SHIP MOTIONS 



209 



locating the stabilizing tanks high in the shi])'s stiiic- 

 ture. Necessaiy damage stability may be secured at 

 the same time by (luickly draining the tanks in emer- 

 gency. 



The need for roll stal)ilization is increased by demands 

 for damage stability. Competitive commercial con- 

 siderations should further increase the use of stabilizing 

 devices (at present activated fins) on passenger ships. 

 Naval architects, shipowners and ship operators may 

 become accustomed to these added complications. It 

 is possible that consideration will be gi\-en to the sta- 

 bilization of cargo ships. Activated tanks (located 

 high) may prove to be the best choice for these appli- 

 cations. 



6.2 Pitching Stabilization. Pitciiing motion of 



ships also can be I'educed by the use of fins. The mo- 

 tions are reduced to a lesser extent than in rolling be- 

 cause damping in pitching and heaving is already large. 

 Nevertheless, theoretical and model research indicate 

 that a significant reduction of the amplitude of pitching 

 can be achie\-ed. 



Quoting from Abkowitz (lU.jo): "Aiitipitchiug fins of 

 hydrofoil shape located at the bow have been investigated 

 at the MIT towing tank. Models of three different type 

 vessels (a) a modern merchant form, (6) an aircraft 

 carrier, and (c) a destroyer, with and without anti- 

 pitching fins, were tested for seakeeping in a series of 

 wave lengths wherein the ratio of wa\'e height to wa\-e 

 length was held constant at Jao- The hydrofoil was 

 located at the bow at the keel depth antl measiu'ed 4 in. 

 along the chord. The models were of the following 

 lengths : 



Model 1 Series 60, block 0.60, ft 5 



Model 2 aircraft carrier, ft 6.25 



Model 3 destroyer, ft 5.5 



"A constant towing force was used in the series of 

 waves and represents approximately the ship at constant 

 EHP At resonance, the pitch angle (double ampli- 

 tude) with hydrofoil is reduced to about }s its value 

 without hj'drofoils for the merchant vessel, to about 3'^ 

 for the aircraft carrier and destroyer. The speed loss 

 is also reduced appreciably for the merchant ship with 

 antipitching fins. The heave is also reduced for all 

 three ship types." 



Abkowitz (1955) presented curves of the data for 

 three ships. The data for Series 60 are reproduced here 

 in Fig. 54. Abkowitz (1957a) furthermore plotted 

 curves showing a good agreement between theoretically 

 computed and measured data for Series 60 with and 

 without fins. 



Pournaras (1956) also presented towing-tank data 

 (obtained at DTMB) for a Series 60 hull with and with- 

 out fins. The model was 10 ft long and was tested in 

 waves 2.5 in. high at X/L ratios of 0.75, 1, 1.25, and 1.5. 

 Tests also were made in these wave lengths at a constant 

 wave length-to-height ratio of 30. A large reduction 

 of pitching motion was demo]istrated. Quoting a part 

 of the summary : 



"The pitch retluction attributable to the fins consid- 

 erably improvctl chyness of the model in head seas. The 

 practical speed range, as restricted by motions of un- 

 desirable magnitude, is also extended. Forefoot and 

 forebody emergence occurring during the test without 

 fins were not observed when the fins were installed." 



The fins just discu.'^sed were fixed. In an unpublished 



theoretical study, E. V. Lewis and W. R. Jacc 



lund 



that no advantage can be expected from controlling bow 

 fins. This is because of the vectorial addition of water 

 orbital velocity, bow vertical velocity and ship forward 

 \-elocity. This addition indicates that fin angles of 

 attack approach the stalling angle. f)n the other hand, 

 a much smaller variation of the relative direction of 

 water flow is foimd at the stern. Therefore, fixed fins 

 at the stern are less effective, but their usefulness can be 

 increased by controlled mi)\-ements. Spens (1958) 

 presented an ach'ance discussi(.)n on experiments with 

 oscillating stern fins. 



The fact that the stalling angle of bow fins is ap- 

 proached in waves indicates the need for certain pr'ecau- 

 tions in model testing and in fin design. Eddies shed as a 

 consec[uence of fin stall may cause fin and hull \ibrations. 

 Since the hull areas at the bow are predominantly ver- 

 tical, the vibratory forces can be exjiected to act in a 

 horizontal direction. Presumalily they can be disclosed 

 l)y placing an accelerometer, oriented to record lateral 

 accelerations, at the bow. It is essential that these tests 

 be conducted in severe sea conditions, as mild conditions 

 may not disclose a potential danger. It is known that 

 stalling occurs at a smaller angle of attack at low Re_v- 

 nolds numbers than at high ones. Therefore, models 

 may indicate lesser effectiveness than can be expected 

 on a full scale. Earlier stalling of model fins may be 

 caused by a laminar separation-^ followed by less violent 

 eddy-making. It may be desii-able, therefore, to ecjuip 

 the models with antistalling devices which are not in- 

 tended for full-size use. Such measures are employed, 

 for instance, in towing-tank tests of seaplane models. A 

 model wing erjuipped with an antistalling leading-edge 

 slot has been found to represent the action of an un- 

 slotted full-size wing. Abkowitz (1958) achieved good 

 results by applying suction on a small stabilizing fin 

 model. This practice can be recommened for small skegs 

 and rudders as well. 



In the case of controllable stern fins, theoretical and 

 model research is needed in order to determine the op- 

 timum phasing of fin motions with respect to ship 

 motions. 



Attention also should be given to the shape of ap- 

 pendages in order to increase their fin action. Ab- 

 kowitz (1955) indicated that bossings may be significant 

 in reducing pitching. Kort nozzles may not only im- 

 prove the pro]3ulsion in heavy weather conditions, but 

 may also reduce ship motions. 



" Davidson Laboratorj', Stevens Institute of Tecluiology. 



'* Spens (1958) indicated that this separation, known to occur 

 in a steady fluid flow, may not necessarily occur in the case of an 

 oscillating hydrofoil. 



