To try to resolve the H-4's hydrodynamic issues, in 1915 Porte carried out a series of experiments on four H-4s fitted with a variety of modified hulls, using the results of these tests to design a new hull which was fitted to the wings and tail of an H-4, serial number "3580", with a pair of 150 hp (112 kW) Hispano-Suiza 8 engines as the Felixstowe F.1. Rather than the lightweight boat-type structure of the Curtiss boats, the F.1's hull was based around a sturdy wooden box-girder similar to that used in contemporary landplanes, to which were attached a single-step planing bottom and side sponsons. Once modified by the fitting of a further two steps, the new hull proved to give much better take off and landing characteristics and was much more seaworthy.