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August, 1910 TAE MarRINE REVIEW 325 THE AVOIDANCE OF EXCESSIVE ROLLING IN VESSELS ENGAGED IN OCEANIC TRANSPORTATION MONG the character- istics of every vessel is the period of time in which she will roll waves from a given inclina- tion on one side of among the upright position to the same inclination on the other side. This is an architectural characteristic which remains fixed under normal condi- tions of load and trim, but which may be varied within considerable limits by changes in the disposition of weights and the consequent affecting of tke vessel's metacentric height and radius of gyration. Fluid resistance operates to limit and tends to extinguish the rolling oscillations of vessels, but whatever the extent of the rolling, within the limit of 15 degrees or 20 degrees from the upright position, the time in which it is performed is prac- tically constant and equal to the peri-- od in which a vessel naturally con- tinues to oscillate when she has been artificially set to rolling in still water. Circumstances of Deepest Rolling In a manner somewhat analogous to that in which the amplitude of the swinging of a pendulum is increased by regularly timed impulses given in correspondence with the period of its swing, the deepest rolling of a vesse! at sea may be expected to take place When she is exposed broadside to waves whose half-period is the same Or nearly the same as her still-water period, of rolling. Among equally mariner, holding in mind the period seem almost insensible to their effects. And so it comes about that the high waves of a different period, she may be comparatively steady © and of rolling of his vessel,and having 2 is quite as important that the vessel knowledge and account of the pe- tiods of the waves met with at sea, May derive great advantage in the Maneuvering of his vessel. In some branches of oversea transportation, it 's quite as important that the vessel be not exposed to unnecessary rolling '8 it is that the gun platform of 2 battleship be not. unnecessarily un- Steady in action, Transportation Dangers Arising from Excessive Rolling. Tn the transportation of coals and *xplosives, and in all those cases in which the attrition caused by ex--- cessive rolling may give rise to igni- tion, occasions arise when it is high- ly desirable soto lay the vessel with reference to the progress of the waves that the rolling may not become ex- cessive; and this is also the case in the transportation of live freight and in those voyages, constantly taking place in many trades, which have to be undertaken with little or no cargo on account of the absence of return freights and in which it is necessary, for seaworthiness and proper immer- sion of the propellers, to sink the vessel by means of water ballast. This has usually been placed in the double-bottom compartments with the result of lowering the center of grav- ity of the vessel so far as to give a largely increased metacentric height and a consequent shortening of the vessel's period of rolling to such an extent as to bring it into approximate coincidence with the half-period ot the particular waves met with on the voyage. A vessel in this ' condition may readily be laid among the waves by an uninformed mariner so that she is actually unsafe. Effect of Change of Heading. While the period of rolling of an individual vessel in a givefi condition of load is practically a fixed quantity, the periods of the waves that are met with at sea vary between compara- tively wide limits, and it is necessary for the purposes of good seamanship that the mariner should be able to find the period of the waves that are running at any given time. If, with a vessel headed in the direction of the wave crests, the half-period of the waves should bz found to be the same as the sti:l-wa er period of roll- ing, a change of heading to a course passing diagonally across the crests of the waves will effect an alteration in the interval of time between the arrival at the vessel of successive waves, and thus destroy that correspondence b:tween the wave period and the vessel's period of roll- ing which gives rise to very deep roll- ing; and it is obvious that, if it should become necessary to destroy this cor- respondence on a. diagonal course, the desired result may be reached by a change of the vessel's speed without changing the course. have been of warships, or still-water pe- Rolling experiments made on most classes and their natural riods have been determined. For small gunboats, torpedo boats, and small craft, the period for a_ single roll is from two to three seconds; for the larger gunboats and smaller crui- sers the average period is four sec- onds; and for the larger unarmored cruisers the average period is from five to six seconds, but some of the - larger swift cruisers have periods as high as eight seconds. Among the armored ships the shortest periods are found in coast-defense vessels of shallow draught, great proportionate beam, and large metacentric heights. One of the United States monitors was found to have a period of only two and seven-tenths seconds, and some of the French floating batteries have periods of three to four seconds, The earlier types of coast defense bat- tleships have periods of five to six seconds, and the larger seagoing bat- tleships and armored cruisers have pe- riods ranging from seven to nine sec- onds. Merchant vessels also differ widely in this respect and have na- tural periods of rolling ranging down- ward from about fifteen seconds in the great trans-oceanic liners to very much smaller periods in those vessels in which a good metacentric height has been secured, while the radius of gyration has remained necessarily small on account of the small dimen- sions of the vessel. It is important that rolling experiments should be more commonly performed with merchant. vessels, and that mariners should be accorded the benefit of be- ing made conversant with the result- ing periods of rolling. Dimensions and Periods of Sea Waves. Many observations upon the dim- ensions of waves at sea have been col- lated and discussed, and it is found that, in different conditions of the weather and different configurations of the waters of the globe, their pe- riods vary from three or four seconds to fifteen or sixteen seconds. In 1839 the officers of the United States ex-