Abstract:
An analysis is made of data on the variation of hull air drag with length/beam ratio and degree of local fairing, and of the maximum beam loading permitted for reasonable hydrodynamic performance. The effect of length/beam ratio on hull structure weight is also very briefly discussed. Considerable reduction of surface-area drag coefficient is shown to be possible by extending the length/beam ratio above that in normal use, keeping the height and waterborne load constant. The length/beam ratio can usefully be defined in terms of the forebody-length/beam ratio. If this be increased from 3.5 to 7 the surface drag coefficient with an unfaired two step hull decreases from 1.6 to 1.35 times that of a body of revolution of the same length and maximum cross sectional area. With a transverse step faired with a 10 : 1 straight fairing in elevation the reduction is from the order of 1.15 to 1.10 and with a step faired in plan-form and elevation from the order of 1.20 to 1.15. The usefulness of the last step form is high because no applied ventilation is required to make it operationally acceptable. Also its drag could be further reduced if applied ventilation were permitted. A similar order of total drag reduction is also possible if the forebody-length/beam ratio be increased so as to keep constant the product of the beam and the square of the forebody length. Under these conditions there is negligible increase of overall surface area, and the water performance for a given water load and hull height is reasonably constant as beam loading is increased. The limit to which the length/beam ratio can be increased will probably be determined by the minimum hull volmne or width required for stowage purposes since this decreases fairly quickly, or seating width in passenger aircraft. Hull weight is probably decreased a little with increase of length/beam ratio when the product of the beam and the square of the forebody length is kept constant. Charts are given to help in the selection of the best hull form for any given design conditions. From these may be estimated dimensions, air drag, and tile maximum beam loading to give a reasonable standard of water performance.
