In the number for December, page 189, we promised a few remarks on connection with the spar draft of the clipper ship Lightning, to show that the model was not at fault, whatever discrepancy might exist in the splendid performances of this vessel. Owing to the absence of one of the editors from the city, this article arrived too late for publication in the number for January, as we had calculated. Having given a draft of masts and spars in the preceding number, we will refer the reader to page 292, while we proceed to disclose the calculations of the rig, which we find as follows:--
Area of leading sails, viz.: two jibs, two courses, spanker, three topsails, and three topgallant sails = 17,838 square feet.
Centre of propulsion located 2.65 feet forward of mid-length on the loadline, and 4.89 feet forward of the centre of buoyancy, or centre of gravity of displacement. Altitude of the centre of propulsion, above load-water-line, fifteen (15) feet above base, equal to 60 feet. Vertical moment of sail, equal to 1,068,900; which is equal to the moment of stability (see page 188) multiplied by 1.37, (one, and thirty-seven hundredths,) showing that her surface of sail is only moderate when compared with the enlarged capacity for carrying it. We think very few ships, if any, have ever been built with so great stability for an equal cubical displacement; and we are sure that they are no less few, whose good judgement would have been contented to refrain from over-sparring a vessel of such prime qualities. Many a builder would have furnished her with a taunter rig, and thus sacrificed some of the finest elements of speedy voyages. Masts and spars, as well as canvas, when adjusted to the demands of utility, will be found capable of stowing a reef. We always have too many spars in a gale of wind, and seldom find a sufficiency in light breezes; hence the propriety of studding-sails and topsails, with which we can double the area of canvas in a few hours. We seldom stop short of over-burdening our ships with spars, when we endeavor to spread a large area of canvas for leading sails, one half of which cannot be used when the winds list to our full desire.
In the aggregate, the Lightning is lightly sparred, for a ship of her class and dimensions. But, for some reason, it is found that she is hard on her forward spars; and having lost a fore-topmast on her first voyage, it was advised in Liverpool to essay and improvement on her fore-body, by filling it out with "vertical pieces of soft pine" spiked to the planks, in order to enlarge the displacement, and, as the patchers think, to ease her oscillations in a sea-way. Against such an un-mechanical alteration -- such a mediocre expedient -- a block of wood might cry out a protest. Should it happen that the spars remain firm at the end of the next voyage, and the ship makes another short passage, which she is as likely to do as before, depending on the winds, &c., the designers of this alterations will doubtless claim the credit of an improvement -- an improvement on a Yankee model -- very gratifying to Uncle John, we suppose. But when his pine gets water-soaked, adding to the weight more than to the displacement, of all things to be avoided, the difficulty, which was to be cured, will then be aggravated, and he will have to dock her again and seek relief in removing his quack prescription.
With regard to the model of the Lightning, there has never been a finer in the docks of Liverpool; and it would be strange, indeed, if, at this point in the experimental history of clipper-ship building -- in a age, also, that has been characterized by more independent efforts at perfective art than ever known before it, nothing could ever be found on shipboard incapable of improvement, or a higher degree of skillful adjustment by a more rigid application of scientific principles. With equal truth, it may be said that those who would undertake to improve a machine, or a ship, should first be equal to the task of its original construction, and, moreover, be familiar with its science. The calculations of rig will show, that the location of the centre of propulsion, on board the Lightning, is as near the true point as it will be found on most ships of her class, being nearly five feet forward of the centre of buoyancy.
The principles of applying propulsory powers to bodies, should be familiar to every mechanic. The most economical expenditure of effort to move a floating body, be it by steam or atmosphere, will ever be found realized when applied in the direction of its centre of gravity. All effort otherwise directed, must needs be subject to the drawbacks of leverage and one-sided action. Why is it that a steam-tug applies her power to greatest advantage in "towing", when her bow is made fast under the quarter of a ship, rather than alongside? Obviously because she brings her effort nearer to the middle line, and in the direction of the centre of gravity of the ship; and could she be secured to the ship directly astern, the end would be still better attained. When the "tow" is placed astern, there is a considerable loss in current, set in motion by the propelling instruments, inasmuch as the tug has to contend with the adverse motion of the water from her own efforts. The same principle is involved when we apply the moving power of vessels above the centre of buoyancy; and, notwithstanding the crude theories of mathematicians and philosophers in Europe, who have supposed there must needs be a "proper height" for sail above the centre of gravity of the ship, our deference for the plainest principles of mechanics requires us to seek no farther for an exposition of the correct philosophy of locating propulsory power. The lower the centre of propulsion, the better the application; and the closer in coincidence longitudinally with the position of the centre of buoyancy the better, also, provided the model is what it should be, with respect to the distribution of displacement and resistance, both direct and lateral. And these remarks are not only true, regarding the original design for sails, but during the subsequent navigation of the vessel, the commander should be guided by the same rules. We say, therefore, reduce the leverage of propulsion to its minimum limits, if a vessel be calculated for the finer performances of maritime skill. It is only when the resultant of resistance is determined too far forward, or when there is an excess of lifting power forward, when under sail, that the centre of propulsion should be assigned a position forward of the centre of buoyancy of the ship. The shape of the ship and the comparative distribution of displacement on the two ends, should be such as to preserve her from any deviation of trim under sail; and when this is the case, and the centre of lateral resistance predominates in favor of the posterior portion of the body, the centre of effort of sails will be found rightly located in a vertical line with the centre of gravity of displacement. Placing it forward of this point, under such circumstances, will increase the tendency to pitching, because the pressure of sail, owing to its vertical remove from the centre of buoyancy, is then brought to bear on the momenta of the fore-body, constantly maintaining an unequal and unbalanced condition of weights, between the fore and after bodies, which should be in equilibrio, in order to insure a minimum degree of pitching, under sail. In view of these facts, a better location of propulsion, in the case of the Lightning, would, perhaps, be found near five feet further aft. This is with the view of reducing any undue tendency to pitch to a minimum; nevertheless, if she would then have a tendency to carry a strong weather helm, (without increasing the draught by the head), or to settle by the stern, the adjustment could not be made without modifying those influences. In the former case, an addition to the depth of keel aft, and in the latter case, a slight trim by the head, with a lesser movement aft of propulsion, would have secured all that could be desired, in the location of this important point.
