222 OCEANOGRAPHY IX THE UNITED STATES 



the waves from reflecting back into the tank. A rotating bridge spans the basin 

 and permits testing of ship models at all relative headings to the vi^aves. 



Whatever success we may enjoy in reproducing the state of the sea in this 

 tank will be due to the oceanographers who labored long and painstakingly to 

 learn how nature produces waves by wind blowing over the sea surface. 



Consider a stone dropped into still water or a puff of wind on a calm sea sur- 

 face. The waves radiate out from this source of energy. Any one of our 21 wave- 

 makers, as you see, has the same effect. As the wind blows over a wider area 

 and for a long time, the sea builds and produces the waves you see — some short, 

 some long, some hi v. and some low. Long waves overtake short waves and give 

 the surface the appearance of irregularity. In nature, the waves travel in a band 

 of directions giving the illusion of short-crestedness ; that is, one cannot follow 

 along a crest very far before it disappears into a trough. This is accomplished 

 in our tank, by providing individual aiitomatic instructions to each wavemaker 

 so that it operates independently of its neighbors. Often there is swell from a 

 distant storm mixed with locally generated waves. We simulate this by waves 

 produced by the other bank of wavemakers. 



It is believed that this tank has the potential to reproduce the characteristics 

 of almost any kind of wavti conditions observed at sea. As such, it is the only 

 one of its kind in the world and there are no precedents for making waves in it 

 and no techniques for testing ship models. This tenth-scale model affords 

 re.search and development apportunities that would have cost us as much as 2 

 years of experimentation in the prototype, if all such work were to wait until it 

 was built and available. 



Storms and Ships. A Mutual Problem of Naval Architecture 

 AND Oceanography 



(By W. E. Cummins) 



Until recently, it was the practice to design ships, both military and commer- 

 cial, to conform to certain idealized conditions. Model tests were caiTietl out in 

 smooth water, and only rarely were tests performed in waves, completely regular 

 and very unrealistic waves. Structural design took into account the fact that 

 ships would encounter storms, but this was done by assuming the ship to be 

 balanced on a certain hypothetical wave, which might or might not occur in 

 service. AVe must admit that these methods were generally adequate. If they 

 had been unsound, we would have experienced many spectacular failures. Never- 

 theless, in recent years more and more stringent operational requirements are 

 being placed on the ships of the Navy as well as on the merchant marine, and 

 it has become necessary for proper design to relate the ship to its actual environ- 

 ment. For instance, such military opei-ations as searching for enemy sub- 

 marines, launching aircraft, or launching missiles are affected very much by the 

 state of the sea, and the environment becomes a determining factor in the design 

 of the vessel. 



To show you what this environment can be, I would like to project a short 

 film strip. It will tell much more effectively than many thousands (if words, 

 just how violent this environment can be. (There will follow a 3-minute film 

 showing a gmuu of United States cruisers riding out a storm off Cape Horn.) 



Mr. Marks has told you how we are attempting to simulate tliis environment 

 in the laboratory. I will very briefly tell you of certain researcii going on in 

 relation to the ships themselves, in which the environment is taken into account. 



As I mentioned earlier, structural design has been based on a highly idealiztnl 

 calculation of the forces acting on a ship in relation to a certain wave. Before 

 the effect of the true environment can be considered, a better knowledge is re- 

 quired of the forces acting on the ship in waves. The model you see here was 

 const rui-ted for n.se in a research program witii this as its objective. The vessel 

 is of an important coiiiiiKM-cial fyix', tlu> Mariner. It has been cut into .-^even seg- 

 ments, and each scgiiiciil is mounted iiidepeudently on a central girder, or back- 

 b()ne. Tile model lias been instrunieutcd so that it is possible to determine the 

 instantaneous forces acting on each segment a.s it moves through the waves. It 

 is also possible to study vibration induced by the flow around the hull. 



This techni(|ue has proved to be extremely valuable, both for research and 

 for actual design iirohlems. At the bow you see a pair of horizontal tins. These 

 fins are intended to reduce the pitching motions of the vessel, and both model 

 and full scale tests prove them to be effective. However, there are certain unde- 



