122 STRAINS IN THE HULL OF A SHIP AT SEA, 



movement across one of the lap joints exceeded the scale reading of the gauge, and 

 was estimated to be ± .00125 inch. Strains due to the engines were greater than 

 those due to the pitching of the ship, both forward and aft the superstructure. 

 This remark concerning the relative effect of the engines and the pitching of the 

 ship refers to the observations made on the outward voyage. North bound, with 

 a light cargo, the relative effects of the engines and the pitching were reversed 

 over those of the outward voyage, the pitching strains now predominating. 



A comparison of results of the outward and return voyages is shown on 

 Plate 68. Whereas on the outward voyage a strain caused by the engines of about 

 .00125 inch was observed at the lap joint of course C next the bulkhead, on the 

 return trip the effect of the engines was hardly perceptible at that place, but the 

 pitching of the ship caused a movement estimated at three times the scale reading, 

 or ± .003 inch. At other joints and on the solid plates similar results were observed, 

 the pitching strains greatly predominating on the return voyage. All measure- 

 ments were taken along the middle of the width of the several courses of plates. 



Strains due to pitching and those due to the engines were recognized by their 

 periodicity. The engines made 78 rotations per minute and strains attributed to 

 them were developed synchronous with the rotations. 



In explanation of the relatively large movements due to the pitching of the 

 ship when in comparatively smooth water, it appears probable that the lap joints 

 of the deck plates had become loosened in a degree by exposure to repeated stresses. 

 A portion of the movement observed not unlikely represented slip of the joints. 

 Of the effects which came from the engines there would be some 700,000 to 800,000 

 repetitions during one voyage to the Isthmus. 



Places on the deck where the greatest longitudinal strains occurred were not 

 usually accompanied with marked vertical vibrations. At least certain places where 

 vertical vibrations were most pronounced, under the feet, were examined without 

 finding unusual longitudinal strains. On the deck over the ridge bar on the star- 

 board side, and also near the main mast, were places of pronounced vertical vibra- 

 tions, but where the measured strains were of moderate extent. Again, at the 

 stern, near the bitts, the vertical vibrations were very marked, while the longitudinal 

 strains in the vicinity were not measurable. 



The stresses corresponding to the measured strains, observed on the forward 

 deck, ranged from 100 pounds to 650 pounds per square inch. On the after deck, 

 2,250 pounds per square inch was found, all referring to measurements made on a 

 smooth sea. The stresses reached a maximum in the vicinity of the bulkheads of 

 the superstructure and disappeared as the bow or the stern was approached. 



The observations which were made prior to the main series, when a moderate 

 sea was running, showed stresses on the after deck near the bulkhead ranging from 

 2,500 to 3,500 pounds per square inch. The highest stress observed was on the bul- 

 wark rail, where a stress of 5,500 pounds per square inch was found. Fig. 4, 

 Plate 66, shows the place occupied by the strain gauge on the angle of the rail 

 when this observation was made. 



