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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