hurricanes in the Boat US Marine Insurance claims files have to do with failure at the pendant, the part of the mooring lying between the surface of the water and the boat, “everyday” losses — where the boat doesn’t stay put in conditions up through gale force — are just about equally divided between dragged moorings, failed pendants, and problems somewhere between.

But the files show that mooring and pendant failures can be reduced — and maybe even eliminated — with new technologies and techniques that first began to find their way into public mooring fields in the wake of Hurricane Bob in 1991. These new technologies — and annual maintenance on all the pieces in between — make it far less likely your boat will go wandering when the wind kicks up.

On The Bottom

Until recently, mushrooms and deadweight mooring anchors — which can include anything from an engine block to rough-hewn slabs of granite a yard or more across — have been the only mooring options.

But both have serious issues. While a deadweight anchor may gain some advantage from suction in a mud bottom, in most cases its holding power is completely dependent on its weight, or, more exactly, on its submerged weight. Cast iron weighs about 12.5 percent less in the water than out; concrete loses nearly half of its weight underwater. The reduced weight and lack of shape to help the anchor dig into the bottom mean that it takes a very large and heavy deadweight anchor to hold a boat in place in gale-force winds. As you would expect, in various tests deadweight concrete moorings had a holding power of right around half their dry weight.

Theoretically, mushroom anchors can provide substantial holding power — up to 10 times their submerged weight — if they are sufficiently buried in mud. In most harbors, though, a mushroom anchor doesn’t sink more than a foot or so into the bottom, and it is most often oriented pointing away from the prevailing winds. A storm that swings the boat into a different direction will first pull the mushroom anchor upright, and it may not reset when it gets pulled back down in the new direction. In a storm with a couple of wind shifts, the chain can also wrap around the shank of the mushroom anchor, shortening effective scope and decreasing the chance of it resetting. Mushrooms tend to drag through silt and bump along without digging into denser bottoms like clay or hard-packed sand. In tests, the holding power of mushroom moorings has varied from 2.4 to 5.7 times their weight.
In the last year, all of the incidents of moorings being dragged in the claim files involved either mushroom or deadweight anchors.

Weighted mushrooms are designed to overcome some of these issues. A weight at the top of the shank is meant to tip them back on their side when they come upright, and the sharper edge on the bell-shaped bottom helps them to dig in. We don’t have enough data to evaluate their holding power, though.

In the last two decades, two alternatives have come into increasing use and the evidence has been mounting that these are substantially better at staying in one place than the traditional solutions. While all the other mooring anchor types rely on weight or weight and embedment for their holding power, helical screw anchors are screwed directly into the seabed. They have long, high-tensile steel shafts with large screw threads on the bottom and an attachment eye at the top. A barge-mounted hydraulic device is used to install them, and they work in most bottoms.

Comparing the holding power of a helix anchor to that of a traditional mushroom or deadweight anchor is like comparing a wood screw to a thumbtack or paperweight. Helical moorings offer an order of magnitude more holding power than any of the alternatives. They’re also significantly less sensitive to scope to maintain their holding power, which means more boats can be fit into a smaller area. Helical screw anchors are not completely failure proof, though. They must be installed properly, and they need sufficient scope to allow for storm surge and waves.

Dor-Mor anchors look like upside down pyramids with a point at the bottom and a short shank on the flat top. The mooring lands point down and buries deeply in mud, clay, or sand bottoms. The high center of gravity tips the sharp edge down so that it digs in. When the boat swings, the mooring may shift, but it doesn’t come upright like a mushroom. While nothing can compare to the holding power of a helix anchor, in tests, Dor-Mors had almost twice the holding power for their weight as the average of the mushrooms. Dor-Mors now account for 75 percent of the moorings at Woods Hole, an anchorage known for a difficult bottom and strong currents. The Coast Guard has been using Dor-Mors for Aids to Navigation in areas subject to currents. Dor-Mors are more expensive than mushrooms, but their installed cost is similar to helix anchors.

At The Boat

When the wind starts to kick up, in all but the most sheltered mooring fields, waves will soon follow. The cycling loads from waves on the mooring pendant can generate forces anywhere from two to 10 times the static load. If the wave period is four seconds, the mooring pendant will be subject to those shock loads over 20,000 times per day.

In these conditions, the mooring pendant provides elasticity to limit the shock loading that could destroy the boat’s hardware or jerk the mooring anchor from the bottom. But that same elasticity results in chafe and may cause the pendant to part if the line passes over any hard spot as it lengthens and shortens. Normal chafe protection can allow heat to build up in the nylon strands, leading to a failure when the internal stresses cause the fibers to melt. There are many of these failures in the claim files. What’s needed is a mooring pendant that provides a great deal of elasticity without chafe — a seemingly impossible combination.

After Irene, Seaworthy reported on a new mooring pendant designed to separate the pendant into two lines, one with high elasticity and one with, for all practical purposes, no elasticity and very high chafe resistance. Nantucket Moorings, in conjunction with MIT, developed Cyclone Mooring Pendants, a two-part pendant with the upper part made from New England Ropes STS-12 line coupled to a standard nylon double-braid lower. STS-12 is made from Dyneema fibers, which are incredibly strong, abrasion resistant, and UV resistant. Soft eyes are spliced into each of the two lines, and they’re then connected together. A second, longer Dyneema line can be added to make a bridle and act as a backup if the first Dyneema line were to fail.

Dennis Metcalfe, manager of Nantucket Moorings, told us that in the three years Nantucket Moorings had been using Dyneema, he had never seen any signs of chafe. You can buy a pre-made pendant or make your own.

What Lies Between

The anchor mooring and the pendant are only two pieces in the mooring system that makes up the mooring. Traditional systems use heavy chain on the mooring to ensure that the pull on the mooring remains at an appropriate angle even in strong winds, and lighter chain above to reduce the weight on the mooring pendant. Eye-to-eye swivels and shackles are used to connect the two chains. The entire system needs adequate scope for the type of mooring anchor. Failures between the traditional mooring and the pendant almost all have to do with corrosion. The key to preventing failures is a comprehensive maintenance program that includes an annual inspection of every element in the mooring system.

Unfortunately, it’s not enough to make sure your mooring is perfect. Your boat’s safety also depends on the condition of every mooring upwind of you. A professionally managed mooring field with an annual maintenance schedule and specific standards with regard to the mooring anchor, pendant, and everything in between will reduce the chances another boat will ruin your day.