Answer
Modern rudder design focuses on hydrodynamic lift enhancement, reduction of flow separation/drag, cavitation suppression, and propulsive energy saving.
1. High-Efficiency Flap Rudders (Becker Type)
- Design: Features an articulated trailing edge (flap) connected via a mechanical linkage system to the main rudder blade.
- Operating Principle: When the main rudder turns to an angle (e.g., 35⁰), the flap deflects mechanically to a greater angle (up to 60⁰ relative to the centerline).
- Advantage: Dynamically changes the hydrodynamic camber, generating up to 60% to 70% higher lift side-thrust compared to a conventional rudder without stalling. Provides superior manoeuvrability at low speeds and in restricted waters.
2. Schilling / Fish-Tail Rudders
- Design: A low-aspect-ratio profile featuring a specialized fish-tail trailing edge and flat end-plates fitted at the top and bottom.
- Operating Principle: The flared trailing edge accelerates water flow, delaying flow separation, while the end-plates prevent tip-vortex losses.
- Advantage: Generates high lift over a wide range of angles (up to 40⁰-45⁰) and allows thrust vectoring up to 90⁰, pushing the stern laterally without requiring moving parts.
3. Asymmetric Leading Edge & Twisted Rudders
- Design: The leading edge of the rudder blade is twisted in opposite directions above and below the propeller shaft axis.
- Operating Principle: Matches the incoming rotational flow (swirl) created by the propeller blades.
- Advantage: Reduces rudder-induced drag, prevents localized low-pressure cavitation erosion on the rudder horn/blade, and improves fuel efficiency by 1% to 3%.
4. Integrated Propeller-Rudder Systems & Energy Saving Devices (ESDs)
- Rudder Bulb (Costa Bulb): A streamlined bulb mounted directly on the rudder inline with the propeller hub. It smooths out the flow behind the propeller boss, eliminating hub vortex drag losses.
- Rudder Fins / Thrust Fins: Hydrofoil fins welded onto the rudder bulb or blade that capture rotational energy from the propeller slipstream and convert it into forward thrust.
- Gate Rudders: A revolutionary dual-blade rudder system positioned on either side of the propeller rather than behind it. Acting like a ducted nozzle, it reduces hull resistance and generates positive forward thrust while providing exceptional turning capability.