SHIP MOTIONS 



179 



4.1 Towing Tanks and Equipment. Towini;; tanks 

 first were Imilt for the purpose uf nieasuring resistance 

 and propulsion characteristics. Since these are affected 

 by model scale, tanks and models as large as possible 

 were used. Models 16 ft long were considered the small- 

 est acceptable. It was subsequently shown by David- 

 son (1936) that, by using artificial turbulence stimula- 

 tion, satisfactory results also can be obtained in smaller 

 tanks, with models 5 ft long, or even smaller depending on 

 the type of ship. Since then a fairly large number of 

 small towing tanks have been constructed and operated 

 throughout the world. These tanks have been particu- 

 larly active and useful during the recent upsurge of in- 

 terest in the behavior of ships in waves. As early as 1861 , 

 W. Froude showed that wave-making was a primary 

 cause of energy dissipation and damping in rolling of 

 ships, thus indicating that viscous forces are secondary. 

 As a result of all comparisons of test results with calcula- 

 tions based on the potential theory, it appears clear that 

 potential hydrodynamic forces predominate strongly. 

 This being the case, small models can Ix' expected to 

 give as good results as large ones in the investigation of 

 ship motions, certainly as far as heaving and pitching 

 are concerned since these depend primarily on the shape 

 of the hull it.self and not on appentlages. The (jnly 

 direct experimental studies in this connection were made 

 by Szebehely, Bledsoe, and Stefun (1956) and by Ger- 

 ritsma (19576). Their work appears to corroborate 

 the foregoing statements, but is not extensive enough 

 to be conclusive. 



Probably the earliest data on model motions from 

 small tanks were obtained at Newport News Shipbuild- 

 ing and Dry Dock Company. Results were published 

 by Hancock (1948) and Niedermair (1951). Recently 

 a large amount of research in wa\'es has been conducted 

 in a 140-ft towing tank at the David Taylor Model 

 Basin (Todd, 1954), a 100-ft tank at Stevens Institute 

 of Technology (various papers by E. V. Lewis), a 100- 

 ft tank at Massachusetts Institute of Technology 

 (Abkowitz and Paulling, 1953), and a 'iOO-ft tank at the 

 University of California (Pauling, 1955). 



Towing tanks generally can be divided into two classes, 

 according to method used to tow a niodel. One is by 

 carriage and the other by means of a long cord and a 

 falling weight. Recent descriptions of the latter type 

 were given by Todd (1954) and by Abkowitz and PauJl- 

 ing (1953). Model motions in such cases usually are 

 obtained by analysis of a series of photographs. Ab- 

 kowitz (1956b) described a method of recording pitch- 

 ing by using gyros in the model. Tanks using carriages 

 have the advantage of a plane of reference. Motions 

 are recorded either mechanically or more often by using 

 electric pick-ups at the model and recording galvanom- 

 eters on the carriage or on shore. The apparatus is 

 designed to permit freedom of a model in surging, 

 heavitig and pitching, while restraining it in yaw and 

 preventing it from rolling. Descriptions of this type of 

 apparatus have been given by F. H. Todd (1954), 

 Korvin-Krouk(.)vsky (1954), Pauling (1956) and Gcr- 

 ritsma (1957a). 



A self-])ro])ell(>d model may be left completely free to 

 surge, but usually the model has to be towed so that a 

 certain degree of resti'aint is jircsent. In the apparatus 

 described by Todd, K((r\'iii-Kr(nikovsky, and Paulling 

 only a short cf)rd, two pulleys and a weight etjual to 

 model resistance were used, pn)\iding a minimum of 

 inertial or frictional restraint. However, occasionally 

 it is neces.sary to u.se light springs to limit excessive drift- 

 ing of the model due to unbalance between towing weight 

 and resistance. In systems using towing weights at 

 the ends of the tank and a long towing cord, the masses 

 in\'olved are larger and tlie cord connecting them to the 

 model has a certain degree of elasticity. Sibul (1956) 

 and Reiss (1956) have shown that in certain cases par- 

 tial, and particularly elastic, restraint of a model in surg- 

 ing may seriously affect the record of model motion. 

 This happens, however, only at certain model speeds or 

 frequencies of wave encounter. 



Additional measuring instruments also may be in- 

 stalled in the model itself. K. V. Lewis (1954a, 19566) 

 described the apparatus and the procedure for measur- 

 ing bending moments acting on a ship model in waves. 



4.2 Tanks for Tests in Oblique Waves. Until re- 

 cently only tests in head or following seas were possible 

 because of the long and narrow tanks originally designed 

 for resistance testing. During the past 4 years there has 

 been .some activity on the design and construction of 

 towing tanks in which models can be tested in oblique 

 waves. Five such facilities exist or are under con- 

 struction at the time of writing. The first fully equipped 

 one in operation is at Wageningen, Holland (van Lam- 

 meren and Vossers, 1955; van Lammeren, 1957). 

 Seaplane and ship models had been tested earlier in 

 oblique waves in an outdoor pool (Schulz, 1954), but the 

 model guidance means and the recording system used 

 at that time are not believed to have been of sufficient 

 precision to permit vahiable quantitative measure- 

 ments.'* Next such a facility was constructed at the 

 Davidson Laboratory (ETT) of Stevens Institute of 

 Technology (E. V. Lewis, 1956a). Two very large in- 

 stallations are being built at the David Taylor Model 

 Basin (Brownell, 1956; F. M. Todd 1957 NSMB 

 Symp.) and at the Admiralty Experiment Works, 

 Haslar, England. 



4.3 Wave Generation. Many types of wave-making 

 machines have been used in towing tanks with essen- 

 tially equal success. Some of these were described by 

 Kent (1922), Abkowitz and Paulling (1953), Caldwell 

 (1954), Bisel (1954) and Allen (19.57). Waves of un- 

 usually good regularity are produced by wave-makers of 

 the pneumatic type (Todd 19.54; Brownell, Asling and 

 ;\Iarks, 1956; Cerritsma, 1957a). Until recently only 

 (nominally) regular wa\'es were produced, the degree of 

 regularity depending on the design and the condition 

 of the wave-maker, its control system, and the shape of 

 the tank. These often left much to be desired. In- 

 terest in producing irregular waves in towing tanks arose 



'* These have since been improved hut apparently" no new de- 

 scriptions of the facilities have liccii i)ublished. 



