Special Rudder
Mariner Fish Tail Rudder
An Increasing Demand for Fish Tail Rudders
A Fish Tail Rudder is a rudder in special shape, having sectional profile of fish-tail-like trailing edge. This rudder remarkably improves ship’s maneuverability and course keeping quality without any propulsive loss, and significantly contributes to safety and economy of ships. Fish Tail Rudders have established high appraisal in Japan, and have already been installed on board more than 3,000 vessels. Recently, maneuverability of large blocky vessels comes into question, and importance of Fish Tail Rudders is steadily rising.
Development of a “Mariner Fish Tail Rudder”
In case of a hanging rudder, rudder stock diameter becomes large for large vessels or high speed vessels. By this reason, development of a Fish Tail Rudder of Mariner type has been demanded. Japan Hamworthy & co., Ltd. succeeded in development of “Mariner Fish Tail Rudders”, in which investigation was given to rudder in terms of performance and construction, under subsidy from the Ship & Ocean Foundation in 1993 and 1994. The “Mariner Fish Tail Rudders” have been installed on board such vessels as a high speed container ship (188m, 24kts), roll-on/roll-off ships, a supply ship of the Defense Agency, 145,000㎥ LNG carrier, etc., and obtained good results in their actual operation.
Model tests were carried out at the Osaka University and the Akashi Ship Model Basin Co., Ltd. related to a 260,000DWT type tanker with a "Mariner Fish Tail Rudder".
Test by 4m Self-propulsive Model Ship
［At Osaka University］
- Prototype of Models
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Item Mariner Fish Tail Rudder Conventional Mariner Type Rudder Spec.of Ship L×B×D×d（m） 313.0×56.6×28.6×19.6 Ship Speed（kt） 14 Propeller φ9.35m×1（FPP） Rudder Size（m） 10.0×7.0 13.8×9.2 Rudder Angle 2×70° 2×35°
- Turning Test
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A “Mariner Fish Tail Rudder” is very excellent in turning performance, compared with a conventional Mariner type rudder.
Particularly, it showed extremely good performance in turning at 65 degrees rudder angle.Locus of Steady Turning Mariner Fish Tail Rudder Conventional Mariner Type Rudder 
- Zigzag Maneuver Test
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In both 10 degrees and 20 degrees zigzag maneuver tests, it was proved that a “Mariner Fish Tail Rudder” shows smaller overshoot angle than a conventional Mariner type rudder, and thus it is excellent in course stability.
Zigzag Maneuver Test at 10° Port 
Zigzag Maneuver Test at 20° Starboard 
Special Features of a “Mariner Fish Tail Rudder”
A “Mariner Fish Tail Rudder” is such that sectional profile of a movable part of a Mariner type rudder is made as that of a Fish Tail Rudder. So to say, it is a Fish Tail Rudder having a rudder horn.（The following Illustration shows sectional profile of a “Mariner Fish Tail Rudder” and a conventional Mariner type rudder.）
Compared with a conventional Mariner type rudder, a "Mariner Fish Tail Rudder" exhibits remarkably high lift coefficient, and at the same time, high drag coefficient when the rudder is turned. Accordingly, it gives excellent maneuverability. On the other hand, there is little difference between both in drag coefficient in a neutral rudder angle zone.
Like a case of a Fish Tail Rudder, it is sufficient for a “Mariner Fish Tail Rudder” to have such rudder dimension as to catch and deflect a propeller slip stream. Accordingly, it is possible to reduce a rudder area. Despite of a reduced rudder area, a ”Mariner Fish Tail Rudder” still has remarkable maneuverability, course keeping quality and yaw quick respond ability.
Mariner Fish Tail Rudder
Conventional Mariner Type Rudder
Independent Test（Wind Tunnel Test）
［At Osaka University］
A “Mariner Fish Tail Rudder” showed very high values both in lift coefficient and drag coefficient, compared with a conventional Mariner type rudder, except smaller drag coefficient in a neutral zone.
- Lift/Drag Coefficient
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Lift/Drag Coefficient 
Resistance and Self-propulsive Test by Large-scale Model ship（7.2m）
［At Akashi Ship Model Basin Co., Ltd.］
Required Propulsive horsepower at the test is as shown in the table, and it was proved that, even in case that a “Mariner Fish Tail Rudder” is adopted, its required propulsive horsepower is nearly equal to that of a conventional Mariner type rudder. Furthermore, an additional test where trailing edge angle of a “Mariner Fish Tail Rudder” is made as 5 degrees (“Ocean Mariner Fish Tail Rudder”) showed that the required propulsive horsepower is less than that of a conventional Mariner type rudder.
- Extract from Test Results for Required Propulsive Horsepower
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Kind of Rudders Mariner Fish Tail Rudder Ocean Mariner Fish Tail Rudder（Trailing Edge Angle 5°） Conventional Mariner Type Rudder Ship Speed 13kt 15,493BHP 15,365BHP 15,660BHP 14kt 19,786 19,560 19,790 15kt 25,067 24,654 24,975
An Example of a “Mariner Fish Tail Rudder” Installed on Board of a Full-scale Vessel
Container Ship "WANHAI 301"
- Owned by Wan Hai Lines Ltd. Built at Setoda Shipyard of Naikai Zosen Corporation
- Overall Length 188.0m
- Normal Speed 21.5kts
