92 NOTES ON LAUNCHING. 



boards where the crushing strips bear, which adds to the adaptabiUty of the design 

 to distribute the pressures. 



Further refinements may be made in the calculations, if desired, to obtain full 

 advantage of this method of distributing the pressures. On the other hand, the 

 crushing strips are able to stand almost any amount of overloading without disin- 

 tegrating, and their use cannot fail to help distribute the pressures, even if fitted 

 without reference to any calculations at all. 



WAY-END PRESSURE. 



The minimum moment against tipping is primarily the measure of safety 

 against the lifting of the fore poppets from the ways and the bodily tipping of the 

 vessel backwards as the center of gravity of the vessel passes the end of the ground- 

 ways. Many of the smaller vessels have "tipped" in launching because it was con- 

 sidered too expensive to extend the ways a suitable distance under water. Usually, 

 no damage has resulted from this practice, as the pressures have been compara- 

 tively small, and the ways were so supported as to "give" appreciably during the 

 tipping. 



On larger vessels, however, even with a considerable moment against tipping, 

 damage to the floors under the flat of the bottom has resulted, caused by the con- 

 centration of pressure over the outboard end of the groundways as the part of the 

 vessel which was damaged passed over that spot. This concentration of pressure 

 is not indicated directly by the calculations as usually made. Special calculations 

 are often made, however, in an endeavor to determine these pressures. In these 

 calculations, it is assumed that the vessel and ways do not change shape and that 

 the distribution of reaction on the ways can be represented by the ordinates of a 

 trapezoid, having a length equal to the overlap of the ways, an area equal to the 

 total reaction corresponding to the particular overlap selected, and with the center 

 of gravity of the trapezoid at the center of gravity of the reaction. If the center of 

 gravity is nearer the end of the groundways than one-third of the overlap, the 

 trapezoid becomes a triangle whose base is three times the distance of the center of 

 gravity from the end of the groundways, and there is assumed to be no pressure at 

 all at the forward end of the fore poppets. This method of determining the pres- 

 sures under the bottom of a vessel was used in preparing for the launch of the 

 Lusitania, was described in volume 12 and was illustrated in volume 22 of the Trans- 

 actions of this Society. 



The method of determining pressures under the bottom, above described, is 

 based on an assumption which can hardly be realized in practice. The condition 

 which controls would seem to be that the vessel does not remain straight, but yields 

 to the bending forces which come into play during the launching. The natural im- 

 pression which one gets of a large vessel afloat is that there is no longitudinal bend- 

 ing. Both theory and actual measurements, however, show that vessels are flex- 

 ible beams, and records of measurements are given in our Transactions. In the 



