312 THE INFLUENCE OF SHAPE OF 



In regard to the system of notation I think naval architects, who are working on the 

 job all the time, have methods of their own and reduce data given in various papers under 

 different notations to some form they are accustomed to and thus are able to make more 

 ready use of it. 



In the appendix I notice a formula Professor Bragg gives to develop from a given test 

 the effective horse-powers for the same speeds of vessels of greater or less displacements due 

 to an increase or decrease of from 10 to 12 per cent in length. It is seldom that, within a 

 range of 10 or 12 per cent, one could not almost guess at the power for the same speed. In 

 most cases it is necessary to jump over a much greater range to handle the miscellaneous 

 problems confronting us almost every day. 



If model tests could be presented on four different draughts, by drawing cross curves, 

 an almost unlimited number of modifications of length to draught and breadth to draught 

 ratios would be available. Any formula or method by which this data could be extended 

 over a greater range than 10 or 12 per cent in length would greatly enhance its value. I 

 have used a method for over twenty-five years with satisfactory results, as far as my experi- 

 ence has gone, without going astray very badly. We all know that skin resistance does not 

 follow the same law; of similitude that the residuary resistance does. If it did, corresponding- 

 powers and speeds of similar models could be readily found by using the ratio of displace^ 

 ments as a function of the power and the 6th root of this ratio as a function of the speed. For 

 a range of only 10 or 12 per cent, this would probably produce results sufficiently close for 

 practical purposes and involve no greater error than is often made in determining the pro- 

 pulsive coefficient. I have found that, by using a slight variation of the 6th root of the 

 ratio of displacements as a function for corresponding speeds, very accurate results can be 

 expected over a much greater range. The modification of the 6th root will vary a little with 

 different types but will not fluctuate much above or below. With data now available it is 

 not difficult to determine these values, after which corresponding powers and speeds close 

 enough for all practical purposes can be derived — at least this has been my experience. 



Mr. H. C. Towle, Member: — It is some five or six years since I departed from a naval 

 architect's office, where I spent ten years applying these principles, and I probably look at 

 some of these things a little differently from the naval architect who is actually practicing. 

 These papers are like education — they form a valuable background for the practicing naval 

 architect to work from; they give him, you might say, his limits, but enough emphasis is sel- 

 dom placed on two points referring to the construction of vessels. One is that the naval 

 architect tries to design a theoretical vessel as far as resistance goes, and tries to maintain 

 the maximum efficiency in resistance, without adequate knowledge from the owners of the 

 route over which the vessel is to trade. We have had recently, if I may criticise a little, some 

 $60,000,000 worth of ships designed with U-shaped forward sections, very fine in the region 

 of the propellers, but with very full waterline aft due to the cruiser stem, the natural result 

 being that in a seaway these vessels dive unmercifully into the water forward. This proba- 

 bly would have been foreseen and guarded against if they had been designed for the particu- 

 lar trade they are in. The architect, I think, if possible, should make a voyage or two on the 

 owner's vessels before he is given the job of designing a boat for that owner's trade. I thinly 

 that more attention should be paid to that matter. 



Mr. H. H. Schulze, Member: — Without attempting to criticise Doctor Sadler's experi- 

 ments, which are a very welcome addition to Constructor McEntee's previous paper, I would 



