A Study upon Simplified After Poppet Constructions

Abstract

The fore and after poppet constructions which consist of wooden pillars reinforced by ribbands and rashing wires are customarily used for the launching of ships. The after poppet is considered as an important construction supporting stern weights of the ship and is usually arranged so as to allow stern overhang about 10% of the ship length. The author investigated its mechanism and contrived a simplified construction which directly supports the keel by transversely positioned strong steel girders instead of usual wooden pillars. A typical construction is illustrated in Fig. 1. Up to this time, three ships surely launched by the new method and labour required for constructing launching cradle was satisfactorily reducted The sizes of girders are theoretically determined taking the distribution of ship's weight, rigidity and displacement of cradles into consideration. According to the experimental results at launching, the theoretical prediction is in safe side for girder loads, however, this suggests that large force was acting at usual wooden cradle, especially at the aftermost block, because the sum of the loads is constant. The extent of size the application of this method is, then, discussed from the viewpoint of strength of the aftermost wooden cradle and launching way. Weight and rigidity distribution which provides most severe condition are obtained from many ship's data as shown in Fig. 5∼9. Stresses in the aftermost cradle under such distribution can be estimated by the formula (21), the result of which is shown in Fig. 10. Extremely high stresses of wooden pillars and sliding ways are found from experiment by old materials. Cradle stresses in Fig. 10 are bounded by broken lines obtained from the experiment. From the above investigations it is clear that large ship can be launched by using the new method so far as her length lies within the safe zone shown in Fig. 10. A design manual of the new construction is also presented.