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The Masthead
Planing Craft Analysis: Static vs. Dynamic Trim
Sept. 2012 Page 19
forces have to be considered for its equilibrium analysis,
which are “Lift,” “Drag,” and “Thrust.”
Lift is the “Vertical” component of all acting forces over the
hull (e.g., buoyancy, dynamic lift), except Weight, whose
principal purpose is to balance the craft’s Weight.
Drag, also known as Resistance, is the “Horizontal” compo-
nent of all acting forces over the hull that, for the craft to be
running at a steady speed, has to be balanced by the
effective” Thrust; otherwise, the craft will slow down. Usu-
ally, the “effective” Thrust is to be provided by the propellers,
or any other propulsion system under consideration, such as
waterjets, etc.
Figure 3 shows the craft in a static equilibrium condition,
where the Vertical and Horizontal forces are in balance, and
there is zero net moment for the present system of forces
acting on the craft.
This is, again, a simple static analysis, since we have re-
duced a complex dynamic problem into a simple static one.
Notice also that, for this equilibrium condition to be
achieved, the craft has to maintain a “Trim” angle. The as-
sumption made here to simplify the analysis is to neglect
any dynamic effects due to things like waves and wind (i.e.,
the boat is assumed as running on “calm waters”).
The Savitsky Method
The Planing Analysis module available in Orca3D, developed
and licensed by HydroComp, Inc., is based upon the very
popular “Savitsky” method.
The Savitsky method relies upon a 2D “Static” representa-
tion of the problem; that is, all forces are considered lying in
the Vertical centerplane of the craft. For each speed to be
analyzed, the method first makes an estimation of the dy-
namic forces acting over the hull, mainly friction and pres-
sure forces (magnitude, direction, and location) and, second,
finds the equilibrium condition among them, including the
Weight and Thrust.
In order to find the balance between the acting forces and
force moments, the hull is rotated (or “trimmed”) to vary the
angle of attack, and moved vertically to vary the wetted
area. The lift and drag force magnitudes, directions, and the
position in which they act vary as a function of the angle of
attack and the wetted area, and the program iterates
through various combinations until the balance between the
forces and moments is achieved. This will result in the
steady “Running Trim” angle.
Continued next page