Maritime History of the Great Lakes

Marine Record (Cleveland, OH), August 22, 1895, p. 6

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S THE MARINE RECORD. “SQUATTING” OF SCREW VESSELS. BY I. McKIM CHASE. Various explanations have been offered as to the cause of the phenomenal increase in resistance to ves- sels when traversing shallow water, and also as to the settling of the stern when propelled at a high rate of speed; but none, so far, have met with general accept- ance. Some writers look upon the latter as the result of the water not filling in with sufficient quickness the cavity at the stern which is cattsed by the forward movement of the vessel. This explanation is probably but partly correct, as similar phenomena are. less no- ‘“ ticeable with paddle-wheel steamers. With very fast “screw vessels this settling of the stern, or squatting, as it is called, is very conspicuous. Any vessel driven by screw propellers meets with an increase of pressure at the bow, the result of the displacement of the water there, and which has a tendency to lift the forward part of the vessel ; while at the stern the pressure is dimin- ished by the action of the propellers, or propeller. The action of screw propellers causes a reduction of pressure on the hull immediately before them, creating a ten- dency in the water there to flow towards the screws. Some call this a suction action. Aft of the screw pro- pellers the pressure on the hull is diminished in conse- quence of the velocity with which they discharge the water, this being in some Cases, considerable ; and the greater this velocity the greater the reduction of the pressure. This is in accordance with a principle of hy- draulics wherein, under a given head, the lateral pres- sure of a flowing stream becomes less as the velocity of flow becomes greater. The increase in resistance to a vessel that results from the action of the screws has been termed by Dr. Froude “‘ resistance augmentation.” -™The water when discharged by the screw naturally follows the path of least resistance that is immediately in contact with the after part of the vessel when the screws are situated beneath,as twin screws are. The discharged water flows towards the surface and is there met by the water flowing in to fill the cavity caused by the advance of the vessel, rises above it, forms a crest and gives to it the appearance of a wave following the vessel (see fig. 3). In some cases this, as before said, is excessive, and inshallow water it is believed to increase. Instances are recorded (in the case of fast torpedo boats) where the discharged water from the screws actually rolled over and broke upon the deck of the vessel. Fig. 2 illustrates the vessel at rest. As to the increase in resistance experienced by an ad- vancing vessel when meeting with shallow places in its course, analogy is found in the retardation in velocity of the current of a flowing river caused by its bottom. The flow of a river is in obedience to the laws of grav- itation. The velocity of its current throughcut is not uniform; the lowest part in contact with the bottom has its velocity retarded by reason of the friction that re- - sults from the particles of water rubbing over the bottom. ; A perfect fluid is considered to be frictionless with regard to the movement of its particles among them- selves, but water is not a perfect fluid; consequently friction ensues when it moves over another substance, and aretardation of motion is the result. The small amount of viscosity possessed by water assists in com- municating this retarding effect among its particles. The water of a river may be considered as divided in- to layers (fig. 1). The layer in contact with the bottom has its velocity reduced by the rubbing action while flowing over the bottom. The next layer immediately above is retarded in consequence of the friction between its particles and those of the lowest layer ; this in turn similarly effects a retardation of the next layer above, andsoon. Each succeeding layer from the bottom to the surface is similarly retarded, but with gradually diminishing intensity, the greatest velocity being at the surface in mid-stream. When the stream varies in depth the velocity of the most retarded in the shallowest places in consequence of the smaller distance throuh which the retarding effect of the bottom acts. Assuming that the quality of the bottom is uniform, and that the current along it is retarded the same at the shallow and deep places, and that this retardation uni- formly diminishes towards the surface of the stream, then the velocity of flow at the surface over the deep and shallow places, will. be represented, respectively, by the distances ¢ and 3. An object, as a vessel, moving along the surface of water, especially still water, in consequence of the fric- tion resulting from the movement of the particles of water over the surface of the vessel, imparts motion to a body of water extending some distance from the ves- sel, and this water is carried along with the vessel, From a similar cause to that of the bottom of a river affecting the curreht upwards, the water will affect for some depth the water downwards. The layer of water in contact with the surface of the vessel is caused to ad- vance with it in consequence of the resulting friction, and this in turn causes the next layer to move with it, and this again produces motion in the succeeding layer, and thus, with diminishing effect, the, water is given motion for some distance from -the vessel in the direc- tion from which it moves. ‘What this distance is has not yet been determined, but it will probably vary with the form of vessel, its velocity and the nature of its sur- face. From the retardation that is observed when ves- sels running at high speed pass over shoal places, it is believed to extend to depths exceeding three or four times the draught of the vessel. The greater the speed of the vessel the greater will be the body of water car- ried