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2.4 External Force
As for external force, the wind force is considered in this study XE,YE aud NE are expressed as follows:
where ρa : density of air. Ax : transverse projected area above-water hull. AY : longitudinal projected area above-water hull. WT : true wind velocity. χT : true wind direction and Ψ : heading of ship. Cx(χa), Cγ(χa) and CN(χa) are the coefficients of wind forces and moments, obtained from a wind tunnel test or estimated from the databases of Isherwood (1970)7) or Tsuji (1971)8).
3. INFLUENCE OF OUTRIGERS
As mentioned above, outriggers are so slender and these are located a far distance from the hull to maintain the transverse stability. It seems to be a very little interacted force between the hull and outriggers in maneuvering motion. On this assumption, the derivatives of the hydrodynamics forces due to outriggers could be described as follows:
where FH: effect of the hull, ΔF: effect of the outriggers and Δf: effect of the distance between outriggers.
Circular Motion Test (CMT) was conducted to investigate the effect on these terms in the National Research Institute of Fisheries Engineering (NRIFE), using a 1/4 scale model as shown in Fig. 2. Type-H is a hull without outriggers. Type-A is a standard type in the Philippines, and distance between outriggers is about half ship's length. Type-C is a wide model to examine the effect of the distance between outriggers. The model arrangements of the craft are shown in Fig. 3, and its principal dimensions are shown in Table 1.
Table 1 Principal dimensions
| Items |
Ship |
Model |
| Scale |
1 |
1/4 |
| Lpp (m) |
10.940 |
2.735 |
| BH (m) |
1.260 |
0.315 |
| dm (m) |
0.468 |
0.117 |
| W (kg) |
3702.4 |
57.850 |
 G (m) |
aft0.196 |
aft0.049 |
| Kyy/Lpp |
0.242 |
0.242 |
 |
0.298
(6.0knots) |
0.298
(1.543m/s) |
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Fig.2 Body plan of the outrigger craft.
Fig.3 Schematic illustration of model arrangements.
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