The drag coefficients of the baits were derived from experiments that used model baits of different shapes, sizes, and sinking orientations. The experiment with model baits was performed in the flume tank under still water conditions (length of observation window was 21.3 m, width was 8 m, and depth of water was 2.7 m) at the North Sea Centre in Hirtshals, Denmark.
In Norway, bait is available as frozen blocks and must be thawed properly before it is cut. Bait (i.e. mackerel, herring, and saury) is commonly used in 20–40 g portions (Prado, 1990; Bjordal and Løkkeborg, 1996), and the specific gravity of bait is slightly greater than 1.0 (Johnston et al., 1994). In the theoretical analyses, model baits were based on the aforementioned weights and densities and were designed to simulate the shape and density of real baits. As shown in Figure 2, fillet and elliptical baits tested in the theoretical analyses corresponded to frozen and thawed baits used in fishing. The baits used in the experiments were made of wood. Lead was added to model the specific gravity, as can be seen from the data in Table 1. Thin twine was used to attach the sinker (as specified in Table 2) to the model baits, as shown in Figure 3.
The mean sinking speed of the model baits was calculated by measuring the sinking time and water depth. The mean sinking times for different experimental conditions (i.e. bait, sinker, and twine, or sinker and twine) and depths were measured under stationary conditions throughout the sinking process. The flume tank measured height was 2.7 m. Video recordings revealed that in still water the oscillation of a bait with orientation A was minor and could be ignored. The results were based on full-scale tests; thus, similar oscillations may occur in practical conditions.
The drag coefficient of a bait as a function of bait shape, size, and sinking orientation was calculated. The total drag force (bait, sinker, and twine) was calculated using Equation (15), and the drag forces on the sinker and twine were calculated from Equations (16), (17), and (18). The drag force of the bait was calculated by subtracting the drag forces due to the sinker and twine from the total drag force, as shown in Equation (18). Finally, the drag coefficients of the baits were calculated from Equation (19). Equation (15) is valid only when the sinking speed is constant. The resulting set of equations was:
where the subscripts T, S, R, B in the equations denote Total, Sinker, Rope, and Bait respectively, ρw is the density of water, Rt is the total drag force (sinker, twine, and bait), m(S + R + B) is the total mass (sinker, twine, and bait), V(S + R + B) is the total volume (sinker, twine, and bait), R(S + R) is the resistance of the sinker and twine, A(S + R) is the projected area of the sinker and twine, U(S + R) is the sinking speed of the sinker and twine without the bait, C(S + R) is the resulting coefficient for the resistance of the sinker and twine, AP(B) is the projected area of the baits, U(S + R + B) is the sinking speed of the bait, including the sinker and twine, RB is the resistance of the bait and CD(B) is the drag coefficient of the bait, g is the gravitational acceleration.