Speed/Power Analysis

Orca3D contains methods from the HydroComp Drag Prediction Library for predicting the resistance of hulls.

The HydroComp Drag Prediction Library v2 is a calculation engine for the prediction of bare-hull drag. Two prediction methods are available - Savitsky (planing hulls) and Holtrop (displacement or semi-displacement hulls).

The methodology employed in the library for the prediction of bare-hull drag must initially be determined in the context of the vessel's principal hull form. Marine craft follow a number of different physical principles to support and move the mass of the hull through the water. Each vessel's basic motion is useful in defining the technique that is most appropriate to predict its bare-hull resistance.

A vessel can typically be categorized by the way the hull is vertically supported while in motion (lift), which in turn points to a particular methodology for the lateral resistance to motion (drag). If a hull's lift is entirely supported by hydrostatic buoyant forces and its drag is horizontal, it has traditionally been known as a "displacement" hull. For the purposes of drag prediction, however, we call it a CT-based hull (due to the numerical calculation formats). A craft principally supported by hydrodynamic bottom pressures is called a Planing hull. Hulls operating in an intermediate regime are traditionally known as "semi-displacement", "semi-planing" or "pre-planing".

Resistance methodologies for the two principal categories – CT-based (for displacement and semi-displacement hulls) and Planing – have seen much development over the years, but the most useful approaches generally fall into one of two forms – a geosim drag coefficient (CT) method for CT-based hulls, and an equilibrium state Planing hull technique.

It is important to mention that both of these methodologies have evolved over the years, particularly with respect to the prediction of viscous (frictional) drag and model-to-ship correlations. As techniques change, however, it is common to see older corrections improperly applied to newer techniques, and the accuracy of the results has suffered. In other words, all parts of the process must be compatible to be successful. With this in mind, the drag prediction methods in the library reflect contemporary thought, as well as corrections and improvements to the behavior and accuracy of the methods.