Chapman remarks on this important subject, that the area and moment of the canvass for merchant ships ought to be determined in the same manner as for ships of war, although the circumstance of their tak ing cargoes of variable density must occasion a cor responding variation in their moment of stability. There may, nevertheless, be supposed at all times a fixed point for the centre of gravity of the ship and its lading, from which the moment of the canvass may be calculated.
It is most usually the case to proportion the height of the masts to the breadth of a ship, and the length of the yards to the length of the same; and from which it may be inferred, that ships of the same length and breadth, but possessing different degrees of stability, must have the same extent of canvass; whereas the extent of canvass should rather seem to be propor tioned to the stability. True as this rule may be for armed ships, there may be reasons of a very strong kind why the same principle should not be followed in merchant ships.
When it is considered, says the celebrated Swedish architect, that the weight of the anchors of a ship is proportional to the length and breadth, or to the square of the breadth, and that the act of weighing the anchors requires a certain number of men, as also the working of sails of a certain size; that large sails require a numerous crew, and that numerous crews are expensive to maintain; it appears that, for a mer chant ship it is advantageous to have as small a crew as possible; or that it is most consistent with good management, that the number of the crew should be suited as well to the magnitude of the sails as to that of the anchors.
Hence it appears, that it is the number of the crew which confines the area of the sails to definite limits.
Let us inquire, for a moment, how far the ordinary proportions of masts and yards are proper. Suppose t WO ships of the same length and breadth, and having, according to the usual practice, the same extent of canvass, but that one of the vessels carries sail better than the other; the usual remark in such a case is, not that this has too much or that too little canvass, but that the former has greater stability than the lat ter. Hence we might conclude that it would be better to effect some alteration in the form of the sails, or to make them smaller with relation to the stability, (pre serving in other respects the ordinary proportions,) than to augment the number of the crew, in order to be able to use a greater quantity of canvass. At the same time, however, it may be remarked, that when the surface of the sails, according to the usual pro portions, is too great with respect to the stability, it should rather be an object to place the parts of the lading which possess a greater specific gravity lower, than to diminish the area of the canvass, particularly if the number of the crew cannot be decreased on ac, count of the anchors.
Moreover, it is worthy of remark, that in different circumstances the same area of canvass may be as proper for a vessel of greater stability, as for one of less. In one case, the surface of the sails may be increased by means of studding sails and stay sails, and di minished in the other, by taking in reefs according to the state of the weather. Hence there is great reason. continues Chapman, to use the rule according to which the masting is proportioned for merchant ships, as that gives most nearly those proportions for the masting which have already been found by expe rience to be the best. So that the moment of sta bility, according to which large ships have masts higher, and small ones lower, than the result of the usual rule. twill not serve to found thereon the pro portions of masts and yards for merchant ships.
As the breadth of ships has the greatest influence on the stability, the lower masts and top masts should be proportioned to the breadth, whence not only the height of the sails, but also the altitude of their com mon centre of gravity, will be in proportion to the said breadth. With respect to the breadth of the sails, or what is the same thing, the length of the yards, it should be proportioned to the length of the ship, and from which it follows, that the moment of the sails will he as the square of the breadth, multi plied by the length. Small ships will, therefore, have a greater moment of canvass, in proportion to their stability, than large ones; and it is a received practice in small ships to increase the height of their lower masts still more, but at the same time to diminish the altitudes of the top masts. If we assume the breadth of a trading ship equal to If, the height of its main mast, according to Chapman will be 3.23 and the height of the main top mast, reckoning from the upper side of the cross trees, that of the main mast 11 11 L1'" Itobeing denoted by L, will be for frigates, and 2.73 2.84for barks. By a reference to Fig.1,Plate CCCCXCIV. the line BNN will be found to represent the height of 11 the masts in the proportion of the element B". The length of the bowsprit, outside the stem, for frigates, is 1.15 B, and for barks 1.1 B, where B denotes, as before, the breadth or the ship.