Eighty years ago, a naval architect would have smiled had anyone suggested that it would be a wise idea to try his design on a scale model before translating it into a full-size ship. Times have changed, however, and today no designer in the Bureau of Ships would give his approval to a ship design which had not been model- tested. He wants and expects proof of his paper plans from an exhaustive series of tests performed on an exact miniature version of the future fighting ship.
To enable it to stage these tests, the Navy ten years ago constructed a model basin at Carderock, Md., near Washington D. C. The basin was named for Rear Admiral David Watson Taylor, CC, U.S. Navy (1864- 1940), formerly Chief Constructor of the Navy and considered the first American to realize the vast potentialities of ship model testing. Here, in a cavernous tunnel a half- mile in length, a ship’s design can be accurately evaluated and its performance predicted months before the keel is actually laid. If necessary, alterations can be made to the model, the tests verified, and the results adapted to the ship.
The Midway class aircraft carrier presents a case in point. The Model Basin ran nearly a dozen separate tests on the Midway model. Seven propeller combinations alone were tried before the best was selected. Some propellers were three-bladed, some four-bladed, some even five-bladed.
From these tests, which, incidentally, were completed in the record time of eleven days from the date that President Roosevelt signed the authorization, it was possible for the Model Basin to present the design section of the Bureau of Ships with a fully documented evaluation of its future carriers. The stern of the Midway today shows the results of these tests. It is primarily of Model Basin design.
This dependence of the naval architect on model testing to insure him accurate results serves to call attention to the great strides made in this field in this country in the past half-century. It was scarcely fifty years ago that the United States built its first test basin—an experimental one—at the Washington Navy Yard.
It was only thirty years prior to that that a talented Englishman, William Eroude, laid the groundwork for all ship model testing by formulating his famous Law of Comparison. This law states roughly that the performance of a full-scale ship can be accurately "predicted from the performance of a scale model if the model is built to exactly proportionate size and if it is towed at a certain corresponding speed. Eroude substantiated his law by making a comparison of the results of tests performed with a small model in a tank in his laboratory and similar tests made aboard a British man-of-war, the H.M.S. Greyhound, which he got the Admiralty to tow in the open sea for purposes of the test.
Rear Admiral Taylor acknowledges our debt to Froude. In his book, The Speed and Power of Ships, he states, “Present day ideas of the resistance and propulsion of ships have been derived almost in toto from the theories and methods evolved by the elder Eroude dating back to 1870. By judicious application of the Law of Comparison, we are able to estimate with fair accuracy the resistance of a full-size ship from the experimentally determined resistance of a small model of the same.”
It is interesting to note in this connection that a famous American, Benjamin Franklin, also took an interest in ship model testing. Riding one day in a canal boat in Holland, Franklin observed that the horses had to pull harder than usual when the vessel passed over the shallow sill of a canal lock. Franklin’s scientific curiosity was aroused. He reasoned that the shallowness of the water must have something to do with it. Therefore, on his return to America, he built a crude wooden trough, fashioned a model boat, fastened it to a string which he passed over a wheel with a weight on the end, and worked out calculations which confirmed his belief. In so doing, he discovered an important principle in hydrodynamics: that the depth of the water has distinct effect upon the resistance of a ship.
In 1907, as a result of experiments based on the principles laid down by Froude, the Washington Basin recommended the first basic change in the design of a U. S. ship to be derived from model predictions. This was the bulb-shaped bow which was incorporated into the design of the battleship Delaware. This bulb bow has proved so successful that it has been continued as a design feature on most large American naval vessels.
Two World Wars and the growth of the United States into the foremost naval power served as potent spurs to the spread of ship model testing facilities across the country. A number of universities and research institutions now have their own model basins. In 1940, the Navy Department itself undertook to construct the David Taylor Model Basin, the most completely equipped model testing and experiment plant in the world. The Model Basin covers some 200 acres of land in the valley of the Potomac River, twelve miles from the center of the capital.
To gather material for this article, the writer spent several days among the white buildings and in the great tunnel of the Model Basin talking with the key engineers, designers, physicists, and technicians, who do their daily work at what a former director of the facility liked to call the “Mayo Clinic for ships.”
Last year marked the tenth anniversary of the Model Basin. No one can say those years haven’t been eventful. Scarcely had it been completed late in 1940 when World War II with its many complex problems requiring immediate solutions broke upon it. When the war finally ended, Basin scientists could for the first time turn their attention to the great number of problems of pure research that had been by-passed during the war. Today, however, with rearmament once more upon the country, the emphasis at the Model Basin is shifting again from the theoretical to the practical.
