NOTES ON LAUNCHING. 91 



the mean pressure per foot run on this bearing will be —^- The value of this 



mean pressure is plotted in Plate 63 for two values of C-3.6 inches and 4.5 inches, 

 corresponding to 30 per cent compression on 12 inches and 15 inches depth of 

 crushing strips respectively. A convenient method of determining the distribution 

 of the crushing strips would be to take the maximum pressure as thus determined 

 and lay it off at the after end of the overlap in bearing corresponding to the posi- 

 tion of the vessel where this maximum occurs (17.3 feet aft of forward end of 

 fore poppet for C=4.5 inches from data on Plate 63), and consider it as the pressure 

 at which crushing should just start. At the forward end of the fore poppets lay 

 off a pressure at which crushing should just start if, when the crushing corresponds 

 to the value of C, the pressure is the same as the pressure laid off at the after end 

 of the length in contact. This will be about 70 per cent of the mean pressure if 

 flat grain yellow pine is used and 30 per cent compression is allowed in determining 

 the value of C. When these two spots are joined by a straight line, we have the 

 distribution of pressure at which crushing should just start to give a uniform pres- 

 sure over the length of bearing when account is taken of the crushing. 



It will be found that the pressures at which crushing should start for points 

 aft of this region may be laid off to advantage the same as the highest point just 

 determined, and for a distance aft such that the area of the pressure diagram is 

 equal to the pivoting pressure. The diagram will show the distribution of pres- 

 sures at pivoting, as a greater' pressure at any point would crush the strips, and 

 this cannot occur before there has been a change of angle due to pivoting. 



The curve of maximum pressure per foot run can be obtained by laying out 

 the distribution of pressure for intermediate positions of the vessel up to the jump 

 off the ways, although, as previously mentioned, the excessive pressures on the tip 

 of the fore poppets would be experienced under static conditions only. Such a 

 curve is indicated in Plate 64. This curve shows a certain amount of crushing at 

 points where later there is no crushing or less crushing, and the elasticity of the 

 wood will probably enable the crushing strips to partially accommodate themselves 

 to this condition, provided the percentage of crushing is not too great. 



The pressures obtained by this method would probably be reduced somewhat 

 by the bodily lowering of the vessel due to the crushing, unless the pivoting point 

 assumed in the calculations corresponds to the point of no crushing. 



Plate 68 is from a photograph of the fore poppets of a vessel with crushing 

 strips arranged somewhat as described in the preceding example. The crushing 

 strips seemed to serve their purpose admirably, distributing the reaction over the 

 strap supports and distributing the pressures over the standing ways so that no 

 trouble at all was experienced from the high pivoting pressures. The same 

 arrangement was repeated for a sister vessel with similar success, the material of 

 the crushing strips being white pine, which has about the same crushing curve as 

 yellow pine. It will be noticed that the crushing strips are in four layers, which 

 gives stability to the structure; and there is a further crushing of the division 



