206 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1919, 
Now look at the planes again. You will notice that they are not 
set horizontally but have a slight lateral angle, or dihedral angle, 
as it is called in the parlance of the aeroplane. It was found in the 
early experiments that when the planes or hydrofoils were arranged 
parallel to the surface of the water, a noticeable irregularity occurred 
when changing speed or when traveling in choppy water, due to the 
effect of the entire plane entering or leaving the water at once. On 
the present machine the lower end of one plane is about on a level 
with the upper end of the plane below it and for this reason the 
reefing process becomes smooth and continuous. Furthermore, it was 
found that the dihedral angle of the hydrofoils greatly increased 
the stability of the machine. 
The hydrofoils are arranged in three sets to give three point sup- 
port like that of an ice boat, which obviates the twisting effect always 
present in a structure supported at four points. The fourth set 
shown at the bow, or “ preventer,” as it is called, is merely to keep 
the bow from diving and to help lift the machine when getting under 
way. At full speed it is entirely clear of the water. 
When looking at the planes for the first time, your impression is 
that they are ridiculously small to support such a large hull. But 
remember that the area of the supporting surface is in inverse pro- 
portion to the density of the medium in which it acts. The specific 
gravity of salt water is nearly 800 times that of air, which means 
that the area of the submerged hydrofoils need be but 34, of the wing 
area of an aeroplane. It means also that the structural difficulties are 
insignificant compared to those encountered in aeroplane design, 
where the designer’s troubles increase as the cube of the dimensions 
of his machine. 
We said before that a surface hydroplane was inefficient in that it 
used only the lower and less important surface of the plane to obtain 
the lift. Few people seem to realize that it is the upper and not the 
lower face of the plane, say of an aeroplane, that does most of the 
work. The results made public recently by the British Advisory 
Committee on Aeronautics show that never, even with simple flat 
planes, does the air impinging on the lower surface exert more than a 
quarter of the total lift. It is the camber of the upper surface, over 
which a partial vacuum is created, that is the important factor. What 
is true of one medium is more or less true of another and, therefore, 
it would seem that a boat depending solely on the lifting effect of the 
water impinging on the sloping bottom is not the ultimate solution 
to the problem of obtaining speed on the water. 
The steel planes of the HD-4 are cambered according to the re- 
sults of countless experiments, in order to take advantage of the 
lifting possibilities of both surfaces. The ratio of “lft” over 
