High Speed Displacement-Type Hulls 589 
LWL B on WL Max. d A 
Round bilge 2084 100 16.0 5.48 100 
Hard chine 2117 100 18.8 4.8 100 
Smooth water resistance data for the two forms are given in Fig. 25; the round-bilge 
form behaved well up to 30 knots (V//L = 3.0), and a spray strip fitted as described pre- 
viously helped to keep the hull clear at higher speeds. 
Experiments with 1/15 scale models were also made in head seas. In these each 
model was towed by a bridle held by hand on the towing tank carriage. Hand towing was 
preferred to a rigid attachment to allow slight retardation of the model when its resistance 
increased in passing through a wave. 
Figures 26-28 show extracts from continuous film records taken of both hulls in the 
wave conditions indicated in the figures. These film extracts have been chosen to show 
the vessels in their worst position, low in the wave with spray being thrown upwards or to 
the side. 
The chine form is “stiff” in waves and tends to slam violently. The low chine forward 
throws water forward and up, obscuring wheelhouse vision and producing a wet ship. The 
round bilge form pitches more but this reduces slamming. The flare forward, which was 
designed with particular care, is very effective in throwing water away from the hull. The 
film records clearly show the superior wave-performance of the chineless, round-bilge form. 
Although the behaviour of the hard chine form could be improved by raising the chine line 
forward, it was not possible to reduce its resistance to that of the round-bilge form. It is 
hoped that a vessel having this round-bilge form will shortly be built. 
SHALLOW WATER EFFECTS 
Many builders of small boats do not appear to be aware of the marked effect of depth of 
water on wavemaking resistance. Trials are frequently run in river estuaries over accurately 
measured distances in water between 10 feet and 15 feet deep, which is considered to be 
ample for vessels with drafts of 2 feet to 3 feet. Not surprisingly, often the designed maxi- 
mum speed is not achieved. 
The 51-foot twin screw launch mentioned earlier demonstrates this shallow water effect. 
Its draft was 3.42 feet, and trials were run in about 20 feet of water, for which the critical 
speed (v = VgD ) is about 15 knots. Figure 22 shows that the measured and predicted 
power curves differ most at about 13-1.,/2 knots. Saunders [2] quotes a method for assessing 
the depth of water effect both below and above the critical speed; this is based on data from 
trials of a German destroyer, the only available information for speeds above the critical 
value. Using this method, and taking the depth of water as 20 feet for the 51-foot launch, 
the shallow water effect given in Table 2 has been calculated. 
The considerable differences between the power ratios directly measured for the 51-foot 
launch and those deduced from Saunders’ data are not surprising, since Saunders states that 
other model and full-scale data do not agree with the German results. It is clear that further 
information is necessary, and a series of NPL trials in different depths of water with a 
27-foot launch are being completed. 
646551 O—62——39 
