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Fig.4 10deg/10deg Zigzag Manoeuvre of Ships for Simulation
2.2 The overshoot angle in modified Z manoeuvre
The overshoot angles in modified zigzag manoeuvres by the simulations are shown for every loop width of three ships in Fig.5-7. The combination of helm angles and heading ones are 10deg/5deg, 10deg/1deg, 20deg/10deg, 20deg/5deg and 20deg/1deg. In addition, 1st overshoot angle and 2nd overshoot angle in 10deg/10deg Z manoeuvre is respectively shown in Fig.5 and in Fig.6. Moerover, 1st overshoot angle in 20deg/20deg Z manoeuvre is shown in Fig.7. All Z manoeuvre carried out using helm angle to starboard side at beginning.
It is shown from these figures that the overshoot angles increase in proportion to increasing the loop width. It is moreover clear that the overshoot angles in 10deg/10deg and 20deg/20deg Z manoeuvre are more remarkable in the difference and the value for loop width than in modified zigzag manoeuvre. It is therefore concluded that the overshoot angles in 10deg/10deg and 20deg/20deg Z manoeuvre are the better items to evaluate clearly the difference of the course keeping and yaw-checking ability.
Fig.5 |
1st Overshoot Angle in Modified Z Manoeuvr by 10deg Helm Angle |
Fig.6 |
2nd Overshoot Angle in Modified Z Manoeuvr by 10deg Helm Angle |
Fig.7 |
1st Overshoot Angle in Modified Z Manoeuvr by 20deg Helm Angle |
2.3 The effect of rudder area to course keeping and yaw-checking ability
When a ship cannot comply with the resolution A.751(18) or MSC137(76), a method modifying the performance of course keeping ability is to increase the ruder area. Since ship A has a wide loop width and doesn't comply with the resolution, her rudder area is purely increased from the prototype and 10deg/10deg and 20deg/20deg Z manoeuvre are simulated to find the modification of the overshoot angles.
The results are shown for rudder area in Fig.8-10. It is found from the figures that the overshoot angles decrease in proportion to the increase of the rudder area and that ship A can comply with the resolution A751(18) when rudder area is increased 20% from prototype. Then she has 6deg in loop width. It is moreover found that the criterion of 2nd overshoot angle in 10deg/10deg Z manoeuvre is the severest in the criteria of course keeping ability. Accordingly ship A can comply with the resolution MSC137(76) by about 15% increase of rudder area. In that case, her loop width is about 7deg.
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