ANALYSIS OF ROLLING BEHAVIOR IN HEAVY QUARTER-ASTERN-SEAS
Effectiveness of Fin Stabilizers
Fin stabilizers have been known as one of the most effective among anti-rolling control systems, particularly for relatively high-speed ships such as this cruise ship. An example of the effectiveness is shown in Figure 4 (c). The solution was obtained in numerical simulation analysis using rolling motion equations with stabilizer control systems, This result compares encouragingly with observations of rolling motion at that time on the ship.
This large magnitude of outreach gives an increased stabilizing effectiveness as follows:
1. An Increase in Lift Due to Large Aspect Ratio
Lift force generated by the fin is a function of its aspect ratio λ. The aspect ratio for fins on the ship is 4.38 / 1.8 = 2.43, which is considered to be a relatively high magnitude for fins. The fin stabilizer is protruding from a huge ship hull that affects the flow around the fin. Accordingly, with inclusion of the image effect due to the hull, the hydrodynamic aspect ratio is two times the actual geometrical aspect ratio. This increases the value of hydrodynamic aspect ratio to 4.86. This high aspect ratio value gives large lift and stabilizing effect.
2. Large Lift Due to Flap
Each fin has a flap on its trailing edge as shown in Figure 2. The flap is similar to that used on airplane wings during take-off and when greater lift is required. It has been known from theoretical and experimental studies that a wing with flap generates approximately two times greater lift than a regular wing without a flap. In a similar manner, the flap increases lift on the stabilizer's fin and its stabilizing effect.
Quarter Astern Seas and Rolling Behavior at 18 knots and 9 knots
(a) Wave Spectrum
(b) Wave Height
(c) Quarter Astern Sea, 18 knots
(d) Quarter Astern Sea, 9 knots