The standard version of Orca3D Marine CFD predicts the calm water resistance and powering performance of ships, yachts, boats, and planing vessels by simulating "resistance" (i.e., towed) and "powered" (i.e., self-propelled) performance. The differences between towed and self-propelled simulations in OMCFD are described in the KB article, Resistance vs. Powering (Self-Propelled) Simulations. We are sometimes asked about how the output information created by OMCFD relates to the overall power generated by the vessel's propulsion system. To answer this question we refer to the image below which shows the losses/efficiencies involved in going from the power source, engine brake power, to the effective power that is ultimately put into the fluid(s) by the propulsor (e.g., the propellers).
The Brake Power is generally defined as the power provided by the vessel prime mover, typically an internal combustion engine, electric motor, turbine (gas or steam), some combination of these, or another principal power source. This is often measured at the engine driveshaft flange as shown in the figure above, but of course can require a different interpretation in cases like outboard engines and electric drives. In an ideal scenario, all of this power from the prime mover could be delivered to the fluid (e.g., water) by the vessel propulsor. Of course in reality there are numerous system losses, generally described by efficiencies, as discussed in the following paragraphs.
The following discussion is focused on the traditional inboard powered vessel driven by screw propellers rotating on one or more propeller shafts. However, the concepts also apply to other propulsion configurations such as waterjets, outboard engines, etc. The Shaft Power is generally thought of as the power provided to the main propeller shaft(s). There are generally power losses from the principal power source to the propeller shaft which are mechanical in nature. For a propulsion system containing a transmission, the greatest losses are usually in the reduction gearing and clutch. However, even direct drive systems have small losses. Although the mechanical losses in most systems are not large, they are not insignificant and must be considered in the system analysis through an estimate of the mechanical efficiency, etaM.
The next point along the propulsion drivetrain is the propulsor itself, in this example one or more screw propellers. From the transmission side of the propeller shaft(s) to the propeller(s) themselves, there are losses due primarily to bearings at struts and stern tubes but also to other factors such as torsional flexure in the shaft itself. These losses are generally represented by a shaft efficiency, etaS, and the power delivered to the propeller(s) is not surprisingly referred to as Delivered Power.
The conversion of rotational energy into a longitudinal thrust force is usually where the largest loss of power occurs in the system. Propeller efficiencies for screw propellers are typically on the order of 70%. The power output from the thrust-producing propulsor is known as the Thrust Power. For screw propellers the efficiency associated with this energy conversion is often decomposed into two components, The first is the propeller open water efficiency, etaO, representing the efficiency of converting from rotation to thrust in ideal open water inflow conditions. Of course in reality the propeller is often located in a flow environment that is anything but ideal, and the losses associated with the propeller operating in this disturbed flow environment is usually termed the relative rotative efficiency, etaRR. The actuator disk model used in the self-propelled simulations of Orca3D Marine CFD include an approximation of the propeller open water efficiency. However, it is not possible to compute etaRR using this simplified propeller representation as described in the KB article, Can I get the calculated 1-t, 1-w, η0 and ηr in the self-propelled simulation? Fortunately, etaRR values for most vessels are often close to 1.0, and the CFD reporting function available in OMCFD allows the user to enter a value manually for etaRR if an estimate is available
Finally, the power the power that is put into the water is the Effective Power. The effective power is related to the thrust power by the hull/Propeller interaction coefficients w (wake fraction) and t (thrust deduction). The hull efficiency, etaH = (1-t) / (1-w). Thrust deduction is automatically accounted for in the self-propelled simulation through the changed in the flow field around the propeller due to axial and rotational acceleration of the flow. Wake fraction is not accounted for directly in the self-propelled simulation but can be estimated using a "nominal wake fraction" computed from a wake survey analysis as described in Simulating a Propeller Wake Survey. To incorporate the results of a wake survey into the self-propelled analysis, see Including the Effects of a Nominal Wake Fraction.
For a steady speed, the effective power is balanced by the total resistance of the vessel according to the relationship, Effective Power = Total Resistance * Steady Speed. The total resistance includes all components of hydrodynamic and aerodynamic drag and is usually computed from a "resistance" simulation with Orca3D Marine CFD. It is important to recognize that the CFD resistance simulation only computes the drag of what is modeled. There may be other drag components that need to be considered as described in the KB article, Factors beyond CFD Resistance.