562 W. J. Marwood and A. Silverleaf 
(c) Appendage resistance 
(d) Effect of changes in scale. 
2. Propulsion 
(a) Components of propulsive efficiency 
(b) Ship-model comparison. 
3. Running Performance 
(a) Effect of spray strips 
(b) Design of a round-bilge form for good seakeeping. 
4. Shallow Water Effects. 
This account of the information available at present is followed by an outline of the 
programme of research now commencing at NPL, and by a description (in the Appendix) of 
the first torsionmeter specially designed at NPL for power measurements on small boats. 
High speed hydrofoil boats are not uncommon, and are being actively developed at 
present. However, the use of hydrofoils to give partial lift to heavy displacement-type high 
speed boats operating at speeds less than the planing speed is not well-established. Some 
preliminary estimates of the value of such devices are given in this paper. 
RESISTANCE OF ROUND-BILGE FORMS 
Normal round-bilge hull forms designed to operate at speed/length ratios between 1.2 
and 3.5 (F, 0.4 to 1.2 approximately) have the following distinctive characteristics: 
1. The afterbody has a flat or convex bottom having a dihedral angle (or rise of bottom) 
up to 45 degrees which runs smoothly into almost vertical sides with a rounded-bilge shape, 
not a sharp-angled chine. 
2. The buttock lines are usually straight and almost parallel to the centre buttock line 
which usually runs aft at 10 to 15 degrees to the waterline. 
3. The stern has a flat or rounded transom, except for low speed/length ratio forms 
which may have a cruiser-type stern. 
4. The bow above water is designed for minimum interference with the bow wave. Any 
chine in the forebody is high in profile so as to reduce the breadth at the deck line, and 
will not extend as far aft as midships. 
High speed round-bilge forms incorporate an unusually large number of design param- 
eters. Besides such basic parameters as beam/draft and length/beam ratios, block and 
prismatic coefficients, features such as depth of transom and angle of rise of afterbuttocks 
may have considerable effects on resistance and propulsion characteristics. 
