Dock (structure)

Dock (structure), a type of harbor structure used for loading, unloading, or repairing ships. In accurate usage the term applies either to the water channel in which a ship is berthed beside a pier, or to a dry dock, in which a ship is placed for repairs. Commonly, dock refers to a pier or quay. A pier is a structure that extends out into the water, usually perpendicular to the shoreline; a quay is constructed parallel to the shoreline.

In harbors that have a large tidal range, ships are usually berthed in wet docks. These docks are actually basins that can be shut off from the rest of the harbor by movable gates that hold the water in the dock at the high-tide level. Wet docks are a necessity in most ports of the British Isles and in other localities where the height of the tide is more than about 3 m (10 ft). The London wet-dock system is the largest in the world; the combined length of the quays and piers in London amounts to about 72 km (45 mi).

In most United States seaports the rise and fall of the tide is small enough to permit the use of tidal docks. Such docks, used almost exclusively in the United States, consist of a series of rectangular water spaces between projecting piers. Because changes in water level are relatively small, there is no need to isolate the dock basins from the rest of the harbor.

Today, enormous container ships are primarily responsible for transporting cargo overseas (see Shipping Industry); as a result, major shipping ports of the world have container ship docks. Container ships transport the trailer components of tractor-trailer trucks, or large standard-sized rectangular boxes. The cargo is already packed in these large boxes, so at the docks these containers need only be placed on or removed from the ships. Container ship docks are often fitted with large cranes that can load and unload the containers. The docks must also be wide enough to accommodate vehicles that deliver and pick up the containers.

Dry docks are generally used to make repairs beneath a ship's waterline. The ship is maneuvered into a dry dock, then the structure is closed and pumped free of water, leaving the entire ship exposed for repairs. Dry docks are of two types: graving docks, fixed basins lined with concrete; and floating dry docks, usually made of steel. Floating dry docks have certain advantages over graving docks in that they can be built more quickly and economically than fixed docks, and can also be towed into place to meet a ship unable to make it to a port.

Caulking

I. Introduction

Caulking, process of sealing joints in wood or steel structures against leakage of liquids or gases. Caulking is commonly used in the shipbuilding, metalworking, and construction industries. The joints between planks on a wooden ship, for example, are caulked to prevent water from leaking into the ship; riveted seams in steam boiler drums and air tanks are caulked to prevent steam, hot water, and air from leaking out.

II. Caulking Wooden Ships

The planks of a wooden ship are grooved to allow space for the insertion of caulking material between adjacent pieces. There are several materials used for caulking planked wooden ships. Historically, a fibrous material called oakum was used; today a number of different plastics, known as elastomers, are also available.

Elastomers consist of long, tightly twisted molecules that stretch and recoil, much like a spring. Plastic caulking materials are thoroughly mixed with a liquid curing material just before application, and the combined paste is then injected into the clean seam. The outer surface of the seam is smoothed off until it is level with the adjacent planks. When properly applied, plastic materials are far superior to oakum because they adhere to the wooden planking like glue, and they retain their rubber-like elastic properties indefinitely.

Oakum may still be used for recaulking older ships; it is also used in places where plastics are not available. Oakum is made from old hemp rope that has been untwisted and picked apart. It is generally used in the form of loosely spun yarn and may be untreated (white) or saturated with a tarry substance (black). Oakum comes in balls or in rope form.

To caulk a ship using oakum, the seam must first be spread slightly, using wedges. The oakum yarn is then pounded into the seam with special chisel-like tools. White oakum is used for the first layer (nearest the inside of the hull) and black oakum for the rest. Care is taken to eliminate cavities in the oakum, as they may cause decay of the wooden planks. The seam is filled to a fraction of an inch below the outer surface of the planking, and the remainder of the seam is then filled with pitch or marine glue.

Small ships and boats built of plywood or plastic require little or no caulking. Plastic adhesives for plywood and other wood laminates are highly resistant to seawater and result in hulls that are extremely watertight.

III. Caulking In Building Construction

Caulking is also used in the construction of buildings. Joints between wood, metal, and plastics may be filled to prevent the leakage of water or air. Examples of such applications include sealing glass panes into a wood or metal window sash, sealing the edges around sinks and bathtubs, and sealing material around vent pipes, chimneys, and walls. Plastic materials are superior to older materials, and many plastic caulking substances are available in ready-to-use cartridges for use in caulking guns or other convenient applicators. These “mastics” are based on urethanes, acrylics, or polybutene or polysulphide elastomers. The formulations used are dependent upon the special properties necessary for specific applications.

IV. Caulking Riveted Seams

Riveted steel drums, tanks, and ships are caulked after riveting is completed. If a seam is to be caulked, it must be designed with that in mind. The spacing of the rivets along the joint must be sufficiently close so that the outer plate will not bulge between rivets, and the distance from the caulked edge to the first row of rivets must be small enough to prevent bulging in that direction.

Special chisel-shaped tools, operated with air hammers, are used for caulking riveted seams. A tool with two parallel edges is first used to score the edge of the outer plate and the face of the under one. Next, a blunt or square-edged chisel is passed along the scored grooves one or more times to seal the joint. Water or air pressure is then applied to the inside of the tank and the joint is examined for leaks. When water is used for testing, leaks are easily detectable through visual examination. When air is used for testing, leaks may be made visible by applying soap and water to the seam and looking for the expansion of the soap bubbles. If leaks appear, the joint is hammered with the caulking tools until all leaks have been stopped.