But perhaps an investigation of the stations of the masts would be instructive with a view to discover the cause, or pretext, if there really be no cause, for the extraordinary exercise of mechanical fogysim, which we have referred to in the history of this ship.
Masts are usually stationed according to the length taken "on deck", or at the load line; and there are almost as many rules as builders, and not infrequently a builder varies his "rules" to suit a particular ship or his own ideas at the time. Principles are more than rules; and in this country, if not in Europe, every intelligent mechanic depends on his own application of them to secure the result which rule-followers would fear to seek, except by the stereotyped mode. It is with reference to the position of the centre of buoyancy, and to its distribution, that we would proceed in placing the masts of a vessel. We will, therefore, show the position of the masts of the Lightning and the packet Aurora, built by Wm. H. Webb, in 1854, and 195 feet long on the load-line. (For our remarks on this fine ship, with a lithograph, refer to page 124, vol. 2, No. 2.) It will be seen that the Lightning is 32 feet longer that the Aurora.
|Forward distance of foremast, from centre of buoyancy||68 ft.||59 ft.|
|Aft distance of mainmast, from centre of buoyancy||6 "||13 "|
|Aft distance of mizzenmast, from centre of buoyancy||72 "||66 "|
The fact that any alteration in the form of a ship has been thought desirable by her master, or others, is far from conclusive with us that it is required. In the contrary, owners will often find another mode of procedure the best to free a vessel from the charge of obnoxious qualities; and that is the discharge of the master and the employment of another, better qualified to command the particular combination of the qualities of the ship in question. The principle upon which we base this remark is of universal application among men and machinery. Seamanship and engineering are only different names applied to different pursuits, which have alike their foundation in natural mechanical principles; and without depreciating the qualifications of a mariner or machinist, we may still prefer one of two first class men for one particular branch of a pursuit, and the other for another branch. The difference between the opinions of shipmasters respecting the best mode of sailing a given vessel is very great. We will give an instance. A three-masted vessel of our construction was furnished with two centre boards, one forward and the other aft of the centre of buoyancy, and so placed with reference to this point and to the centre of propulsion, as to require, with an even trim, the use of both in oblique courses.
The vessel has been commanded by two masters, one of whom prefers to use the forward board only, while the other is equally confident that the after board is the proper one to work with! "Extremes meet" in every study and pursuit of man. But more than this is true. That a ship-builder should fall into the hands of the "Philistines" only once in five years, is a subject for special self-gratulation at his hands.
The experience of every man who has built vessels, will furnish illustrations of "grounding on the flats" -- flats that are entirely ineligible for dredging or removal, by "appropriations" from Congress -- and which sink and destroy more property for underwriters and owners in the United States, annually, than the whole class are worth, or ever earned. We have known a man promoted to command a vessel -- a fore-and-aft schooner, who run before a gale of wind with the foresail stowed, and with reefed mainsail and jib, and afterwards gave her a name for steering badly! The same fellow at another time was taken by a squall on the weather beam, instantly put his helm "hard up", with two men to hold it, and after ordering the foresail settled away, hung on to the weather main rigging in a fit of terror, and shouted "hard up!" "hard up!" while the schooner, which was empty, and having hatches off, careened rail under, and sweeping a circle to windward, lost headway and stability, and capsized. On being righted and towed into port, the dangerous (?) qualities of the unconscious vessel were forever removed by adding a piece of wood to the back of the rudder, and cutting down the masts six or eight feet! That the Lightning has not suffered in like manner, we have not at this moment the date for a full judgment. It is most certainly true, that many of the most successful ships at sea, are less entitled to the first place in perfective rank; and in the hands of men fully equal to her design, it is our opinion, in the present state of marine art, very little fault should be found with her qualities. Where genius makes one mistake, she is one thousand times misunderstood.
Transcribed by Lars Bruzelius.
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Copyright © 1996 Lars Bruzelius.