The propulsion system of any vessel is considered an important investment; it can increase the ability to compete in the market, improve effectiveness and reduce operational costs.
When optimizing your vessel, it is crucial to look at the complete propulsion line - the engine, shaft, bracket, gear, rudder, propeller etc.
In this blog article, we will have a closer look at the rudder for high speed vessels, and how twisting your rudder can improve effectiveness of your propulsion system.
Challenges with a conventional rudder
The rudder is one of the most heavily strained components on a high speed vessel. Waterflow with a higher velocity and a slight angle over a rudder blade will, under certain conditions, result in cavitation which will increase fuel consumption and cause erosion on the rudder’s surface.
Conventional rudders are placed behind the propeller with the rudder cross section arranged symmetrically on the vertical rudder center plane.
However, this arrangement does not consider the fact that the propeller induces a strong rotational flow impinging on the rudder blade.
This results in areas of low pressure on the blade, inducing cavitation.
In addition, a conventional symmetrical rudder is often thick, thus providing drag as the water passes.
Read more: What's important when selecting propulsion technology
Advantages with twisting your rudder
In general, a twisted rudder can have about 20% higher steering performance than conventional rudders of same size. This is mainly due to the slim and twisted rudder sections, which are design to utilize the propeller slipstream for reduced drag, high steering performance and to raise the propulsion efficiency.
A twisted rudder itself will benefit your propulsion efficiency, however there are other components to consider when improving your rudder and the complete propulsion line.
A twisted rudder designed as an airfoil will provide a lift in the water flow.
Due to the angled flow from the propeller slip stream, the rudder lift force will have a forward component and hence contribute to increase the propeller thrust. This will increase the vessels total propulsion efficiency.
Casting the rudder in one piece in high quality stainless steel material will reduce blade thickness and still secure required strength.
A slim rudder design will reduce the generated drag from the rudder.
To secure the required rudder angle range of +/- 35 degrees, the rudder must be lowered slightly from the hull. Having an exposed rudder stem will provide a drag and generate significant turbulences towards the stern. Adding a rudder fairing between the upper section of the rudder and the hull will secure optimum water flow past the rudder stem, consequently reducing the drag and improve the vessels steering performance.
Read more: Efficient propulsion in a green environment, what are the options?
Conclusion
Optimizing your rudder can lead to reduced drag, higher steering performance, a lift in the water flow contributing to increase the propeller thrust and increase rotational efficiency. A twisted rudder compared with conventional rudders for high speed vessels, indicates about 10-15 % reduced fuel consumption, corresponding to approx. 1,0 – 1,5 knots.