During these ten years, the Model Basin has progressed well beyond the original concept laid down by Congress—that it should “conduct the work of investigating and determining the most suitable and desirable shapes and forms to be adopted for U. S. vessels, and the investigation of other problems of ship design.”
The Model Basin has not only fulfilled this primary mission of investigating new shapes and forms for ships but has also extended its investigation into the problems posed by the form or shape of all bodies which can be propelled, towed, or projected on or through the water—paravanes, mines, torpedoes, ahead-thrown weapons, bombs, depth charges and periscopes—all of which are tested in the dim interior of the mammoth basin.
In addition, examples are prolific of instances in which engineers at the Carderock basin provided solutions to “other problems of ship design.” Early in the war, to cite one, the Navy was faced with the question: Could an Essex-class carrier safely negotiate the locks of the Panama Canal? The necessary measurements were known—the carrier’s beam and the width of the locks. By simple arithmetic, the carrier should have been able to squeeze through with a clearance of several inches as it had been built to do.
But there was a complicating factor. Due to its lopsided construction, with the flight deck overhanging on the port side, the carrier would have to be maneuvered into the lock in an off-center position. Some said that this off-center position would create a suction or sideways creep powerful enough to pull the ship into the side of the lock, damaging the carrier and possibly plugging the lock.
This was something the ship’s designers hadn’t bargained for, and an exact answer was necessary. To provide it, engineers had ready within a few days an exact replica of the canal locks. At the same time, the woodworking shop had produced a scale model of the hull of an Essex-class. Technicians had adapted instruments to measure the pull exerted upon the model as it passed through the miniature locks.
Instruments were mounted on the “carriage.” This carriage is virtually the heart of the basin’s operations and looks like a spiderweb of silvery pipes spanning the water. The model was trundled out to the basin on a dolly, removed and attached beneath the carriage. Technicians climbed aboard the carriage and it moved slowly forward. Beneath the carriage, the model cut the water smoothly, then passed through the lock. As it did, the moments of force and the drag were measured by delicate dynamometers.
Two days after the test, Captain Harold E. Saunders, CC, U.S. Navy, the officer who was primarily responsible for planning and laying out the Model Basin and who was then serving as its first technical director, called the Bureau of Ships and said, “All right, you can take her through.” The complete report was rushed by special messenger to Washington. There it was put aboard a fast plane and delivered to the captain of the carrier for his information. Result: a few minor alterations were made to the locks and the ship went through—using hawsers instead of the customary fenders.
Another example is to be found in the launching of the battleship Alabama. The Alabama was constructed at Norfolk and was to be launched into the narrow Elizabeth River. To be certain that enough preventive gear would be rigged to keep the ship from going aground on the opposite shore, shipyard authorities requested the Model Basin to make an estimate of the situation.
After a full-dress rehearsal of the proposed launching, using a model of the Alabama, a scaled-down way complete with scaffolding, underpinning, keel blocks—and water—the Basin predicted how far the momentum of the battleship would carry her before drag chains would halt her forward motion. Altogether, it was the largest scale model launching ever performed. Moreover, the estimate was correct to within a dozen feet.
But although predictions such as these are important, they do not constitute the Basin’s primary task which is the testing of proposed ship shapes.
In making these tests, engineers are looking for two basic facts: how much resistance the hull will offer to the water, and how much power it will require to push the craft through the water at the desired speed. Through such test voyages, engineers have been consistently adding speed, cutting horsepower, and saving the taxpayer millions.
Resistance is the keystone to the problem of the ship design. Resistance does not show on blueprints. Moreover, it costs energy. Lower a ship’s resistance and you increase its speed or enable it to carry a bigger pay-load or to improve its maneuvering characteristics or to reduce its horsepower, or a combination of these. Increase a ship’s resistance and you decrease these favorable factors.
In determining the power needed to drive a ship forward at a given speed, two types of resistance must be considered-—frictional resistance and pressure resistance. Frictional resistance is that induced between a hull’s wetted surface and the passing water. It accounts for most of the total resistance at low speeds. Pressure resistance, which consists in turn of wave-making resistance, eddy-making resistance and cavitation, becomes highly important at high speeds. Pressure resistance is markedly affected by the hull contour.
In a typical test to determine the resistance forces acting upon a ship, the model is set afloat in the basin, the smell of pine and glue still strong upon it. The sleek-looking model is an exact replica of the proposed ship except that it has no superstructure. Instead, the hull is hollowed out so that instruments and propulsion machinery can be inserted.
