CHAPTER IV. FREEBOARD. In the following tables the word Freeboard denotes the height of the side of a ship above the waterline at the middle of her length, measured from the top of the deck at the side, or, in cases where a waterway is fitted, from the curved line of the top of the deck continued through to the side. The freeboards and the corresponding percentages of reserve buoyancy necessary for flush-deck steamers not having spar or awning decks and for flush-deck sailing vessels are given in Tables A and D for vessels of these classes and of various dimensions and proportions. The freeboards necessary for spar- and awning-deck steamers are given in Tables B and C. The latter are determined by considerations of structural strength, and they denote the limitations to depth of loading which are thereby imposed upon first-class vessels of these types. The freeboards and percentages of reserve buoyancy thus obtained being in excess of what would otherwise be required, the amount of such percentages are not given in Tables B and Ć. The exact freeboard required for a given ship of standard proportions belonging to either of the classes comprised in Tables A and D may be calculated by constructing a displacement scale to the height of the deck to which the freeboard is measured, so as to give the whole external volume up to the upper surface of that deck. The percentage of the total volume which is given in the tables as the reserve buoyancy for a vessel of given type and dimensions will be the amount of volume that must be left out of the water. If a waterline be drawn up upon the displacement scale aforesaid to cut off the given percentage of total volume, the height of side above this line will be the freeboard required. In order to simplify and reduce the work that would be involved by the above mode of determining the waterline and the consequent freeboard that correspond to a given percentage of reserve buoyancy, an approximate method is adopted in the following tables, which enables the freeboard of a vessel to be calculated with a sufficient degree of accuracy for all ordinary The use of this method not only saves the time and labor that would be involved by making a complete displacement scale for the whole external volume of the ship, but, what is much more important, it makes the tables easily and directly applicable in cases where such a displacement working purposes. scale for a vessel is not at hand, or where the data requisite for constructing one are not procurable. In this approximate method the form of the ship is taken into account by means of proportionate quantities, which are termed coefficients of fineness, instead of by the exact volumes that a displacement scale would give. It is found that the whole internal volume of a ship as measured for register tonnage divided by the product of the length, breadth, and depth, measured as described in the following clauses, 1, 2, and 3, gives a fractional quantity of coefficient which bears a nearly constant relation to the quantity that would be obtained by dividing the whole external volume below the upper surface of the deck by the product of the length, breadth, and depth. This fractional quantity is called the “coefficient of fineness” for freeboard purposes, and it serves the same practical object, when combined with the dimensions of the ship in the manner explained in the tables, as the volume itself would do. In applying such an approximate method as the above, it is necessary to connect the coefficients of fineness given in the tables with a standard sheer and round of beam. The standard scales for sheer and round of beam that have been adapted for this purpose are given in Clauses 18 and 19 hereafter. Descriptions are also there given of the corrections that should be made for deviations from these standard amounts. The freeboards given in the tables are for flush-deck vessels in all cases. Such reductions in freeboard as may be allowed for deck erections of various kinds and sizes in steamers not having spar or awning decks and in sailing, vessels are described in paragraphs 11, 12, 13, 14, 15, 16, and 17. No reduction of freeboard should be allowed on account of deck erections in spar-deck and awning-deck steamers, except in spar-deck vessels in which an allowance may be made for a long bridge house, see pp. 21 and 22. Tables A and D give the minimum freeboards for first-class iron and steel vessels, the strength of which is at least equal to the requirements of the 100a class in Lloyd's Register for threedeck and smaller vessels. The freeboard of all other iron and steel vessels, classed or unclassed, should be regulated by the same standard, the increase of freeboard required in each case being determined by the limit at which the stress per square inch upon the material of the hull amidships shall not exceed that of the standard class, of the same proportions, form, and moulded depth, when loaded to the freeboards required by Tables A and D. Tables B and C give the freeboards for vessels built in accordance with, or equal to, the requirements of Lloyd's Register for the spar- and awning-deck classes, and are subject to the conditions just stated for any modifications of strength in excess of diminution of the requirements of their respective classes. 1. LENGTH. — The length of the vessel is measured on the load line from the fore side of the stem to the aft side of the sternpost in sailing vessels, and to the aft side of the aft post in steamers. 2. BREADTH. — The breadth used in obtaining the coefficient of fineness is the extreme breadth measured to the outside of plank or plating as given on the certificate of the Ship’s Registry. 3. DEPTH OF HOLD. — The depth used in obtaining the coefficient of fineness is the depth of hold as given on the Certificate of the Ship’s Registry. This dimension is subject to modification in determining the coefficient of fineness as explained in Clause 4. 