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A

question that often arises for Orca3D users is whether

or not the “Static” and “Running” Trim of a fast planing

craft are somewhat “geometrically” related to each other.

The quick answer is “No,” but it is worth studying the ques-

tion more closely to understand the answer.

Static Trim

Whether it’s about a slow displacement vessel or the fastest

planing “Hot Rod” boat, the “Trim” angle, as reported by Or-

ca3D when performing a hydrostatic analysis, is related to a

“

Static” condition of the hull only. That is, throughout the

analysis performed the hull is considered to be “at rest,” or

in physics terms, “in static equilibrium.” No dynamic effects

(

i.e., speed/acceleration-related forces) are taken into ac-

count under this type of hydrostatic analysis.

More specifically, when performing a “Free-Float” hydro-

static analysis in Orca3D, the reported “Trim” angle corre-

sponds to the longitudinal (i.e., Fore-Aft) hull orientation in

3

D space that results from the balance of the simple system

of forces considered by this analysis, that consists of two

forces only. These are the “Weight” of the craft, acting

through its “CG” (Center of Gravity), and the “Buoyancy”

force that acts through the “CB” (Center of Buoyancy

1

).

As we all know from basic engineering mechanics, for any

“

structure” (i.e., our craft, yacht, or vessel) to be in a “static

equilibrium” condition, it has to be verified that the net sum

2

of all acting forces must be zero (i.e., the net resultant force

is zero), and also that the moment is zero. This is known as

“

pure” static analysis.

Orca3D, through its built-in algorithms, is capable of finding

the final hull orientation in 3D space, after finding the solu-

tion to this both “simple” and “complex” problem.

Given a hull geometry for analysis, once a craft’s weight and

CG are specified by the user, the software solves the system

of equations for static equilibrium with the hull free to trim,

heel, and translate vertically, until an orientation is found

that results in an immersed volume, and its corresponding

buoyancy force, that counteracts (i.e., is equal and opposite

to) the craft’s weight.

Furthermore, the resulting solution is such that the relative

position between the CG, which is “fixed” to the craft, and

the CB that results from the “immersed geometry” is such

that no net moment will result from this situation. In other

words, this also means that Weight and Buoyancy forces are

also acting through the same line of action, or that their di-

rections are mutually coincident.

Figure 1 illustrates a typical situation before performing a

“

Free-Float” hydrostatic analysis. That is, the boat’s geome-

try is modeled with reference to a known “Baseline” (usually

a horizontal, or the “x” axis line

3

).

The boat’s weight and CG

4

are known by the designer’s preliminary estimations, and a

“

preliminary” Design Waterline is drawn, usually parallel to

the Reference Line at a height (or Draft) that produces a

displacement (buoyancy force) that equals the boat’s

weight.

Notice that in this condition what usually happens is that,

even when the boat’s weight and displacement for the given

preliminary Design Waterline are equal, both forces are not

aligned, since, as we can see in Figure 1, there is a horizon-

Sept. 2012 Page 17

Planing Craft Analysis:

Static vs. Dynamic Trim

——————————————————————-

1

The CB, or Center of Buoyancy, is the centroid of the wetted volume.

2

Here we refer to a “vector” sum.

Continued next page