Appendix B Intact Ship Information Information
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info_mart S0300-A8-HBK-010 APPENDIXB INTACTSHIP INFORMATION B-1 INTRODUCTION To be effective, the salvage engineer needs a general knowledge of ship form and construction, supplemented by information specific to a particular casualty. The characteristics of ships can be grouped by broad classes, because similar service requirements lead to similar forms. This appendix describes the sources of vessel-specific data available to the salvage engineer, including a short description of the NAVSEA drawing numbering system. Summary tables of pertinent characteristics of Navy ships are also included. B-2 SHIP-SPECIFIC DATA A great deal of information is generated and recorded during a ship’s design, construction, and trials. Access to tabulated ship data can greatly simplify and speed the salvage engineer’s work. The documents described in this paragraph are particularly useful. Naval ships generally carry a greater body of ship’s data than commercial ships. Documents carried onboard are normally kept in the engineering log room or technical library on naval ships. Documents for naval ships are also available from the parent squadron and repair activities; each ship class is assigned to a planning shipyard that maintains complete drawing files for assigned ships. Paragraph B-4 describes the NAVSEA ship’s document numbering and classification system, gives planning yard assignments for Navy ships and craft, and describes likely sources for ship’s documents. Commercial vessels usually carry fewer documents than naval vessels, but the information contained in them is often quite detailed, especially in regard to hydrostatic characteristics and cargo capacity and stowage. Documents for commercial vessels are obtained from the ship’s officers, owners or shipping company, agents, port engineer, building shipyard, or ship designer. Drawings for U.S. flag vessels can also be obtained from Chief, Naval Architecture Branch, Marine Technical and Hazardous Materials Branch, Headquarters, USCG, Washington D.C, 20593. Most current drawings are accurate and reliable. Drawings and other documents describing ship’s characteristics are revised to reflect changes to ship and component characteristics and to correct errors. Documents for Navy ships are normally revised during overhaul or major maintenance availabilities to reflect changes made during the overhaul/availability and previous changes or discrepancies reported by the ship’s force or other organizations. Salvage personnel should verify that they are using the latest revision, as listed in the Ship’s Drawing Index (SDI), and should be aware that the issuance of revised drawings may lag completion of the alteration by many months. Ships that have been inactive for many years are often objects of salvage or wreck removal; drawings may not reflect alterations made after the ship entered inactive status. When drawings and other data for a specific ship are not available, documents for similar ships are used. In such cases, the data should be used only as an indication of probable conditions, to be verified as the work progresses. Even drawings for ships of the same class may not be entirely accurate, especially in the particulars of component structures and systems. Design modifications are often made before a shipbuilding program is completed; only the later ships will be built with the modifications. Subsequent alterations may not be accomplished on all ships of the class; modifications cannot be made simultaneously to all ships. Shipyards are allowed some latitude in determining final details—ships built at different yards will usually have differences. The following are some typical differences between ships of the same class built at different yards: • Tanks or compartment lengths, which may vary by a foot or more with attendant differences in tank capacities. • The exact routing of piping and wiring systems. • Arrangement and location of machinery room auxiliaries. • Relative position and arrangement of staterooms, passageways, and other minor compartments not bounded by major structural or watertight bulkheads. • Precise location of doors, hatches, fireplugs, and similar fittings. The relative importance of the differences between documented and actual characteristics depends on the nature of the salvage operation and data required. Discrepancies should be noted and compiled to give a subjective evaluation of the data’s reliability. For ships that will be returned to active service, discrepancies in published data should be included in the final salvage report and/or forwarded to the cognizant authority. B-1 info_mart S0300-A8-HBK-010 B-2.1 Curves of Form. Curves showing hydrostatic characteristics of a ship’s hull are prepared by the designers. These curves are normally presented in a single document called the Curves of Form, Displacement and Other (D & O) Curves, or Hydrostatics Curves. This set of curves is often the single most useful