The effects of changing scale on ships and semi-submersibles are explored in this paper. In the early part attention is drawn to the wide differences in design practice which exist for large ships with, in particular, a tendency for a lower length to breadth ratio as size is increased. Theoretical reasons for this are given, the principal ones being that this leads to a reduction in steel weight per ton displacement without any large increase in power. It is suggested that this tendency will continue until the weight of the transverse material in the hull approaches that of the longitudinal material. Many other characteristics of the ships improve with lower L/B ratios but manoeuvrability declines and in order to reap the benefit of low L/B designs greater research should be applied to improving this aspect of large ships.

Much greater flexibility in design exists for semi-submersibles since there are no draft restrictions. The paper examines the effects of geometric variations in semi-submersibles and concludes that the stability, dynamic motions and wind and hydrodynamic drag all improve with increasing size but that structural steel weight may not be a significantly lower percentage of the displacement unless design changes are made with changing scale. Attention is drawn to the big improvements in rolling (pitching) performance which would enable large floating platforms to have a very low level of motion. This could have great significance for the siting of petrochemical and other plants at sea.