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Performance Analysis FAQ
- 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.
- 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.
- 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.
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