THE " HD-4 " NUTTIJTG. 207 
" drift," that is to say, the ratio of the useful vertical component to the 
head resistance or horizontal component, is 8, whereas the best results 
thus far obtained with the surface hydroplane type, I believe, is 
about 6. In aeroplane work it is higher, but we are not claiming 
perfection for the HD^'t at this early stage of its development. 
The most efficient angle of incidence of the planes was a problem 
that was decided only after a long series of experiments. The highest 
ratio of lift over drift was obtained with the chord of the planes 
inclined at an angle of l-|-°, which is used on the two forward sets. 
For the tail set it was found that the best results were obtained with 
the chord or flat under surface of the plane absolutely horizontal, 
which proves the contention that the under surface is comparatively 
unimportant. 
At present the hydrofoil surfaces of the HD-Jf. are supporting 
approximately 2,000 pounds to the square foot at 60 miles an hour, 
which is 200 times the load carried per square foot of wing area in 
aeroplane practice. What the limit is we do not know, as the subject 
is absolutely unexplored beyond this point. The theoretical limit to 
the lifting eilect of the upper plane would be at the point where an 
absolute vacuum was created above it. Just below the surface of the 
water this would be slightly above 2,000 pounds per square foot, but 
the effect of another atmosphere could be obtained by submerging the 
planes to a depth of 32 feet which would be impractical. 
It takes a thrust of about 2,000 pounds and a speed of about 20 miles 
an hour to get the hull clear of the water, beyond which point the 
thrust required drops to about 1,500 pounds and rises very slowly, 
due principally to the resistance of the air, the resistance of the water 
dropping from 1,900 pounds at 15 miles an hour to 1,300 pounds at 
34 miles an hour and remaining practically constant above this point, 
due to the reefing of the supporting surfaces. 
Since this article was written the writer has received from Mr. 
Baldwin the following interesting data : 
When the main set of hydrofoils is set at an angle of 1J° and the rudder set at 
0° the machine rises at about 20 miles per hour on roughly 40 square feet of 
surface, which equals about 275 i>ounds per square foot. At 40 miles per hour 
about 10 square feet of surface are immersed, which equals 1,100 pounds per 
square foot, and at 60 miles per hour the entire macliine is carried upon about 
4 square feet of surface, or 2,470 pounds per square foot of hj'drofoil im- 
mersed. 
Now that we have explained the theory, let us look over the ma- 
chine itself. The hull of the craft is a torpedo-shaped affair GO feet 
in length, with two outrigger hulls or pontoons each 16 feet in length 
connected to the main hull by a deck. The deck supports the two 
Liberty motors, which are mounted on either side just abaft the cock- 
pit. It is designed in the form of an aerofoil, with flat under surface 
