NOTES ON LAUNCHING. 93 



process of launching, the bending moments are quite pronounced and can be ap- 

 proximated with some degree of certainty, although considerably more data must 

 be accumulated before they can be calculated with accuracy. Measurements taken 

 during launching show the effect of these bending moments on the upper deck, as 

 the stringers show considerable stretching during the passage of the vessel down 

 the ways. 



The effect of this bending of the vessel is to tend to concentrate the reaction of 

 the ways prior to pivoting at two points — the outboard end of the groundways and 

 the forward end of the fore poppets. The relative intensity of these two forces 

 would vary to suit the center of reaction, and the sum of the two forces would be 

 the total reaction on the ways, or land-borne weight. The reaction at the outboard 

 end of the groundways would be a maximum before the position of minimum mo- 

 ment against tipping is reached, and is equal to 



Wa — By 

 I 



in which Wa is the moment of the weight about the forward end of the fore pop- 

 pet; By is the moment of buoyancy about the forward end of the fore poppet; / is 

 the overlap of the ways. 



The settling of the ways, compression of the packing, and deformation of the 

 bottom of the vessel all tend to distribute this way-end pressure, but it is manifestly 

 unlikely, if not impossible, that the distribution of pressure can be as indicated by 

 the trapezoidal formula. The use of crushing strips in the packing under the bot- 

 tom has suggested itself as a means of distributing this bottom pressure, just as it 

 has served as a satisfactory means of distributing the pivofing pressure. The cal- 

 culations to determine the amount of the crushing strips required are not so sim- 

 ple as in the case of the fore poppet strips, but the following method would seem 

 applicable. 



From the launching calculations, determine for several positions of the vessel 

 the reaction on the ways and the moment of this reaction about a point chosen, say 

 lo feet aft of the forward end of the fore poppets. Determine, as indicated in 

 Plate 65, the length from the after end of the cradle over which a certain mean pres- 

 sure per foot run must be applied to afford a reaction which will have the same 

 moment as above described. Obtain similar lengths for other mean pressures, and 

 plot the series of curves through the points so obtained (see Plate 66). Note the 

 points on these curves at which the reaction at the fore poppet becomes zero, as 

 this gives the minimum mean pressure which is practicable. The highest point of 

 the curve joining these points of minimum mean pressure gives the maximum mean 

 pressure which need be experienced if the crushing strips are suitably arranged. 



If the strips are arranged so that they will just begin to crush at the forward 

 end of the bearing thus determined ; and if they are arranged at the after end of 

 the bearing, over the end of groundways corresponding to this position of the ves- 

 sel, so that they will give this pressure per foot run after having crushed sufficient 



