Ship Construction And Stability
Separate streams of plates and rolled sections converge toward the prefabrication shop, where they are used to build structural components or subassemblies. The subassemblies are transported to an area nearer the berths, where they are welded together to form large prefabricated units, which are then carried by cranes to the berth, to be welded into position on the ship.
Ship stability oow pdf
Strength and Stability of a ship or any other marine structure are of major concerns for a Naval Architect. Ships, which are designed to give lifelong operations should have strength and efficiency as well as smooth performance. Stability is defined as the general tendency of a vessel or any other floating body to remain upright. A ship is said to be ideally stable if the line of action of the buoyancy coincides with the vertical centreline, i. e; the centre of buoyancy and the centre of gravity of the ship lies in the same line. Figure 1: A heeled ship ( Courtesy: Googleimages) However, invariably in all seas, the ships have to face the same problems of waves, environmental vagaries and sometimes interplay of both in worse case scenarios. Moreover, internal factors like improper distribution of loads, structural breach or sometimes problems in maneuvering and course-keeping can drastically alter the stability of the ship; i. e its tendency to remain upright! Stability of a ship has to be calculated for every situation a ship have to face, whether it is sailing in normal conditions or facing with storms or even on the jetty/port.
(2) Exercises (i) What is the weight of a steel block 1m x 1m x 0. 5m having a density of 7500 kg/m 3? (ii) According to Archimedes's law, what is the weight of this same block when it is immersed in water having a density of 1000 kg/m 3? (iii) Calculate the TPC of a box shaped barge 20m long by 5m large floating in water of 1000 kg/m 3. (iv) What will be the change in the mean draft of this barge when 500t of cargo are discharged? (v) Draw a diagram of the midship cross section of a general cargo vessel and show the relative positions of the Centre of gravity (G), Centre of buoyancy (B) and Metacentre (M). Give the typical values in meters relative to the keel (K). (vi) Draw the same diagram at an angle of 10° and show the righting arm. Give the value relative to the ship's displacement. 7. 4 Ship Construction (i) Define the following terms relating to a ship's structure: (a) Frame (b) Longitudinal framing (c) Transverse framing (d) Web frame (e) Stringer (f) Floor (g) Deck Girder (h) Beam knee (j) Pillar (k) Bracket (l) Bilge plate (m) Double bottom (ii) Briefly describe the following: (a) Stern trawler: (b) Multi rig trawler (c) Long line vessel (d) Bollards (iii) Exercises (i) Explain the purpose of double bottoms.
This will tend to undo the heel. The points shown in the figure are very important. Point M (Metacentre point where the line of action meets the centreline of the ship), is most important, many of the calculations which are done deals with M. The moment relation used for the righting arm (GZ) in the condition of heel is as follows: GZ = sin θ*GM where GM is the metacentric height measured from the Centre of Gravity and the Metacentre. GZ Curves and Calculations These curves are drawn With GZ on the Y axis and Heeling angle on X axis. If we see the general GZ curve, for small angles, righting lever GZ is proportional to heeling angle and thus a tangent can be drawn through origin which gives GM. Till the maximum GZ value, there is a variation in the rate of growth of GZ value, the point where rate tends to decrease is point of contraflexure and the angle is angle of contraflexure. Now, above points are valid only when neglect many factors which contribute to ship instability. Figure 4: GZ Righting Curve of Stability ( Courtesy: Basic Ship Theory) Area under the graph gives the energy stored.
Ship Construction & Basic Stability Course (MITPMI-499) | MITAGS
7. 3 Ship Stability (1) Basic knowledge and definitions (i) What is the Centre of Gravity of a ship? (ii) What is the Centre of Buoyancy? (iii) What is the transversal Metacentre? (iv) What is the longitudinal Metacentre? (v) What do the acronyms TPC and TPI stand for? What do they mean? (vi) What do the acronyms MCTC and MCTI stand for? What do they mean? (vii) For a specific ship, where can we find the information on the above? (viii) Define briefly the following terms: (a) Displacement (b) Deadweight (c) Coefficient of form (d) Waterplane area (ix) What is Free Surface Effect? Generally speaking, how does this affect the stability of a ship? (x) What is meant by the trim of a ship? What makes it change? (xi) With regard to a ship's hull, what do Hogging and Sagging mean? How are they measured? (xii) How do we find the stress in a ship's hull? How can one find the maximum allowable stress? (xiii) Define the propeller pitch. What is meant by Propeller slip? (xiv) State and explain Archimedes's law.
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