90 NOTES ON LAUNCHING. 



have served the purpose in some cases, notably in the launching of the Lusitania 

 and Mauretania. A special rotating device or rocker formed as a segment of a 

 trunnion has been used successfully, as described in volume 22 of the Transactions 

 of this Society. Curving or cambering the forward end of the fore poppet crib- 

 bing to introduce a clearance for the adjustment of the bearing surfaces has been 

 used, as noted in the discussion on the paper above referred to. Strips of soft 

 wood especially proportioned to crush under pressure were used in the fore pop- 

 pets of certain vessels launched on the Pacific Coast, as described in volume 12 of 

 the Transactions of this Society. 



The use of crushing strips in the fore poppets seems the most simple and the 

 most effective method of distributing the pivoting pressure, and the calculations 

 to determine the number, size and spacing of the crushing strips are not very 

 elaborate. It is desirable, of course, to select for the crushing strips material which 

 is uniform in quality ; and white pine, flat grain yellow pine, or Port Orford cedar 

 would seem suitable, as fairly uniform material can be obtained and the compres- 

 sion curves show the proper characteristics. In Plate 62 are shown average com- 

 pression curves for flat grain yellow pine and Port Orford cedar. These curves 

 indicate that crushing begins at about 650 pounds per square inch for yellow pine 

 and at about 750 pounds for Port Orford cedar, for which pressures the compres- 

 sion is less than 4 per cent, and that for a compression or crushing of 30 per cent 

 the pressures are about 950 pounds per square inch and 1,200 pounds per square 

 inch respectively. These curves are based on samples of width equal to or greater 

 than their height, and the variation of pressure from the mean at 30 per cent com- 

 pression was less than 10 per cent for a large number of samples, when care was 

 taken to eliminate resiny and unseasoned yellow pine. Slightly different values 

 were obtained when the width was less than the height. 



In order to determine the most suitable arrangement of crushing strips, it is 

 necessary to calculate the reactions and the change in grade of the keel or sliding 

 ways for several positions after pivoting. In Plate 63 the reactions for a certain 

 vessel are plotted in tons, and the angle between the sliding way and the ground- 

 way in inches per foot of length on base of distance slid and of length of overlap 

 of ways. The curve of minimum mean pressure per foot run is obtained by divid- 

 ing the reaction by the length of overlap of the ways, and it is evident that such 

 pressures can only be obtained if the whole of the overlap of the ways is in bear- 

 ing. They are based on a static launch, and are modified materially when the over- 

 lap is approching zero by the launching velocity of the vessel as compared with the 

 pitching period of the vessel. 



The height and general construction of the fore poppet will determine what 

 depth of crushing strips can be fitted, and the maximum crushing in inches which 

 can be counted on will regulate the portion of the overlap of the ways which will 

 be in contact. If we call the reaction R, and the angle between the ways expressed 

 in inches per foot of length as a , and the maximum crushing at the forward end 

 of the fore poppet as C, then the length of overlap in bearing will be C-4- a , and 



