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6 THE MARINE REVIEW do not collect dust. These, however, should be well above the eye line to have a good effect. Panels should be large and, when made of several ply wood or one of the many kinds of composition, will not warp or crack. Small panels or those running in a brick fashion have a gingerbread look and require more labor to put to- gether, which adds to the cost and lacks appearance.” Hog and Sag of Vessels A paper by T. M. Cornbrooks, chief engineer and naval architect, Maryland Steel Co., Sparrows Point, Md., en- titled “Data on Hog and Sag of Mer- chant Vessels”, is presented in full elsewhere in this issue. Prof. H. A. Everett, Massachusetts Institute of Technology, discussed the paper and gave some interesting information re- garding observations at the plant of the Fore River Ship Building Co., Quincy, Mass., to determine the effect of temperature on the hog and sag of ships. Prof Everett’s work indi- cated that the deflection of a large vessel varies considerably throughout the day, due to expansion and con- traction resulting from changes in temperature. To support his con- clusions he presented the following figures: Difference in temperature Changein Time between deflection of ship’s deck due to day. and water, temperature, a.m. deg. Fahr. ft. Be SOG uicie Gs stabs sieves 9* 0.00 GO BU ae ie cc cate esau ecers + 77 0.04 TST | hogs so MR ees Agee em 32 0.08 DOH Oe iran aaa 39 0.12 13:30 40 0.13 Se Ne 41 oiag potst BOs oe deb ciateie ste 33 0.11 *Hull temperature below temperature of water. {Hull temperature above temperature of water. Maximum, Elmer A. Sperry, Sperry Gyro Co., New York, said that vessels also are subject to lateral deflection due to heat. Francis B. Smith, chief engineer, Pittsburgh Steamship Co., Cleveland, called attention to the excessive hog that frequently develops in long lake vessels due to the great difference in temperature between the deck exposed to the summer sun and the underbody immersed in almost ice cold Lake Superior water. The deflection some- times becomes so great that the ship cannot enter the Soo canal and the difficulty is overcome by turning a hose on the deck and cooling it off before entering the lock. This causes the ship to straighten out. The max- imum hog of lake vessels, said Mr. Smith, is about 6 inches, and not 10 inches. A paper entitled “Period of Vibra- tion of Steam Vessels”, by William Gatewood, naval architect, Newport News Ship Building & Dry Dock Co., Newport News, Va., gives the results of an investigation of the vibrations which occurred in service on an oil tanker built at the Newport News yard. “This tanker is 460 feet long and is fitted with a quadruple expansion en- gine installed in the stern, designed to indicate about 2,800 horsepower at 78 revolutions per minute. When run- ning in ballast condition, no vibration was experienced. When loaded, how- ever, longitudinal vibrations in a ver- tical plane were experienced with the engines at revolutions between 72 and 76. These vibrations were of the two-nodal character, and their period corresponded with the revolutions of the main-engines. After several trips, the pitch of the propeller was in- creased so that the revolutions in ser- vice might be less than 72, and under the altered conditions no vibration was experienced. In order to investigate the matter more thoroughly, however, observations were taken on the vessel after the pitch of the propeller had been increased. The methods of tak- ing observations were somewhat prim- itive and consisted of a device for measuring variations in the shape of the vessel in a vertical plane and an- other device for measuring the change in length of the deck plating. “A vertical batten marked off in inches was set up at mid-length of the vessel, in line with the inner edge of the shelter-deck stringer plate. A horizontal batten was located at the height of the rail abreast the after end of the engine casing. An observ- ing station was established at the out- board end of the breakwater near the bow. The distance from the observing station to the vertical batten was 192 feet 8 inches, and from the vertical batten to the horizontal batten was 189 feet 2 inches, a total distance of 381 feet 10 inches. This was the longest fore-and-aft sight obtainable. The method of observing was to sit on the breakwater with head against the rail and with binoculars to watch the motion of the horizontal batten past the divisions on the vertical batten. Observation Stations “Five stations were located for ob- serving the stretching and compression of the shelter-deck stringer plate. No. 1 station extended from the _ break- water for 90 feet aft on the line of the inner edge of the _ shelter-deck stringer plate. No. 2 station over- lapped No. 1 station, and extended 90 feet from just forward of the bridge- house to nearly amidships. No. 3 station overlapped No. 2 station and extended 90 feet over the mid length of the vessel. No. 4 station overlapped No. 3 and extended 90 feet from for- ward of the mainmast to aft of the after cofferdam. No. station over- lapped No. 4 and extended 90 feet from after cofferdam to after end of engine casing. The method of observ- ing was to secure one end of a steel tape to the heel of a rail stanchion and to hold the other end in the hand with constant tension, watching the motion of the 90-foot mark past a scratch on the deck plating. “The observations took place in the Gulf of Mexico in July, 1914, with the vessel loaded with about 11,000 tons, deadweight. The revolutions of the January, 1916 main engine were increased from 68 to 74, when a slight vibration became noticeable. It could not be measured by the battens, but at No. 3 station the motion amounted to about 1/32- inch in 90 feet. When the revolu- tions were increased to 76, the vibra- tions were quite appreciable. The motion on the batten amounted to about %-inch total. It was difficult to determine this with exactness, as it varied slightly, but it never appeared to exceed 34-inch, and Y%-inch seems near the true reading. The motion in 90 feet as determined by the tape line was 1/64-inch at No. 1, 1/32-inch © at No. 2, 1/16-inch at No. 3, 3/64-inch at No. 4, and /64-inch at No. 5. The number of vibrations corresponded with the revolutions of the engine and could be easily counted by the motion of the tape. The type of vibration was of the two-nodal character, in which the bow and stern drop as the midship ‘portion of the vessel rises, and rise as the midship portion drops. The nodes seemed to lie near the after end of the bridge deck-house and at the after cofferdam, but it was difficult to decide where the nodes were. Compressive Stress “The vibration in stress in the shel- ter-deck stringer corresponding to 1/16-inch variation in length in 90 feet amounts to about 1,700 pounds per square inch. The deck was already in compression, and the additional compressive stress therefore amount- ed to about 850 pounds per square inch. During the tests the sea was smooth. When the engines were slowed down, the vibrations ceased. When the revolutions were raised to 76 again, so that indicator cards might be taken, the vibrations reappeared, and of the same intensity as before. Here seemed to be a case of syn- chronism between the revolutions of the main engine and the period of vibration of the hull structure occur- ring in the working range of revolu- tions of the engine on a vessel of low speed.” The discussion of this paper brought out the opinion that vibrations can be killed, in a practical manner, by regulating the engines, by altering the propeller blades, and sometimes by a redistribution of the water ballast. Capt. A. P. Niblack, U.S. No stated that the control of vibrations through water ballast might be of great as- sistance in the navy where excessive vibration makes accurate range finding difficult and where speed must be maintained to keep the ship in fleet formation. Capt. W. S. Smith, navy yard, Philadelphia, discussed vibra- tions on war ships and expressed the opinion that in many cases they can be eliminated by stiffening the en- gine foundations. Considerable discussion took place regarding the, relative vibration aris- ing from three and four-bladed pro- pellers. F. B. Smith, chief engineer, Pittsburgh Steamship Co. Cleveland, cited an instance of a lake vessel that ee Set