Introduction to Naval Architecture
Ship's Lines and Model Testing

Ship's Lines and Model Testing

As stated in reference (1), "The shape of a ship's hull, whether it is slim and graceful or full and bulky, is the very essence of its character. This form determines the power required to drive it; it reflects directly the ship's speed; it determines the quantity of payload and the comfort and habitability within the ship; more important, it largely establishes the limits of safety and stability as well as the motion of the ship among waves." A ship is a three dimensional form and in order for a naval architect to produce drawings of the ship a conversion to the two dimensional medium is required. In the past, naval architects were required to produce these drawings by hand, but today computers can perform this function much more efficiently and accurately. There are three projections used for the ship's hull, as depicted in the drawings directly above this paragraph. They are the sheer plan, the half-breadth plan, and the body plan. The sheer plan shows the hull from the side, the half-breadth plan from the top, and the body plan from the front. The body plan shows the forward sections on the right and the after sections on the left of the center line. In order to make measurements between plans, reference lines are drawn on each plan. The reference lines are waterlines, sections, and buttock lines. The waterlines are generally spaced one to two feet apart and are numbered from the keel. The one waterline that is not just a reference line, but an actual design feature is the design waterline (DWL), which represents where the ship is designed to float at a set load. The section halfway between the first and last is known as the midship section and designated by the symbol seen in the SNAME homepage. The first section is called the forward perpendicular and the aftermost one is called the after perpendicular. The longitudinal plane splitting the ship equally and vertically is known as the centerline plane and the line as the center line.

After the lines drawing is produced, a table of offsets is developed and a ship model created for testing. An example of a model testing basin is located at David Taylor Model Basin. Model tests are run to determine the resistance of the model, which can then be scaled up to determine the resistance of the ship. The resistance can then be used to determine the power required to operate the ship. The theory behind using model tests to determine ship resistance was proposed by William Froude and is too extensive to discuss here, but can be found in most of the books listed in the reference section.

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Stability and Ship Characteristics

Stability and Ship Characteristics

The diagram presented above may look confusing, but I intend to present enough information for you to learn more than you may want about it. Many of the requirements that a ship must fulfill are common to all types whether they are warships, passenger ships, cargo ships, or any floating vehicle. An exception to some requirements is submarines, but I will not discuss those topics here. The first requirement is that the ship must satisfy stability principles. The two forces that are required to maintain equilibrium are the force of buoyancy and gravity. Buoyancy is a fundamental physical law that was defined by Archimedes in the second century B.C. and is known as Archimedes' Principle. In the diagram above the buoyant force acts through the center of buoyancy,point B, and as you can see the arrows indicate an upward force. As the ship is heeled, the position of the center of bouyancy will shift, as point B1 in the diagram above indicates. The force of gravity acts through the center of gravity, point G, where all of the weights of the ship may be said to be concentrated. The other major point in determining stability is the metacenter, point M. The key points to know for introductory stability is: (1) M must be above G in order to be in stable equilibrium, (2) if M is equal to G then you have neutral equilibrium, and (3) if M is below G then you have unstable equilibrium and the ship will not remain upright and you have failed your objective. The distance between G and M is known as GM, or metacentric height. The keel, point K on most diagrams, is used as a reference for measuring the other points. A pneumonic that I have used to remember the positively stable order of the points is MGBK, Mother Goose Beats Kids. I know this sounds bad, but I never forget it. Vessels must also satisfy damaged stability, but that is an extensive subject to research using the references listed.

Ships are presented by the naval architect with given characteristics. These characteristics are terms that all should know to be able to talk the language of the naval architect. They include the length overall (LOA), length between perpendiculars (LBP or LPP), displacement, gross tonnage, beam, draft, and depth. In addition, there are several coefficients of form which must be understood by the naval architect, but is not required for this simple presentation. The traditional weight term used in naval architecture is the long ton, as discussed in the past section.

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