174 THE LONGITUDINAL STRENGTH OF RIGID AIRSHIPS. 
LIEUTENANT CoMMANDER H. E. SAunpeErs (C. C.), U. S. Navy, Member:—I suppose 
I am one of those to whom Commander Land refers when he says that the submarine fellows 
should take a day off and read over Professor Hovgaard’s paper and learn how to build sub- 
marines properly. 
I think with propriety, and without revealing any confidences or secrets, I can say that the 
Bureau of Construction and Repair, especially during the past year, has been making a com- 
plete study of the question of strength of submarines. We started away back in the beginning 
with a clean slate, and we have checked up everything that has developed, everything we knew 
of, and all the experiments that have been made—and we have been able to get a great deal 
of information from Germany, both directly and indirectly, and from a study of the vessels 
taken over after the armistice. In this connection I desire to say that Professor Hovgaard, 
who has done so much work along this line, has been assisting the bureau in the researches 
on the strength of submarines, and I would call the attention of the Society to the fact that 
we are doubly indebted to Professor Hovgaard for the extensive work he is going on with, 
and the Navy Department hopes, after we have progressed to a certain point with this work, 
and if we are able to obtain funds with which to carry out experiments, that we will be per- 
mitted to give out this information for the benefit of all naval architects who can use it. 
Tue PRESIDENT :—If there is no further discussion, Professor Hovgaard will have an 
opportunity to make any replies he cares to. 
Mr. HENry GoLpMARK, Visitor:—Mr. President, I appreciate very much the oppor- 
tunity of saying a word or two on this subject. During the last summer a committee of five 
engineers, of which I have the honor of being the chairman, has been engaged upon the ques- 
tion of the structural strength of rigid dirigible airships, with special reference to the airship 
ZR-1 now being built by the U. S. Navy. I would like to emphasize the fact that Profes- 
sor Hovgaard, a member of the committee, gave us most novel and valuable information 
from the studies he has made on this subject. The structural frame of a dirigible airship is 
very different from any other form of structure. The requirement of great lightness, as well 
as strength, has made it necessary to use a new material. The metal used, duralumin, an 
alloy of aluminum, with 4 per cent of copper and small percentages of magnesium and man- 
ganese, is a very wonderful mixture. It has a tensile strength of fully 55,000 pounds per 
square inch—i. e., as much as soft steel—with an elongation of 18 per cent in 2 inches. The 
compression members which are used are extremely light and, at the same time, have a 
compressive strength comparable, at least, with what we get from steel. 
There is no doubt that in this field, as in all other similar fields, the test of actual service 
is the ultimate criterion. These airships have been constructed since the beginning of the cen- 
tury, but most of them have been built in the last ten years. The experience with them has 
been very largely in Germany, to some extent in England, and the ships under the very severe 
conditions of war service have stood up remarkably well. 
The consequence is that it has been possible to arrive at something in the way of stan- 
dardization and at factors of safety which can be safely used in structures of that kind. 
They are somewhat lower than in structural steels, such as are used in bridges and buildings 
and ship work, but not so very much lower, if we consider the fact that the compression 
members in ordinary structural work are usually strained to about half the actual compres- 
sive strength as shown on full size tests. In airships the factors of safety are lower than 2, 
