In addition, on management contracts, the works will be carried out by ' works contractors' and on construction management contracts the works will be carried out by ' trade contractors', although technically trade contractors are not sub-contractors as they are contracted by the client, and only managed by the construction manager.Ī nominated sub-contractor is one that is selected by the client to carry out an element of the works. The use of sub-contractors enables the main contractor to undertake more complex projects whilst not unacceptably increasing their risk, however, concerns have been expressed about the prevalence of sub-contractors because of a perception that the main contractor has less control over the skills and training of sub-contractor employees and so there may be a negative impact on quality and health and safety on site. As construction has become more complicated and more specialist construction techniques have been developed, it has become increasingly common for contractors to sub-contract others rather than employing a large workforce themselves. Sub-contractors are appointed by main contractors to carry out part of the works on their behalf. 7 Related articles on Designing Buildings Wiki.In an emergency, the ballast tanks can be filled quickly with high-pressure air to take the submarine to the surface very rapidly. The hydroplanes are angled so that water moves up over the stern, which forces the stern downward therefore, the submarine is angled upward.

When the submarine surfaces, compressed air flows from the air flasks into the ballast tanks and the water is forced out of the submarine until its overall density is less than the surrounding water ( positive buoyancy) and the submarine rises. In addition, some submarines are equipped with a retractable secondary propulsion motor that can swivel 360 degrees. The submarine can steer in the water by using the tail rudder to turn starboard (right) or port (left) and the hydroplanes to control the fore-aft angle of the submarine. Water is also forced between the bow and stern trim tanks to keep the sub level.

When the submarine reaches its cruising depth, the hydroplanes are leveled so that the submarine travels level through the water. To keep the submarine level at any set depth, the submarine maintains a balance of air and water in the trim tanks so that its overall density is equal to the surrounding water ( neutral buoyancy). The hydroplanes are angled so that water moves over the stern, which forces the stern upward therefore, the submarine is angled downward. In addition, the submarine has movable sets of short 'wings' called hydroplanes on the stern (back) that help to control the angle of the dive. A supply of compressed air is maintained aboard the submarine in air flasks for life support and for use with the ballast tanks. As the submarine dives, the ballast tanks are flooded with water and the air in the ballast tanks is vented from the submarine until its overall density is greater than the surrounding water and the submarine begins to sink ( negative buoyancy). When the submarine is on the surface, the ballast tanks are filled with air and the submarine's overall density is less than that of the surrounding water.

To control its buoyancy, the submarine has ballast tanks and auxiliary, or trim tanks, that can be alternately filled with water or air (see animation below). Unlike a ship, a submarine can control its buoyancy, thus allowing it to sink and surface at will. This displacement of water creates an upward force called the buoyant force and acts opposite to gravity, which would pull the ship down. A submarine or a ship can float because the weight of water that it displaces is equal to the weight of the ship.

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