NOTES ON LAUNCHING 

 By William Gatewood, Esq., Member. 



[Read at the twenty-sixth general meeting of the Society of Naval Architects and Marine Engineers, held in 



Philadelphia, November 14 and IS, 1918.] 



In connection with the launching of large and heavy vessels, for which launch- 

 ing calculations are made, two conditions of the operation usually receive especial 

 attention : — 



The pivoting pressure, or reaction on the ways when the stern begins to lift. 



The minimum moment against tipping. 



The calculations as usually made are well known to naval architects, and may 

 be found described in the transactions of this and kindred societies, and in text- 

 books on naval architecture. The calculations assume that the level of the surface 

 of the water is not disturbed by the entrance of the vessel, that the vessel itself 

 remains rigid, and that the alignment of the groundways is not disturbed by the 

 passage of the vessel. The pressures and moments obtained by the calculations 

 must necessarily be approximations only, since the assumed conditions cannot exist 

 in the actual launching. It is probable, however, that the results are relatively 

 comparable, and that the calculations can be relied on with confidence in making 

 the essential preparations for the launching. 



PIVOTING PRESSURE. 



Suitable provision for taking the pivoting pressure on the fore poppets was 

 early recognized as essential, because the concentration of pressure received visual 

 confirmation at almost every launching. It was recognized, also, that some means 

 of distributing this pressure is necessary, as theoretically the pressure is concen- 

 trated on the extreme forward end of the sliding way. In the early days, the fore 

 poppets were built of timbers on end, but it was soon noticed that this was not a 

 good practice, as the compression of end grain wood is quite small compared with 

 the compression when the grain is flat. By building up the fore poppets with the 

 wood laid flat, an appreciable amount of compression occurs at the forward end 

 when the vessel pivots, and the under surface of the sliding way is enabled to 

 remain in contact with the upper surface of the groundway over an appreciable 

 length, thus distributing the pressure between the two pairs of surfaces. 



As the launching weight of vessels increased and the pivoting pressures 

 changed from a few hundred tons to more than two thousand tons, some more 

 definite means of distributing this pressure became necessary. The introduction 

 in the fore poppets of a considerable depth of soft wood with grain flat seems to 



