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Propeller Optimization

Objective Function​

In marine propeller optimization, objective functions can vary widely, including

  • maximizing propeller efficiency,
  • minimizing cavitation,
  • reducing noise,
  • maximizing acceleration,
  • optimizing top speed or
  • minimizing stress in critical areas.

The specific objective depends on the application. Often, the optimization is approached as a multi-objective problem, requiring the use of a Pareto front, which adds complexity. A common approach is to focus on maximizing propeller efficiency while imposing certain constraints.

Constraints​

The constraints may include:

  • limiting the cavitation area or volume,
  • achieving the required thrust,
  • managing blade stress and
  • ensuring maximum torque or minimum thrust.

In ship propulsion, hull resistance is typically calculated through model tests in towing tanks, full-scale sea trials or CFD simulations. To maintain a constant speed, the propeller must counteract the hull resistance. This resistance can be increased by 20% if the hull is fouled or by 40% if both fouling and poor weather conditions are present; this adjustment determines the required thrust [15]. Additionally, managing blade stress is crucial to ensure the propeller's structural integrity and durability.

Targeting Thrust​

As shown in the open water characteristics diagram, different advance coefficients yield varying thrust and torque values. To target the required thrust, it's effective to run CFD simulations for each propeller design at 3-4 different advance coefficients (J values) and create an open-water diagram for each design. By combining these diagrams with total ship resistance as a function of speed, you can evaluate and optimize efficiency under operating conditions.

You can adjust J by

  • varying the propeller's RPM at a constant speed,
  • changing the speed while maintaining constant RPM or
  • modifying both speed and RPM while keeping a constant Reynolds number at 0.7 R.

Generally, operating on the left side of the peak of the propeller efficiency curve is preferred.

Surrogate Modeling​

Surrogate modeling is an essential technique in optimizing marine propellers [16]. Surrogate models provide fast approximations of complex simulations, reducing computational costs and enabling efficient exploration of design options. By using these models, designers can quickly assess various configurations and trade-offs between different objectives, such as efficiency and cavitation.

Response Surface Method (RSM)

Involves developing mathematical models that approximate the relationship between design variables and performance metrics [17]. This approach systematically analyzes how changes in design parameters affect propeller performance. By combining surrogate modeling with RSM, effective multi-objective optimization is achieved, leading to improved design decisions while optimizing computational resource usage.