Untitled - CNR

Marine research at CNRFigure 5: Statistical models at 0° of heel for the drag-force coefficient (CD), lift-forcecoefficient (CL), efficiency coefficient (Eff(CL/CD)) and reaction force (RZ[kg]) withangle of attack (Alpha): comparison between the flume-tank (continuous line) and fullscale(dotted line) obtained on the Cambered vee AR (AR) and Clarck-Y (CY) otterboards.The bold line represents the ratio between the full-scale and the flume-tank test.For RZ[kg], the data of the flume-tank experiment carried out at 2.2 kn (FT2.2) and fullscaleattained at towing speed of 3.2 (ST3.2) and 3.8 kn (ST3.8) have been underlined.times the otterboard tended to be unstable(heterogeneous measurements of doorspread and tensions). Probably for thisreason the drag of the Clarck-Y was verylow compared to the AR door (Figure 4)and hence the higher efficiency (Figure 4).Fine adjustment of the attachment of thechain backstrop brackets, and consequentlyof the angle of attack, carried out just beforethe third cruise proved to be necessaryas the instability disappeared and the doorspread improved (Table 2), conversely insuch conditions, the Clarck-Y provided evidenceof lower performance than AR (Table2). In Figure 4, we can observe asdown-force coefficient C’z is towing speeddependent and, for a given speed, it is similarin the two otterboards. The absolutevalue of C’z ranges between 0.31-0.50 atthe towing speed of 3.8 kn, and it reacheshigher absolute values (0.60-0.93) at 3.2kn. Table 3 shows in details the comparisonbetween the two full-scale doors. Inthis table, for each warp attachment position(HF), estimated values of attack-, heelandpitch-angles and corresponding drag,lift and efficiency coefficients for both thedoors have been summarized. In bothdoors the attack-, heel- and pitch-angles increaseas warp towing point (HF) movedaft. Moreover, the differences in attackangles between consecutives towing points1881