Rudder performance: how to see the full picture
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Standard rudder performance assessments, including towing tank tests and straight-sailing CFD simulations, evaluate the rudder at its neutral steering angle. This gives a reasonable reading of rudder drag in calm, straight-sailing conditions. What it does not capture is rudder drag or steering performance in typical service conditions, where the rudder is actively working. For that, the neutral angle tells you very little.
This means that the real-world performance difference between a conventional rudder and a twin-foil configuration is structurally invisible to the methods most commonly used to evaluate it. The gap shows up in operation. It does not show up in the tank or simulation.
In service conditions
A sea margin of around 15% extra power is standard practice. It accounts for waves, wind, course corrections and autopilot activity in service. It absorbs the drag cost of rudder use, but it is almost never examined in detail.
Wärtsilä has run full-scale CFD simulations modelling vessel response to rudder steering angles of