Performance Analysis FAQ

Copyright Alion Science and Technology
 

1. Why isn't the lower end of my speed range displayed in the results?
   The Savistky prediction method is applicable when a boat is "fully planing", which is often defined as where the non-dimensional speed parameter known as volumetric Froude number (FV) is approximately 2 or more. An extension to the method provides for the prediction of the pre-planing "hump speed" drag curve down to FV of approximately 1. Speeds below FV of 1, however, cannot be reliably predicted with this method, so they are omitted from the analysis.
2. Why do I have to specify different surface(s) for the Planing Analysis than I used for Hydrostatics?
   The Planing Analysis uses the surfaces to determine certain geometric parameters that describe the planing surface of the hull, such as deadrise angle, chine beam, and planing length. If other surfaces are selected, the Planing Analysis has no way to "know" which surfaces represent the planing surface, and which do not. The Planing Analysis is not "floating" the model like the Hydrostatics function, so you should not include any surfaces other than those that describe the planing surface.
3. What is the difference between Effective Power and the power rating of the engine?
   Effective Power is the power delivered into the water, and is the product of the drag times the speed. In addition, a service margin for additional hull roughness, appendages, windage, and seas is normally added to the computed hull drag to determine total Effective Power. In order to compute the engine power, or Brake Power, necessary to produce the effective power, the efficiency of the propeller, transmission, and shafting must be accounted for. The Overall Propulsive Coefficient, or OPC, includes these efficiency losses. It is not uncommon for the effective power to be half of the engine power (OPC=50%). See About Effective Power in the RhinoMarine Help for more information.