150 CHANGE OF SHAPE OF RECENT COLLIERS. 



less than the recorded weight one time, and the next ten or fifteen tons greater, on a vessel 

 of five hundred or six hundred tons displacement, at the time the observation was made. 

 Consequently it was necessary to take up this question and to attempt to find some way of 

 treating it. 



The matter of loading is quite important to the shipbuilder in running trial trips. 

 Every added ton adds appreciably to the horse-power necessary to be developed to secure the 

 given speed. There is also another matter to which I will refer. On the trials, in loading 

 the vessel for a run, it is necessary to take in water to get the vessel down to the required 

 displacement. We took in five or ten tons of water in the trimming tanks to get the re- 

 quired draughts and the displacement, and then found that the displacement determined 

 from the draught had increased ten or twenty tons. 



The means adopted for the vessel to which I refer, to observe its change in shape, was 

 to fit an additional internal draught gauge, as near central between the other two as prac- 

 ticable, which means a gauge fitted amidships. We then read accurately the draught shown 

 by the three gauges, and took the difiference between the readings of the middle gauge, and 

 the mean of the other two, as giving the amount that the ship was hogged or sagged be- 

 tween the end gauges. A curve of correction for displacement was calculated and plotted 

 to show the amount to be subtracted or added, in proportion to the difference in readings. 

 Of course the ship might show the same difiference in readings of the gauges at different 

 .times, and not have exactly the same displacement, that is to say, may not always be bent 

 into exactly the same shape, between gauges. But if you stop to think of the long flat curve 

 to which a vessel hogs or sags, it is evident that the correction, whether based on bending 

 to a parabola, a circle or even a triangle, will give a more accurate result than to neglect 

 the correction altogether. That is what we found ; in a large number of these comparisons, 

 there was never a time when the application of the correction did not bring the results of 

 different observations nearer together and so that they were much more consistent with 

 one another, and much nearer to the recorded weights. 



Mr. E. H. Riggs, Member: — Mr. Tawresey has given us a very interesting description 

 of the methods adopted on destroyers to find the amount of deflection and to allow for it in 

 the displacement on trial and at other times. It has been my fortune, or misfortune, to have 

 to get that displacement during the trial trips of some destroyers, to Mr. Tawresey's satis- 

 faction, and we have had our fun in doing so. The midship draught gauge he refers to, 

 in the destroyer, as you all know, must come in or near a fire room. In one case it came 

 right in the fire room, and someone started to read it under air pressure; the result was 

 startling. After that experience we did not read it when under air pressure. The value of 

 the midship gauge in arriving at a correct light weight of the ship is indisputable. 



In the discussion on Mr. Howard's paper, some reference was made to changes of 

 shape in lake vessels. These colliers also are single deckers, and anyone who has had any- 

 thing to do with keeping track of the weights, in connection with this type of vessel, will 

 know at once the difficulties of the situation. As an instance of hogging that takes place, 

 due to difference in temperature, I will cite a case which came to my attention — a carfloat, 330 

 feet long, lying at our wharf in the sun, with nothing on her at all. We tried to get a 

 check on the weights, but found from the end draughts and displacement curve that they 

 were not anywhere near what they should be. We noticed that the sun was hot and the 

 water cool, so we then took a transit and shot a line, finding that there was a hog of eight 



