High Speed Displacement-Type Hulls 595 
ratios (say above V//L = 4.0). The hydrofoil craft considered in these comparisons have 
been those in which the complete hull is above the water at the design operating speed, 
only a minimum of foil and support structure remaining immersed. 
It seems worthwhile to consider whether a mixed-type craft has any advantages. In this 
the total weight of the craft and any foil support structure would be balanced partly by 
buoyancy, as in a conventional displacement vessel, and partly by dynamic lift generated 
by hydrofoils below the surface. Such a mixed type craft would thus move through the water 
rather as though it were a displacement vessel operating at a lighter draft than in its deep 
load condition. It seems possible that such a craft might have operating advantages com- 
pared with a fully supported hydrofoil craft, and consequently some preliminary estimates of 
the resistance of such a craft are given here. 
Among the available NPL data for high speed displacement hulls are some results for 
one form at different displacements. These indicate that, at a fixed speed/length ratio the 
displacement/resistance ratio is approximately constant between full and half displacement. 
This enables an estimate to be made of the variation of resistance with displacement over 
this range. For any assumed reasonable value of the lift/drag ratio for a hydrofoil support 
system (which could closely resemble the simple arrangement familiar in ship roll stabil- 
isers), it is then possible to estimate the total resistance of the partly supported mixed- 
type craft. Making a reasonable allowance for the additional weight of the foil support 
structure, it is then possible to compare the total resistances of a series of partly supported 
craft based on a single parent normal displacement-type hull. This simple analysis may be 
expressed thus: 
where r — Resistance of hull remaining in water 
A — Displacement of hull remaining in water 
L, D — Lift, drag respectively of hydrofoil system 
€ — Lift/drag ratio of hydrofoil system 
R — Total resistance of “mixed” craft 
W — Weight of vessel excluding foil and support structure 
S — Weight of hydrofoil and support structure 
k — Weight ratio of hydrofoil and support structure to lift of foil system. 
Figure 29 shows a group of resistance curves derived on this basis from the data obtained 
from a model of a displacement-type craft having a full displacement of 80 tons and length 
100 feet. At each speed/length ratio selected, estimates have been made for L/D = 10, 15, 
and 20 for the hydrofoil unit, and for k = 0.15. Variations in L/D significantly affect the 
result, but reasonable variations in & have little effect. 
The shaded areas in Fig. 29 represent regions in which resistance, and thus power 
reductions are achieved. It is obvious that as L/D increases, so the possible reductions 
also increase. It is also clear that if displacement-type hulls can be designed so that the 
displacement/resistance ratio decreases with the displacement, then significant gains may 
also be achieved; this is now being investigated. 
