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. The next point along the propulsion drivetrain is the propulsor itself, in this example one or more screw propellers. 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. However, even direct drive systems have small losses. For a propulsion system containing a transmission, the greatest losses are usually in the reduction gearing and clutch. There are generally power losses from the principal power source to the propeller shaft which are mechanical in nature. The Shaft Power is generally thought of as the power provided to the main propeller shaft(s). However, the concepts also apply to other propulsion configurations such as waterjets, outboard engines, etc. The following discussion is focused on the traditional inboard powered vessel driven by screw propellers rotating on one or more propeller shafts. Of course in reality there are numerous system losses, generally described by efficiencies, as discussed in the following paragraphs.
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. 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. 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. 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).
We are sometimes asked about how the output information created by OMCFD relates to the overall power generated by the vessel's propulsion system. The differences between towed and self-propelled simulations in OMCFD are described in the KB article, Resistance vs. 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. Modified on: Mon, 11 Apr, 2022 at 7:12 PM
Solution home FAQ Orca3D Marine CFD - General Interpretation of Orca3D Marine CFD Results
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