In the previous papers, the characteristics of directly measured wave resistance, amplitude functions and wave spectra were discussed in comparison with Michell's theory, and some examinations were made how closely the principle of linear superposition of free waves is realized. In the former part of this paper, using the results described in the previous papers, some practical applications are shown for the estimation of wave resistance of ship of given principal particulars, and a method to determine the optimum principal particulars of high speed ships is established. The latter part of this paper is devoted mainly to the investigations of the effect of bow bulb on wave resistance which are examined by the tests of series models with airship type bulbs. Furthermore, a method for applying the results of the series tests to the hull form improvement is considered, according to the principle of linear superposition of free waves, which is confirmed by the experimental fact.