Performance Model

SimWAPS has been created to limit the number of parameters required by the user, and focuses on the effects that have the greatest influence on the potential for wind propulsion. 

Deck-mounted systems are defined by 2 aerodynamic parameters and 2 geometric parameters 

• CL_max, 
• Area, 
• Span, 
• Number of devices. 

For dynamic kites, the following parameters are used : 

• Operating CL, 
• Lift to Drag ratio, 
• Area, 
• Elevation 

Principes of velic propulsion 

The aerodynamic force generated by the ship propulsion system is expressed in terms of the apparent wind. The apparent wind direction is indicated by the ship's windvane. 

The aerodynamic force depends on the setting of the WAPS, in particular the angle of incidence (replaced by the speed of rotation in the case of rotor). 

When the apparent wind comes from ahead, lift is needed to develop propulsive force. When the apparent wind is accross the ship, propulsive force is confused with aerodynamic lift. When the wind is downwind, aerodynamic drag is the propulsive force. 

The aerodynamic force is proportional to : 

• The system aera, 
• The square of the apparent wind speed, 
• The lift coefficient. 

Every system has the capacity to develop maximum lift, which depends solely on the system geometry and can be amplified in the case of active systems. 

In addition to te propulsive force, the velic propulsion also generates a lateral force that must be balanced by the ship hydrodynamics. The result is a drift and a rudder angle which are not present with mechanical propulsion only. 

Computation of the savings

SimWAPS computes power and fuel savings and C02 emissions reductions based on a method that takes into account : 

• The aerodynamic characterisctics of each type of engine. How these characterisctics are obtained depends on each system and is detailed here, 
• The influence of lateral force on the hull, which sets the ship adrift and generates additionnal hull drag, 
• The interactions between the systems in the case of a multiple installation, 
• The route taken by the ship, 
• The number of systems, 
• The system maximum lift coefficient and aspect ratio are used to interpolate an aerodynamic polar for the system. 

These polars are based on CRAIN experience. 

From the aerodynamic polar, the propulsive and side forces are calculated and multiplied by the number of systems. In the case of active systems, fuel consumption is calculated taking into account aerodynamic coefficient and engine efficiency. 

All these forces, together with the meteorological statistics for the choosen route, are used to compute the power saved by using wind propulsion. 

The computation process is illustrated below :