This rough method requires good judgment and practice, as it is obvious from the example given that although 1,709 tons is a fair approximation it is still too heavy. Recognizing this fact and the necessity for a quick approximative rule which would give fairly close results, Mr. J. Johnson (vide Trans. Inst. Nav. Arch. Vol. 39) devised a method based on Lloyd's longitudinal number (modified for some types) and by plotting down known steel weights opposite their numeral, drawing curves through the mean values of each type, he analyzed them and found their equations. By means of curves prepared in this way from actual weights, the amount of steel is easily read off and the increase or decrease due to an alteration in the numeral is readily seen. Johnson's formula is as under, X W=CN or W= K NAX 100 where W = Finished weight in tons of iron or steel used in hull construc tion. N Lloyd's longitudinal number modified as follows: In 3 decked vessels the girths and depths are measured to the upper deck without deduction. In spar and awning decked vessels the girths and depths are measured to the spar or awning decks respectively. In one, two or well decked vessels the girths and depths are taken to the main deck in the usual way. c and K are coefficients varying with different types. x is an exponent, also varying with different types. Table Giving the Mean Values for c, K, and x for Vessels Built to Lloyd's or Veritas' Highest Class. TYPE OF VESSEL. c. K. X. Three deck, with complete shelter deck .00359 .328 1.48 Three deck Spar deck Awning deck. One deck, two deck, and well deck .00167 .00078 .492 1.40 .00115 .576 1.35 .665 1.30 Of course differences in the arrangement of scantlings, extent of double bottom, number of bulkheads or length of erections must be calculated as extra. A complete set of curves based on this method, but extended to embrace the largest types of vessels including complete shelter deck steamers is given opposite. The second part of the finished hull weight, viz.: the wood and outfit, embraces everything that goes to finish the ship excepting fresh water, coal and consumable stores. That is, it comprises all wood work, both shipwright and joiner, masts, rigging, sails, boats, anchors, chains, cables, hawsers, furniture, fixtures, etc., many of the items being extremely difficult of accurate calculation. For this reason it is necessary where these fittings are calculated in detail to carefully check the result obtained by a similar method to that used for the approximated steel weight from actual wood outfit data derived from known ships of similar type. The value of this coefficient for various classes will be seen from the Table of Elements of Ships. Regarding this weight, Johnson states that it will be found to vary almost directly as the longitudinal number. LLOYD'S LONGITUDINAL NUMBER MODIFIED (N) DIAGRAM ON HULL STEEL WEIGHTS, ACCORDING TO JOHNSON'S FORMULA |