CHAPTER II. TYPES OF VESSELS DISTINGUISHED AND ILLUSTRATED. Throughout the discussion of the measurement of vessels it is necessary to refer to different types of ships and to the structural parts of vessels. Frequent mention must also be made of the many spaces into which vessels are divided. Inasmuch as technical terms are used, which, though familiar to shipbuilders and seafaring men, are not a part of the vocabulary of those not connected with ships and shipping, it will be well to distinguish and illustrate the main types of ocean vessels and designate and locate both the main structural parts and the principal spaces within such ships. It is desirable that this report and the measurement rules it contains should be understood by lawmakers and courts as well as by the technical officials who may apply and enforce the measurement rules. The rules may have to be interpreted by the courts from time to time in deciding cases brought by complainants appealing from decisions of the executive officers in charge of the interpretation and enforcement of the rules. It is also possible that vessel measurement may be the subject of future legislation. Congress has authorized the President to prescribe and to change the Panama rules, but the rules by which American vessels are measured for registration and enrollment and by which all ships entering the ports of the United States are, or may be, measured for the imposition of tonnage taxes are established by law. Moreover, the existing national rules, as will be pointed out in the following report, are incomplete in some particulars and not sufficiently specific in other regards. It has seemed best to run the risk of seeming to some readers to be unnecessarily elementary, and to prepare this report with a view to making its contents clear to the layman as well as to the engineer and the nautical expert. An ocean vessel's type is indicated by the number of its decks, by the characteristics of its structures above the main deck, by the fuel it uses, and by the kind of engines and the number of propellers with which it is equipped. There are many variations in the structural details of vessels, but the main types of ships may be designated by reference to the number of their decks, the nature of their decks, their above-deck structures, their fuel, and their engines and propellers. Small freight vessels have two decks, medium-sized freight and passenger ships have three decks, while large freight vessels and those carrying both freight and passengers usually have four full decks, above which there may be one or more decks extending less than the full length of the vessel and inclosing successive tiers of superstructures. In vessels having more than one deck, the main deck (which, in measurement rules, is also called the tonnage deck) is the second deck from the bottom of the vessel. When there are three decks they are designated lower, main (or middle), and upper deck. If there are four full decks, the fourth deck is generally called the shelter deck, above which there may be a bridge deck and a promenade deck, or bridge, promenade, and boats decks. The decks above the shelter deck do not extend the full length of the hull. The sketches (Figs. 1, 2, 3, and 4) of half midship sections of vessels name and locate the several decks. The strength of a vessel depends, first of all, upon the weight and strength of its transverse framing, consisting of the floors and frames (see Fig. 3) to which the plates inclosing the hull are riveted. The minimum size of frames for different types of vessels is prescribed by the rules established by Lloyd's Association and similar organizations which classify vessels and give them the rating upon which insurance rates depend. The transverse frames are placed from 24 to 30 inches apart and the framing may be strengthened in various ways: (1) By "deep framing," i. e., by making the transverse frames deeper and stronger; (2) by substituting web frames or beams for each sixth to tenth transverse frame; and (3) by increasing the number and dimensions of the longitudinal beams or stringers that give longitudinal strength to the vessel. Practically every vessel is now constructed with a double bottom, the space between the inner and outer plating being used to carry water ballast. Certain compartments may be FIGURE 1.-Midship section, showing beams and pillars for lower and main decks. used to carry fresh water for the boilers, and in the case of ships with oil-burning engines a portion of the tank space in the double bottom may be used to store fuel oil. The particular use to which the double-bottom compartments are devoted is an important consideration in vessel measurement and is the subject of special mention in all codes of tonnage rules. The prevailing method of constructing the double bottom of vessels is illustrated in figures 3 and 11. It is called the cellular double bottom. The floors, which are the transverse steel plates extending from the center longitudinal girder (keelson) to the margin plate or longi |