The effect of bilge keel on the roll motion of a naval destroyer

Abstract

In this master's thesis, ship roll damping systems have been examined, and the effectiveness of bilge keel as a roll damping system has been parametrically evaluated using the DTMB 5415 high speed displacement hull form. Roll motions pose a range of problems that can adversely affect the performance of marine vessels. These motions can cause damage to the ship's cargo, decrease crew efficiency, and negatively impact weapon systems on warships. Additionally, roll motions can create issues during aircraft and helicopter landings and takeoffs on warships and reduce the accuracy of weapon systems. Systems used to control roll motions are generally categorized into two main types: active and passive roll damping systems. These systems aim to reduce roll motions through various methods. The roll damping measures discussed are as follows: Passive roll damping systems include bilge keels and passive roll reduction tanks. Bilge keels are a traditional, cost-effective method that effectively dampens roll motions. They are simple to manufacture and are often combined with active systems on warships. Passive roll reduction tanks, on the other hand, achieve roll damping by balancing the movement of liquids within the vessel. Active roll damping systems feature gyro stabilizers, active roll reduction tanks, fin stabilizers, and rudder-controlled stabilizers. Gyro stabilizers are highly effective at low speeds and during anchorage, as they operate within a fully enclosed box, protecting them from external damage. This design allows for safe navigation in shallow waters. Active roll reduction tanks actively balance the ship's motion, while fin stabilizers mitigate rolling caused by waves or external forces. These stabilizers use sensors and control systems to dynamically adjust their fins, ensuring effective roll damping. Fin stabilizers must remain submerged during ship inclination, be located in protected areas of the hull, and minimize water flow disturbance. To address collision risks, foldable fins have been developed, allowing retraction when not in use. Despite their high cost, fin stabilizers are highly efficient, particularly in challenging sea conditions, and are widely used on various vessels, including yachts, cruise ships, warships, and cargo ships. Additionally, they must be installed in a manner that does not disturb the water flow. Active fin stabilizer systems, being mounted on the exterior of the ship, pose a collision risk; thus, foldable fins have been developed. These foldable fins can be retracted into the vessel when not in use, thereby minimizing additional resistance to the ship. Fin stabilizers are particularly effective in challenging conditions such as head seas and cross seas and are considered the most efficient equipment for roll damping. Advanced fin stabilizers are now produced for a wide range of vessels, from yachts and cruise ships to warships and commercial cargo ships. Despite their high cost, these systems are preferred for their effective performance. Rudder roll stabilizers use the ship's rudder to balance roll motions. However, they are generally not effective at low speeds and while the ship is at anchor. Rudder-controlled roll stabilization systems are advantageous because they do not add extra weight or volume to the ship. The history of roll damping systems began with William Froude's free surface tanks and Frahm's U-type passive roll tanks. The use of bilge keels extends back to the early 20th century, with fin systems beginning to be employed in the 1930s. In the subsequent sections of the study, parametric analyses have been conducted using different bilge keel geometries on the 5415 model bare hull to investigate the effect of bilge keels on roll damping. Prior to these parametric analyses, a validation was performed by comparing experimental roll damping results of the 5415 model bare hull with results from AQWA© software. This comparison yielded highly favorable results. Ikeda's viscous roll damping method has been thoroughly examined to account for viscous effects in validation and parametric analyses. The Ikeda method has been used to detail and examine the critical roll damping method and calculation techniques. When viscous effects are incorporated into the AQWA© program, the roll values for the structure are found to be very close to the expected values by experiments. Following the completion of the validation work, parametric analyses were performed using AQWA© software with a total of six different bilge keels, utilizing the bilge keels of the DTMB 5415 model ship. In these parametric analyses, the angle, width, and length of the bilge keels were systematically varied, and the model ship was analyzed with these bilge keels at different speeds. The analyses concluded that examining parameters such as the width, length, and angle of the bilge keels individually would be insufficient; instead, all relevant parameters should be considered together in the analysis.