4. COEFFICIENT OF FINENESS. — The coefficient of fineness in one-, two-, and three-deck and spar-deck vessels is found by dividing 100 times the gross registered tonnage of the vessel below the upper deck by the product of the length, breadth, and depth of hold. In awning-deck vessels the registered depth and tonnage are taken below the main deck. (a) It is of importance in the application of the rules and tables of freeboard that the coefficient of fineness deduced from the under-deck tonnage and the principal dimensions to be a correct index to the vessel's relative fullness of form, and that a change in any of those elements which affect the coefficient, determined in accordance with the rule set forth, should be considered, and the necessary correction, having regard to the special circumstances of the case, introduced. Among the cases that have from time to time come under notice are the following: (6) Vessel Having a Cellular Bottom Throughout, or Floors of Greater Depth than those Usually Fitted. — In such a case the coefficient as determined from the under-deck tonnage is in most instances slightly greater than it would be if the vessel were framed on the ordinary transverse system with floors of the usual depth. No general rule can be given for guidance, but it is not difficult, if the depth and slope of the top of the cellular bottom or floor be compared on the midship section with the depth and slope of an ordinary floor, to determine very closely the amount of the correction necessary. (c) Vessel Constructed with Floors of the Ordinary Kind, but with a Cellular Bottom for a part of the Length Amidships Under the Engines and Boilers. - In such case the registered underdeck tonnage is smaller than it would be if the vessel were framed with ordinary floors throughout, the difference being the tonnage of the space between the bottom of the cellular bottom in the part amidships and the level of the ordinary floor. The depth of hold is also measured by the customs officials to the top of the cellular bottom, and this depth is inserted in the register. Under such circumstances, in order to arrive the coefficient of fineness the vessel would have, if built on the ordinary system throughout and for which the tables are framed, the tonnage of the volume between the top of the cellular bottom and the level of the ordinary floor should be calculated and added to the registered under-deck tonnage. The tonnage so corrected used in conjunction with the depth of hold to the top of the ordinary floor, gives the coefficient to be used in the tables. (d) Vessel Constructed with a Cellular Bottom Throughout the the Fore and After Holds, but with Floors of the Ordinary Kind Fitted for a Part of the Length Amidships Under the Engines and Boilers. In such a case the tonnage of the space between the top of the ordinary floors in the part amidships and the top of the cellular bottom, if made continuous, should be estimated and deducted from the registered under-deck tonnage and the remainder employed in conjunction with the depth of hold to the top of the cellular bottom in determining the coefficient of fineness. (e) Other cases may in practice arise in which the registered under-deck tonnage, or the registered depth of hold, or registered breadth require modification before being used in the determi'nation of the coefficient of fineness, but little difficulty will be experienced in making the necessary correction if it be remembered that the coefficient sought is the coefficient the vessel would have if framed on the ordinary transverse system. 5. MOULDED DEPTH. —- The moulded depth of an iron or steel vessel, as given in the tables, is the perpendicular depth taken from the top of the upper deck beam at side, at the middle of the length of the vessel, to the top of the keel and the bottom of the frame at the middle line, except in spar- and awningdeck vessels, in which the depth is measured from the top of the main-deck beams. In wooden and composite vessels the moulded depth is taken to be the perpendicular depth from the top of the upper-deck beam at the side of the vessel amidships to the lower edge of the rabbet at the keel. (a) The form at the lower part of the midship transverse sec wooden and composite vessels being of a hollow tion of mar character, as in cases where thick garboard strakes are fitted, the moulded depth in such instances should be measured from the point where the line of the flat of the bottom continued cuts the keel. 6. FREEBOARD. — The moulded depth, taken as above described, is that used in the tables for ascertaining the amount of reserve buoyancy and corresponding freeboard in vessels having a wood deck, and the freeboard is measured from the top of the wood deck at side, at the middle of the length of the vessel. (a) On the same principle, in flush-deck vessels, other than spar or awning decked, and in vessels fitted with short poop and forecastle, having an iron upper deck, not covered with wood, the usual thickness of a wood deck should be deducted from the moulded depth of the vessel measured as above, and the amount of reserve buoyancy and corresponding freeboard taken from the column in the tables corresponding with this diminished moulded depth: Example. — In a steamer fitted with an iron upper deck, not covered with wood, and having a moulded depth of 19 ft. 10 ins., four inches, or the usual thickness of a wood deck, must be deducted from this, leaving a depth of 19 ft. 6 ins. The freeboard of such a vessel with a coefficient of fineness of 0.76, taken from the column under 19 ft. 6 ins., is 3 ft. 81 ins., which should be measured from the top of the iron upper deck. (6) In spar-deck vessels having iron spar decks and in awningdeck vessels having iron main decks, the freeboard required by the tables should be measured as if those decks were wood covered. Also in vessels where io, or more, of the main deck is covered by substantial erections, the freeboard found from the tables should be measured amidships from a wood deck, whether the deck be of wood or iron. In applying this principle to vessels having shorter lengths of substantial enclosed erections the reduction in freeboard, in consideration of its being measured from the iron deck, is to be regulated in proportion to the length of the deck covered by such erections. Thus in a vessel having erections covering % of the length, the reduction is % of 31 inches, or 2 inches. 7. For vessels which trim very much by the stern, through the engines being fitted aft, the freeboard, as ascertained from the tables, if set off amidships would not cut off the amount of surplus buoyancy deemed necessary, and in such cases the suitable freeboard amidships could only be determined after full information is obtained regarding the vessel's trim. |