along with it, for the reason that more motion is absorbed by the water from the vessel. Fig. 3 illustrates a vessel advancing and carrying : CIOL. ae along with it a body of water. From this it may be con- ceived how it is possible for a vessel to experience in- creased resistance when passing shallow places where this body of water would be dragged over the bottom and the resistance from this cause be transmitted through the several layers to the vessel. This carrying along with it of a portion of the material through which a body passes appears to prevail generally. It is well known that a bullet passing through air carries along with ita large body of air. In the published accounts of experiments in France on the penetrative effects of the Lebel rifle on ordinary snow, the experimenters were surprised to discover that a penetration of the pro- jectile of more than five feet could not be obtained, even though fired at the same distance at which it had been made to pass through a tree 3-12 feet in diameter. The reason given for so small a penetration is that the projectile entering the snow with a high velocity and a rotary motion due to the rifling, collects particles about it and drives them ahead until the accumulated mass arrests its course; or, in other words, it carries along with it a body of the snow, and. has not only its own resistance to overcome, but also that.of the mass of snow which it forces along. ‘This phenomenon is notice: able even in such an exceptional case as that of a pro- jectile penetrating in armor plate. ‘: It may be urged that it is not»possible for resistance to be transmitted through water in this manner. In an- swer to this, the increase, to resistance to the flow of fluids in small pipes may be referred to. The sm the diameter of the pipe the less will be the propor ate discharge. g ; os A Let A and B (in fig. 4) represent the cross sections < pipes. Assume the former to be double the diameter the latter, or, the former one inch and the latter on half-inch in diameter. Suppose them to be divided cor centtically into equal divisions representing laye water one-sixteenth of an inch thick. Assuming th quality of surface to be the same, (he coefficient of fri tion will then be equal in each case. The flow bein under equal heads, the outer layer in each case will be similarly affected and equally retarded, the flow through the pipe meeting with its greatest resistance th The retardation of the outer layer will affect the lay next:to it in consequence of the mutual friction of th particles, and this in turn will retard the next lay and so on, with diminishing effect, each layer tow the center of the pipe. The larger pipe, having a lar, number of layers will be less affected in the center the pipe than the smaller one, which has fewer layer In other words, the outer layer of the half-inch pip meets with greater resistance than the correspondin layer from the center of the large pipe; consequen the area of one-half inch in diameter in the center of the large pipe will permit a greater flow than the area of the small pipe. 2 So or : NEW PATENTS, ee Following is a list of new patents which have been re cently awarded on inventions bearing on navigation, marine trade and relative matters: ; No. 544,479.—Means for Operating Swinging Beams of Excavating Machines. Wm. Davy, Kenosha, Wis., and Ernest J. Davy, Chicago. Filed June 22, 1895, serial No. 553,715 (no model). ios 544,460.—Dredging Apparatus (suction). Caleb G. Collins, Woodbury, Alburtus C. Hilsinger, Killawog, and Calvin A. Stevens, New York, N. Y.; said Collins and Hilsinger assignors to said Stevens. Filed Mar. 4, 1895, serial number 540,565 (no model). a No. 544,676. Folding Boat, Manfierd U. Loree, Miam- E isburg, assignor of two-thirds to Eugene A. Othmer we and Wm. D. Huston, Dayton, O. Filed March 12, 1895, ~~ Serial No. 541,509. (No model). ’ No. 544,667. Automatic Hatchway Door Operating a Device, James M. Elder, Indianapolis. Filed June 19:29 1894. Serial No. 515,064. (No model). e No. 544,733. Bridge. Robert P. Lamont, Chicago. Filed Jan, 5, 1895. Serial No. 533,901. (No model). No. 544,755... Water Elevator and Motor, Josiah E. Symons, Boise City, Idaho. Filed Feb. 1, 1895. Serial — No. 537,015. (No model.) s No. 544,792, Upright Marine Boiler, William N. Oldman. Buffalo. Filed March 23, 1885. Serial No. 542,889. -(No model). ; No. 544,979. Ship’s Berth, William P. Hoskins, Bir- mingham, Eng. Filed Dec. 6, 1892. Serial No. 454,296. (No model). Patented in England. risk Nos. 544,980, 544,981, 544,982, same, to same. : 544,608.—Life-boat. George Bluemcke, Brooklyn, N.Y., filed Mar. 28, 1895, serial number 543,552 (no model). The claim is for a keel of practically rigid material, a stem — and a sternpost of practically rigid material secured to ~ the keel ; a supplementary stem and sternpost of elastic material firmly secured to the keel; a series of elastic * ribs secured to the keel between the stem and the ’ sternpost; a series of longitudinal elastic side-bars provided with eyes to engage the ribs, and connected at their ends with the supplementary stem and sternpost; two upright standards firmly secured to the keel; a seat guided by said standards and con- structed to move up and down on the same; and braces hinged at their inner ends to the seat, and at their outer ends to the elastic ribs. Also the combina- tion with the keel, the stem, and the sternpost, of a sup- plementary stem provided with guides which engage the stem, and of asupplementary sternpost provided with guides which engage the sternpost. ; A full description of these patents can be obtained by sending to the Patent Office, Washington, 14 cents fo each, and mentioning THR MARINE RECORD. : . Po ore THE new steamship Howard, built at the yard of H land & Hollingworth, Wilmington, Del., and a cut 0: whose hull and engines appeared in the MARINE RECO! I a few weeks since, has gone into commission.

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