The first test is the “bare hull” test. The model is hitched beneath the carriage by means of a balancing arm which enables signals to be transmitted from the instruments in the model to the carriage but which adds no weight to the model itself. As the model is towed down the length of the basin, the resistance being offered to the motion of the model is noted by the instruments, whose signals are then recorded by a stylus which marks on a drum mounted on the carriage. Lines of flow of the water along the hull sides can also be determined at the same time by placing bits of dye in small holes drilled in the bow. As the hull is towed, the dye is drawn from the holes and streaks sternward, leaving tell-tale stream lines on the hull.
These stream lines tell the model men where to place hull appendages such as bilge keels, skegs, and rudder assembly. Since such appendages also have their effect upon the smooth flow of water past the hull, a separate “appendage test” must then be run.
For the third basic test, the power test, the model is fitted with propellers, a rudder and a small electric motor which simulates the ship’s main propulsion machinery. Each propeller is precision-finished by a skilled workman until it is the precise pitch required. Sometimes as many as a dozen propellers, each of different pitch, are tried on a single model.
This time the model is allowed to run free, propelled by its own power. The carriage follows along above the model recording the usual signals. From the power test, bare hull test, and appendage test, figures are collected and interpreted and used to verify or alter the proposed design of the ship.
Here’s a description of one of these ship characteristic tests, the turning test. The model is equipped as for the power test with propellers, a motor, and a rudder. In addition, two small, blinking lights are installed, one fore the other aft, on the model. All lights in the basin are then turned off.
In the dark, the model propels itself down the straightaway and into a J-shaped turn. As it reaches the turn at a predetermined speed, an operator kicks over the rudder to a certain angle and the model commences to swing. As it does, a long jointed steel arm glides out from the carriage, carrying with it electric lines to the dynamometers in the model. High above the basin on a catwalk a camera exposes its plate to the blinks of light from the model. These light pulses describe a pattern which experts will analyze to predict the ship’s prospective transfer, advance, and time to turn at various speeds and rudder angles.
Other tests are as ingenious. In the artificial wave test, the model is buffeted with man-made waves to determine its seaworthiness. In the circulating water channel test, the model is held stationary while gallons of water flow past it at speeds up to ten knots and cameras peer through windows in the tank’s bottom and sides to record the movement of small “tufts” of cloth attached to the model’s sides. In the wind resistance test, the model is analyzed in a wind tunnel.
Ship captains also reap a rich harvest from other Model Basin activities, some carried out within the Basin, others in the field. These auxiliary tests include making gun- blast measurements on combatant ships, measuring the bending of a ship’s structure in waves, measuring the pressures and impulses due to underwater explosions, plotting the behavior of supports and fairings for submarine periscopes, noting the relative performance of rudders of various designs, observing the motion of ships in launching, studying air direction and velocity over a flight deck, testing the effect of deep submergence on a submarine and studying the vibration characteristics of a ship.
The importance of the last-mentioned, vibration studies, was illustrated during the war in the case of the battleships North Carolina and Washington. It is well known that when these ships were launched they were found to vibrate so badly at high speeds that their main batteries could not be fired with accuracy. It is probably less well known that Model Basin engineers, hurriedly called in to diagnose the trouble, recommended certain changes in the size and number of the propeller blades, and the vibration was drastically reduced.
Advances in theory, begun in earnest by William Froude (continued after his death by a son, R. E. Froude) and added to by hydrodynamic specialists in England, The Netherlands, Germany and France especially, have now laid the foundation for the big steps forward in ship performance data and the pre-determination of ship shapes and shipboard power plants.
This advance in theory has been matched by a similar advance in technology. New testing devices such as the dynamometer— most of them electric or electronic—enable research men to make rapid, simple, and exact measurements of the forces which act upon a ship model. Model Basin scientists themselves have developed many of these devices as well as methods to fit the devices. With their precision carriage running down its perfectly smooth track, Model Basin engineers can measure forces accurately to .01 pounds. This is an improvement of tenfold over the old Washington tank.
Mobilizing these advances in theory and improvement in technique, the Basin can, at a relatively small cost, furnish a ship designer with a prediction of the performance of his proposed vessel. It is frequently possible for the designer in turn to make changes in the plans for his vessel which will greatly improve the ship’s performance, and for the basin to confirm the effect of these changes by testing them on the model—all this before the actual construction of the vessel.
If radical new ship shapes are just around the corner and if a great advance into the realm of nuclear power is on the horizon, as some authorities suggest, much of the basic research that must inevitably precede such developments will be carried forward by establishments like the David Taylor Model Basin.