Mooring Block aka Anchor Weight 1500 lbs
A 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.
The term probably stems from the Dutch verb meren (to moor), which has been used in English since the end of the 15th century.
Mooring line of Polish ship Fryderyk Chopin
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. There are several kinds of moorings:
Swing moorings aka simple or single-point moorings, are the simplest and most common kind of mooring. A swing mooring consists of a single anchor at the bottom of a waterway with a rode (a rope, cable, or chain) running to a float on the surface. The float allows a vessel to find the rode and connect to the anchor. These anchors aka swing moorings because a vessel attached to this kind of mooring swings in a circle when the direction of wind or tide changes.
For a small boat (e.g. 22′ / 6.7 m sailing yacht), this might consist of a heavy weight on the seabed, a 12mm or 14mm rising chain attached to the “anchor”, and a bridle made from 20 mm nylon rope, steel cable, or a 16mm combination steel wire material. The heavy weight (anchor) should be a dense material. Old rail wagon wheels are used in some places (e.g. Clontarf, Dublin, Ireland) for this purpose. In some harbours (e.g. Dun Laoghaire, Ireland), very heavy chain (e.g. old ship anchor chain) may be placed in a grid pattern on the sea bed to ensure orderly positioning of moorings. Ropes (particularly for marker buoys and messenger lines) should be “non floating” to reduce likelihood of a boat’s prop being fouled by one.
Pile moorings are poles driven into the bottom of the waterway with their tops above the water. Vessels then tie mooring lines to two or four piles to fix their position between those piles. Pile moorings are common in New Zealand but rare elsewhere.
While many mooring buoys are privately owned, some are available for public use. For example, on the Great Barrier Reef off the Australian coast, a vast number of public moorings are set out in popular areas where boats can moor. This is to avoid the massive damage that would be caused by many vessels anchoring.
There are four basic types of permanent anchors used in moorings:
Dor-Mor pyramid-shaped anchors used in mooring
Dead weights are the simplest type of anchor. They are generally made as a large concrete block with a rode attached which resists movement with sheer weight; and, to a small degree, by settling into the substrate. In New Zealand old railway wheels are sometimes used. The advantages are that they are simple and cheap. A dead weight mooring that drags in a storm still holds well in its new position. Such moorings are better suited to rocky bottoms where other mooring systems do not hold well. The disadvantages are that they are heavy, bulky, and awkward.
Mushroom anchors are the most common anchors and work best for softer seabeds such as mud, sand, or silt. They are shaped like an upside-down mushroom which can be easily buried in mud or silt. The advantage is that it has up to ten times the holding-power-to-weight ratio compared to a dead weight mooring; disadvantages include high cost, limited success on rocky or pebbly substrates, and the long time it takes to reach full holding capacity.
Pyramid anchors are pyramid-shaped anchors, aka Dor-Mor anchors. They work in the upside-down position with the apex pointing down at the bottom such us when they are deployed, the weight of wider base pushes the pyramid down digging into the floor. As the anchors are encountered with lateral pulls, the side edges or corners of the pyramids will dig deeper under the floor, making them more stable.
Screw-in moorings are a modern method. The anchor in a screw-in mooring is a shaft with wide blades spiraling around it so that it can be screwed into the substrate. The advantages include high holding-power-to-weight ratio and small size (and thus relative cheapness). The disadvantage is that a diver is usually needed to install, inspect, and maintain these moorings.
Multiple anchor mooring systems use two or more (often three) lightweight temporary-style anchors set in an equilateral arrangement and all chained to a common center from which a conventional rode extends to a mooring buoy. The advantages are minimized mass, ease of deployment, high holding-power-to-weight ratio, and availability of temporary-style anchors.
Constructing a Permanent Mooring
A permanent mooring must remain secure for long periods while unattended, occasionally under adverse conditions. For peace of mind, it should be properly sized for the job, and determining the minimum size has a lot to do with the conditions under which the boat is moored, the amount of fetch for waves to build up, and whether your mooring is a temporary one, designed for overnight use in fair weather, or a mooring capable of riding out a hurricane. Below are the basic components.
Several types are in common use, and we’ll review them in order of their holding power, from the wimpiest to the most tenacious:
- Concrete Blocks: Many boats use 50-gallon drums filled with cement, concrete blocks, auto engine blocks and other types of dead weight. This type provides the least holding power, working on the principle of sheer weight, but is reliable if pulled out of the bottom. If they drag, they will resist motion with a constant amount of force. Note that concrete loses over half of its weight when submerged in water, so a mooring designed to withstand a 500lb. pull will need 1,000lb. of concrete.
- Mushroom Anchor: the most common type of mooring anchor is the mushroom, which, under ideal conditions, with the right kind of bottom, can dig in, create suction and develop good holding power. Mushroom anchors work best in a silt or mud bottom, and are not as effective in rocks or coarse sand. If a mushroom gets pulled out of the bottom, it is less likely to reset itself completely, and will merely skip along across the bottom. A weight of 5-10 times boat length is a good rule of thumb, as a bare minimum. The heavier the better, as long as you don’t have to move it.
- Pyramid Anchor: The cast-iron Dor-Mor pyramid anchor is a superior alternative to the mushroom. Its smaller size, concentrated weight and pyramid shape allows it to embed itself more rapidly, and its holding power (at a scope of 3:1) is up to about ten times its weight. Recommended by Practical Sailor/Powerboat Reports in 2009.
