Problems of Commercial Hydrofoils 243 
where Lo = lift when travelling at normal waterline 
by = normal wetted span of foil between surface-piercing points 
h = depth of submergence. 
V_ 
< 
& 
x 
~ 
> 
65 
Safe neranatat, 
50 ena 
eee a 
55 
submerged 
foil 
50 
45 
y/ 
V 
40 
35 ol all 
| 
2000 2500 3000 3500 
Motorspeed (r.p.m.) 
Fig. 9. Comparison of the two foil systems on atest boat 
The wing characteristic obtained with a surface-piercing foil in the Saunders-Roe 
towing tank and measurements carried out on the 60-ton Supramar craft (dotted line) is 
presented in Fig. 10. The hump in the curve of the 60-ton boat is produced by the stern- 
heavy trim of the boat at low speed. 
In designing a surface-piercing hydrofoil the value of the characteristic Z can be 
influenced to a certain extent by selection of the dihedral angle, by suitable variation of 
foil chord, foil camber, and angle of incidence in the vicinity of the piercing points. High 
values of Z provide for efficacious stabilization but involve hard riding in a choppy sea. 
Vice versa, reducing the slope of the curve shown in Fig. 10 leads to gentler behavior in a 
seaway but low stability. In order to achieve better riding comfort it is obvious that the 
wing characteristic should be adapted to wave heights and wavelengths; this, however, 
presents difficulties with this type of foil. Nevertheless, trials which have been recently 
undertaken in this respect with a smaller craft and an adjustable front foil of rather simple 
design gave very encouraging results. 
In order to make a vessel with fully-submerged foils stable, variations of lift as a 
function of A must be provided by proper variation of the angle of incidence of the foil or 
