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3.3 Results and Discussion
The derivatives of hydrodynamic forces are obtained from fitting by polynomials of v'and r'. The derivatives of resistance are shown in Table 2. The static derivatives of lateral force and moment are shown in Table 3. The dynamic derivatives are shown in Table 4. Furthermore, the each vale of terms in formula (11) with respect to the linear derivatives is shown in Table 5. On the other hand, lv(=N'v/Y'v) is representing the point of force due to the oblique motion from the center of gravity in the craft, and lr(=N'r/Y'r-(m'+m'x)) is also representing the point force due to the pure turning motion.D(=lr-lv) is the stability discriminate as shown in Table 6. If D would be positive (D ≥ O) , the craft would have a stable characteristic on course keeping ability. As can be seen, the craft is adequate to the course keeping ability (see Table 6).
Table 2 Derivative of resistance.
| Type |
X'o |
X'w |
X'rr |
X'vr+m'y |
| H |
-0.0163 |
-0.122 |
-0.028 |
0.059 |
| A |
-0.0350 |
-0.180 |
-0.045 |
0.059 |
| C |
-0.0353 |
-0.180 |
-0.045 |
0.059 |
|
Table 3 Static derivatives of lateral force and moment in oblique motions.
| Type |
Y'v |
Y'vvv |
N'v |
N'vvv |
| H |
-0.315 |
-4.318 |
-0.068 |
-0.511 |
| A |
-0.458 |
-0.5271 |
-0.084 |
-0.573 |
| C |
-0.467 |
-5.893 |
-0.086 |
-0.614 |
|
Table 4 Dynamic derivatives of lateral force and moment in turning motions.
| Type |
Y'r |
Y'rrr |
N'r |
N'rrr |
| H |
0.078 |
- |
-0.038 |
- |
| A |
0.061 |
- |
-0.043 |
- |
| C |
0.056 |
- |
-0.045 |
- |
|
Table 5 Effect of outriggers with respect to linear derivatives.
| |
FH |
ΔF |
Δf |
| Y'v |
-0.315 |
-0.143 |
-0.126 |
| Y'r |
0.078 |
-0.017 |
-0.070 |
| N'v |
-0.068 |
-0.016 |
-0.028 |
| N'r |
-0.038 |
-0.005 |
-0.028 |
|
Table 6 Stability discriminate.
| Type |
l'r |
l'v |
D=l'r-l'v |
| H |
0.611 |
0.216 |
0.395 |
| A |
0.543 |
0.183 |
0.360 |
| C |
0.534 |
0.184 |
0.350 |
|
The derivatives of the hydrodynamics forces on a part of outrigger could be discussed in this study As an outrigger is made of bamboo, its hydrodynamics derivative seems to be a little variation for most of fishing craft. In the near future, if the hydrodynamics derivatives of main hull could be built a database, the prediction method would be applied to a great many outrigger craft in the Philippines.
4. NUMERICAL SIMULATIONS
The main hull of the craft is so slender, and the propeller and the rudder are located far distance from the hull as shown in Photo1. It seems to be a little interaction among the hull, the propeller and the rudder. Accordingly, the interactive parameter of rudder (tR) is negligible, and the other interactive parameters are estimated from the published database as shown in Table 7.
Meanwhile, the model of diameter of propeller is 0.150 m and the number of blade is 2. The rudder area is 0.015m2 (AR/Ld=1/21.3) and the aspect ratio is 0.67.
Table 7 Coefficients with respect to propeller and rudder.
| Propeller |
Rudder |
| t 0.100 |
γR 0.80 |
| ωo 0.200 |
aH 0.35 |
| ε 1.000 |
lR -1.00 |
| κ 0.135 |
t R 0.0 |
|
Photo.1 Rudder and propeller of outrigger craft
Photo.2 Outrigger craft in the Philippines
4.1 Accuracy
The accuracy of the prediction method is discussed by comparing the predicted motions with results of free running model tests in calm condition. The trajectories of turning motion of type-A and type-C are shown in Fig. 10 and Fig. 11, and compared with the measured results of free running model tests. The rudder angles are 35 degrees and 20 degrees. The results of the prediction agree well with the results of the measurements made at each rudder angles. The trajectories of type-A and type-C are almost equivalent. Accordingly, the influence of the distance between outriggers is slight in the turning motions.
Fig.10 |
Turning trajectories on the outrigger craft with a 1.36m beam in length(Type-A). |
Fig.11 |
Turning trajectories on the outrigger craft with a 1.76m beam in length (Type-C). |
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