- Helical Screw: while the above types rely for holding power on sheer weight or a combination of weight and embedding themselves in the bottom, the helical anchor is screwed into the seabed, usually by a barge-mounted hydraulic device. Helical screws have long, high-tensile steel shafts (8′ length is common) with large screw threads (10″ to 14″ diameter) on the bottom and an attachment eye at the top. These professionally-installed anchors, originating in the offshore oil industry, have gained popularity with recreational boaters since the 1990s, and have the most extreme holding power in relation to their weight.
Chapman’s recommends two sections of galvanized chain: a heavier, primary chain and a lighter, secondary chain. The primary (ground) chain lies on the bottom. Its length should be 1 1/2 times maximum water depth. The secondary (riding) chain, is connected to the ground chain with a galvanized shackle or swivel. It’s usually half the size of the ground chain and equal in length to maximum water depth. The heavier chain is not used for the entire run so that the mooring buoy does not have to support an excessive amount of weight. Chain should be as large as possible—make the riding chain at least double the size of the chain on your anchor rode.
The sole purpose of the mooring buoy is to support the mooring chain. The two preferred designs for mooring buoys are a traditional buoy with hardware or a buoy with a tube through the center. Both offer reliable flotation and will last for several seasons, depending upon the salinity of the water. Obviously, freshwater applications will extend the useful life of any mooring system.
The buoy must have about twice as much flotation as the suspended chain has weight in order for it to ride high enough in the water to be visible. The Taylor Sur-Moor™ T3C™ buoys allow you to pass the chain through the center of the buoy, and attach the pendant on top. Secure the mooring chain at the top using a 4″ galvanized O-ring, such as Model 201105, and add the T3C™ Mooring Collar to protect the buoy from wear by the anchor chain and extend its lifespan.
The pendant (pronounced “pennant”) attaches the chain to the boat. Large-diameter three-strand nylon line is used because its inherent elasticity (stretching about 10 percent under a load equaling 20 percent of its tensile strength) allows it to act as a shock absorber. Polyester line, Dyneema line or stainless steel wire is preferred by some for better chafe resistance. Length should be about 2 1/2 times freeboard. Diameter should be as large as is practical—it must be able to fit through bow chocks and around a bow cleat.
Effective chafe protection is recommended for the point where the pendant passes through a chock. This is critical, as failure caused by chafe at this location is one of the main reasons why boats end up on the beach. A light pick-up buoy at the boat end makes it easy to grab the pendant.
Cyclone Mooring Pendants, was developed by Nantucket Moorings in conjunction with MIT. Traditional pendants are made from three-strand nylon, to absorb shocks by their elastic nature. This stretch, while allowing your boat a comfortable, cushioned ride, allows the line to move across the bow chocks, creating friction and causing chafe.
Cyclone Pendants are attached (using a lunch pail hitch, also called a “cow hitch”) to your standard nylon pendant with its floating pickup buoy, and allow use of a low-stretch upper section. Their high tensile strength allow smaller lines to be used, so they fit more easily on boats with smaller cleats and chocks. Very low elongation result in a dramatic reduction in friction, heat and chafing. Also, because the top and bottom sections are just looped together through spliced eyes, you can replace a damaged section without replacing the entire pendant.
Surviving a storm
Hurricanes making landfall in locations along the Eastern Seaboard have wrecked hundreds of boats over the past twenty years, many of which were driven ashore with their complete and intact mooring systems still attached. Weather forecasters predict that we’re now in a period where we can expect more storms of greater destructive force. Just how large does a mooring system need to be, in case you’re caught by extreme weather with the boat swinging on its mooring?
The design of the mooring block, increased length or weight of ground chain, embedding the mooring block into the seabed, plus the optional use of a nylon spring in the mooring system in water depths greater than two metres at chart datum, and on vessels greater than six metres in length overall, will all make the mooring more effective. However, care must clearly be taken before lengthening the ground chain for an existing swing mooring. If you want to lengthen the ground chain of an existing swing mooring, please contact the Regional Harbourmaster at ECan.
The seabeds in Canterbury harbours are principally marine silts, but in some areas mixed sand is common. In both these conditions it is important that the mooring block is built according to minimum specifications to maximise holding power. To get the best result, the block needs to expel air and liquid as it enters the seabed. To ensure the block is secure, the block may need to be jetted or ‘airlifted’ in. Liquid silts will soon fill the space adjacent to its sloping sides, effectively keying the block into the seabed. This situation increases holding, and decreases the block profile, reducing the prospect of the mooring block being hit by the keels of other boats, or of it being snagged. The sloping sides of the block will:
- Help embed the mooring into the seabed,
- Reduce the chance of snagging, and
- Assist the block to rotate in conditions where the ground chain tension is very high. This will cause the block to dig in (like the flukes of an anchor) and increase its sectional area in the direction of the load.
Minimum deadweight mooring weight
10 – 17 1000 lbs.
18 – 26 1500 lbs.
27 – 35 3000 lbs.
36 – 45 4000 lbs.
45 – 55 5000 lbs.
The pull-out resistance of a dead weight mooring is linked to two principle factors: its apparent weight (dry weight minus the Archimedes force) and the size of the surface in contact with the seabed. The cost and reported breakout force varies according to the size of anchor and between the varieties of deadweight anchors.
Deadweight moorings may include:
Weighted blocks – provides the least holding power, working on the principal of sheer weight. 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] (1500 lbs–8,000 lbs) range from 362–1,812 kg (800–4,000 lbs).
Example 20′ Boat:
For a 20 footer using cement you need roughly a 2730 pound dry weight mooring to attain a 1500lbs submerged weight.
The recommended weight for deadweight moorings is about 150 0lbs (submerged). Eight concrete blocks at roughly 40 pounds each gives you a submerged weight of only 176 pounds, or less than 12% of what is a safe suggested deadweight mooring for a 20 footer.