588 W. J. Marwood and A. Silverleaf 
Design of a Round-Bilge Form for Good Seakeeping 
For a small vessel, less than 100 feet in length, designed to operate at sea, the per- 
formance in waves can be more important than in smooth water. Near-synchronous pitching 
conditions may well be met, and waves no more than 3 feet in height can cause slamming 
decelerations up to 9 g, injurious to both structure and personnel, as recorded at sea [3] and 
observed in model experiments. 
The requirements for a small vessel with good seakeeping qualities include: (a) a dry 
deck, especially forward of the wheelhouse so as not to interfere with navigation, and (b) 
avoidance of slamming, and (c) minimum resistance so that available power is efficiently 
used in maintaining speed in a seaway. Other qualities are also required, but these are not 
discussed here. 
An attempt to design a hull form with these qualities was made some time ago at NPL. 
Initial experiments were carried out with models of hard chine forms similar to those used 
on wartime air-sea rescue launches. However, for a design maximum speed/length ratio of 
3.4, it was soon found that a round-bilge form with a fine bow and a transom stern had a 
superior smooth water resistance. Such a form, designated Model 2084, was designed as 
shown in Fig. 23, and for comparison a hard chine form, designated Model 2117, shown in 
Fig. 24, was also evolved; boats with this hard chine form were known to have had 
Fig. 24. Model 2117, 100 feet x 18.8 feet x 4.80 maximum 
: draft x 100 tons 
to reduce speed considerably in moderate seas. These two forms had the following main 
dimensions: 
