High Speed Displacement-Type Hulls 577 
20% 
ROUND BILGE BOAT 
CHINE BOAT 
7. INCREASE 
IN. RES. 
le) {1-0 20 
CHANGE OF 9%, 
Fig. 14. Comparison of the effect of B/d on 
round-bilge and hard chine boats 
The available information does not permit any reliable estimate to be made of the effect 
of changing the position of the longitudinal centre of buoyancy (LCB), although this is gen- 
erally considered to be an important parameter. As shown in Fig. 15 two forms, otherwise 
similar, with LCB positions 6.25 percent and 8.81 percent of the length aft of midships had 
almost identical resistance coefficients, possibly because the maximum advantage to be 
gained by moving the LCB aft had been achieved in the first of them. In both forms the for- 
ward waterlines were straight, and by moving the LCB further aft the half angle of entrance 
at the waterline, initially 11 degrees, was reduced to 9-1/2 degrees, probably the practical 
minimum. It is suggested that so long as the LCB is far enough aft to permit straight 
waterlines, there is little to be gained by further movement aft. 
Appendage Resistance 
Appendages for high speed displacement-type hulls generally comprise: shaft brackets, 
either A or / type, propeller shafts, usually angled to the flow, stub bossings at the hull, 
skegs, rudders, either single or twin, bilge keels, and bar keels. 
The separate resistances of these appendages were measured in one case at NPL by 
removing each one in turn from a fully fitted model. The measured model resistances, 
expressed as percentages of the naked hull resistance, were: twin “A” brackets, 7.5%; 
shafts and stub bossings, not measurable; twin rudders (large), 10.5%; bilge keels (large) 
4.0%; bar keels, 5.0%; for a total of 27.0%. 
In accordance with current NPL practice for such appendages, this total was halved in 
estimating the additional full-scale resistance. At the design speed/length ratio 1.35, this 
gave an additional resistance coefficient 5 © of 0.27. For two other models the full- 
scale addition 5 © was 0.21; one of these had a single rudder and the other twin rudders, 
though neither had bilge keels, which are not usually fitted to launches and similar vessels. 
In all three cases the appendage resistance coefficient showed little variation with change 