document to a salvage engineer. Curves of Form are carried aboard Navy ships, usually in the custody of the Engineer or Damage Control Assistant. Figure FO-2 is an exact copy of the Curves of Form prepared by the designers for the frigate FFG-7 and other ships of the same class. On newer ships, the Curves of Form are presented on a single drawing with the Cross Curves of Stability and the Bonjean’s Curves. The following information is available from the Curves of Form for Navy ships: • ∆ Displacement in Saltwater ( SW), • Vertical Position of the Center of Buoyancy (VCB or KB), • Longitudinal Position of the Center of Buoyancy (LCB), • Longitudinal Position of the Center of Flotation (LCF), • Tons per Inch Immersion (TPI), • Height of the Transverse Metacenter above the Keel (KM), and • Approximate Moment to Change Trim One Inch (MT1). The Curves of Form drawing for older ships usually include the following additional curves: • ∆ Displacement in Fresh Water ( FW), • Areas of Waterplanes (AWP), • Area of the Midship Section (AM), • Outline of the Midship Section, • Longitudinal Metacentric Radius (BML), • Area of Wetted Surface (S), and • Curve of Sectional Areas. All curves are entered from the ordinate scale with the value for mean draft. The value of the desired characteristic is read from the appropriate horizontal scale, or a factor is applied to the displacement value, as noted on the graph. All characteristics are plotted as a function of mean draft, assuming zero trim. A ship trims about its center of flotation without changing displacement. If the center of flotation is not coincident with the midlength, mean draft differs from draft at the center of flotation; the displacement corresponding to the draft at the center of flotation is the true displacement, while taking displacement based on the mean draft returns an erroneous value. Because of this disparity, entering the curves with a known displacement will give an accurate mean draft only for a ship with no trim. An additional curve is sometimes included to provide a correction to be applied to the value for displacement when the ship is trimmed. If there is no curve, displacement when trimmed is determined by entering the curve with the draft at the center of flotation. From Figure B-1, the difference between the mean draft and the draft at the center of flotation can be seen to be: dt TC = L where: TC = correction to mean draft for trim, in. d = distance from midships to the center of flotation, ft t = trim, in. L = length between draft marks, ft B-2 info_mart S0300-A8-HBK-010 The correction is added to or subtracted from the mean draft, as appropriate. EQUIVALENT UNTRIMMED TRIMMED Alternatively, the draft correction can be WATERLINE WATERLINE multiplied by TPI to calculate a displacement correction to be applied to the displacement returned by the mean draft. LCF For the situation shown in Figure B-1, LCF is abaft midships and there is trim by the bow, so the correction is negative. It is helpful to draw a similar sketch for each situation to determine whether the T correction is added or subtracted. f t TM For Navy ships, the above correction need TLCF d TC not be made if a draft diagram is available. Ta See Paragraph B-2.6.2 for instructions on the use of draft diagrams. If a ship has appreciable hog, the draft at midships is less than the mean draft. Since AFT FWD ships are fuller in the midbody than at the MARKS MARKS ends, the displacement is less than that L indicated by the mean draft. Similarly, if the ship is sagging, the draft amidships is greater than the mean draft and Figure B-1. Correction to Displacement for Trim. displacement is greater than that indicated by mean draft. When displacement is calculated by integration of sectional areas, Bonjean’s Curves can be entered with the actual forward, after, and amidships drafts. Sectional areas at intermediate stations are obtained from the curves by assuming the hull deflection follows a parabolic form, either by sketching a deflected waterline on a profile arrangement of the Bonjean’s Curves, or by interpolating drafts for the intermediate stations. When a displacement curve is used, a common practice is to enter the displacement curve with a corrected midships draft equal to the mean of the forward and after drafts, plus or minus a fraction of the deflection at midships. The deflection is added for sag or decreased for hog—i.e., the correction brings the calculated midships draft towards the observed midships draft. For a rectangular waterplane, the correction is two-thirds the deflection, since the area under a parabola is two-thirds that of a circumscribing rectangle. For most commercial hull forms, 0.75 times deflection is a reasonable approximation. B-2.2 Offsets. Offsets are tabulated as described in Paragraph 1-2.7. They are often included in a set of drawings with the lines plans (e.g., Molded Lines and Offsets for OLIVER HAZARD PERRY). Waterline halfbreadths and deck heights/halfbreadths are tabulated for 21 stations on the FFG-7 Lines Plan (FO-1).