- Mooring (watercraft)
Mooring is often accomplished using thick ropes called mooring lines or hawsers. The lines are fixed to deck fittings on the vessel at one end, and fittings on the shore, such as bollards, rings, or cleats, on the other end.
Mooring by permanent anchor can be accomplished by use of a permanent anchor at the bottom of a waterway with a rode (a line, cable, or chain) running to a float on the surface. This allows a person on the vessel to connect to the anchor.
While many mooring buoys are privately owned, some are available for public use.
As an ancient word, “mooring” (probably stemming from the Dutch verb meren, moor, used in English since the end of the 15th century) has accumulated a number of related uses and terms.
Mooring to a shore fixture
A vessel can be made fast to any variety of shore fixtures from trees and rocks to specially constructed areas such as piers and quays. The word pier is used in the following explanation in a generic sense.
Mooring requires cooperation between people on the pier and on a vessel. For larger vessels, heavy mooring lines are often passed to the people on the shore by use of smaller, weighted heaving lines. Once the mooring line is attached to the bollard, it is pulled tight. On large ships, this tightening can be accomplished with the help of heavy machinery called mooring winches or capstans.
For the heaviest cargo ships, more than a dozen mooring lines can be required. Sailboats generally take 4 to 6 mooring lines.
Mooring lines are usually made out of synthetic materials such as nylon. Nylon is easy to work with and lasts for years, but has a property of very great elasticity. This elasticity has its advantages and disadvantages. The main advantage is that during an event, such as a high wind or the close passing of another ship, excess stress can be spread among several lines. On the other hand, if a highly-stressed nylon line does break, or part, it causes a very dangerous phenomenon called "snapback" which can cause fatal injuries. Mooring lines made from materials such as Dyneema and Kevlar are much safer to use, but the lines do not float on the water, and tend to sink, are costly, so they are used less frequently. Manila rope is preferred.
Some ships use wire rope for one or more of their mooring lines. Wire rope is hard to handle and maintain. There is also a risk of using wire rope on a ship's stern in the vicinity of its propeller.
Combination mooring lines made of both wire rope and synthetic line can also be used. This results in a hawser. This is more elastic and easier to handle than a wire rope, but not as elastic as a pure synthetic line. Special safety precautions must be followed when constructing a combination mooring line.
The two-headed mooring bitt
The two-headed mooring bitt is a fitting often-used in mooring. The rope is hauled over the bitt, pulling the vessel toward the bitt. In the second step, the rope is tied to the bitt, as shown. This tie can be put and released very quickly. In quiet conditions, such as on a lake, one person can moor a 260 tonne ship in just a few minutes.
Permanent Anchor Mooring
There are four basic types of permanent anchor moorings; dead weight, mushroom, screw in, and triple anchor. 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 commonly used to hold dock floats in place.
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 in close proximity.
- Dead weight moorings are the simplest kind of mooring. 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. The advantages are that such moorings 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 moorings are the most conventional moorings for mud and silt substrate. They are shaped like an upside down mushroom which can bury itself in these materials quite readily. The advantage is that it has up to ten times the holding power to weight ratio as compared to a dead weight mooring. The disadvantage is that they're more expensive than dead weight moorings, don't hold well on rocky or pebbly substrates, and they take time to settle in before reaching full holding capacity.
- Screw in moorings are a modern method. The screw in mooring is a shaft with wide blades spiraling around it so that it can be screwed into the substrate. The advantages are a high holding power to weight ratio. An additional consideration is size. Screw in moorings are so small that they are relatively cheap. 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) light weight 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 ease of access to the required anchor components because temporary style anchors are commonly available.
The basic rode system is a line, cable, or chain several times longer than the depth of the water running from the anchor to the mooring buoy, the longer the rode is the shallower the angle of force on the anchor (it has more scope). A shallower scope means more of the force is pulling horizontally so that ploughing into the substrate adds holding power but also increases the swinging circle of each mooring, so lowering the density of any given mooring field. By adding weight to the bottom of the rode, such as the use of a length of heavy chain, the angle of force can be dropped further. Unfortunately, this scrapes up the substrate in a circular area around the anchor. A buoy can be added along the lower portion of rode to hold it off the bottom and avoid this issue.
Mooring line materials
- Steel wire
- Polyester (e.g. used for deepsea mooring of offshore platforms)
High Performance mooring lines
- Mooring mast - a structure designed to hold an airship or blimp securely in the open when it is not in flight.
Chapman, Charles F.; Chapman Piloting: Seamanship and Small Boat Handling, 61st edition, 1994 St. Remy Press; ISBN 0-6881683-3
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