94 NOTES ON LAUNCHING. 



to permit of this length of bearing; the mean pressure per foot run over the whole 

 length of bearing may be obtained by arranging the strips in between so that they 

 will begin crushing at the proper pressure to give this mean pressure after crush- 

 ing to suit the curvature of the vessel in this position. It is assumed in the dia- 

 gram on Plate 66 that a straight line variation will prove satisfactory. 



If we assume that the wedging up has adjusted the cradle so that no bending 

 will occur to localize the pressures until after the after end of cradle has passed the 

 end of groundways, it will be seen that in the after poppets the pressure will be 

 not much different from the mean pressure due to the weight of the vessel.' In 

 that part of the cradle, therefore, the crushing strips can be arranged to give only 

 a slight margin over this mean pressure, say about 25 per cent, to allow for irregu- 

 larities in the strips. 



The crushing strips can be arranged to give uniformly varying pressures be- 

 tween the after end of cradle and the outboard end of the bearing for maximum 

 mean pressures (shown uniform in the diagram) without causing the above deter- 

 mined maximum mean pressure to be exceeded. Between the inboard end of this 

 bearing and the outboard end of the fore poppet bearing, an arrangement of strips 

 to give uniform beginning of crushing would seem satisfactory. 



In order to determine with accuracy the maximum pressures over the way- 

 end when crushing strips are fitted as above outlined, it will be necessary to calcu- 

 late the deflection of the vessel in several positions and plot the pressures corre- 

 sponding to the crushing and spacing of the strips, as indicated in Plate 66, to give the 

 proper reaction and moment. The curve shown in Plate 66 represents the probable 

 maximum pressures which will be experienced if the crushing strips are sufficiently 

 thick to actually cause the sliding way to accommodate itself to the groundway. 



In Plate 67 is shown the distribution of crushing strips which corresponds to the 

 pressures shown on Plate 66. Four rows of 2-inch by 3-inch strips are shown in the 

 fore poppet, one row is shown where the height is limited, and two rows amid- 

 ships and aft where there is sufficient height to fit them. 



One further condition needs to be mentioned. Although the crushing strips 

 do not exert nearly as much pressure in returning to their original thickness as they 

 do in being crushed, the recovery is appreciable, and there will be a considerable 

 pressure exerted between the bottom of the vessel and the sliding way outboard of 

 the end of the groundway. This pressure may be sufficient to break the sliding way 

 if there is an appreciable crushing of the strips, as the sliding way has considerable 

 rigidity. The excessive bending moment on the sliding way may be relieved by 

 cambering the last few feet of the groundway. 



