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By Larry Leibman, Principal Naval Architect, DRS Defense Solutions, LLC
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Surface Normals and
Hydrostatic Calculations
I
n order to compute the hydrostat-
ics properties of your hull model,
Orca3D performs a numerical inte-
gration on the surfaces that you
select for the hydrostatics analysis.
This numerical integration is af-
fected by the orientation of the sur-
face, where in this context
“orientation” refers to the direction
that the surface normals are point-
ing. By convention, Orca3D as-
sumes the surface normals point
into the water (i.e. the normals are
on the wet side of the surface). One
might be tempted to say that this is
the equivalent of assuming the nor-
mals point “outward” from the hull
surfaces; however, this is not al-
ways true. Consider the case of a
bow thruster opening in the hull.
Here, pointing into the water will
have the normals pointing inward.
Surfaces whose normals are out-
ward create positive displacement;
when surface normals are inward,
the displacement of that surface is
negative. For the bow thruster ex-
ample, it’s correct for the buoyant
volume of the surface to be negative.
You can use the Rhino Dir command to determine and/or
change the normal direction of your hull surfaces. However,
an easier way to visualize the surface normals without hav-
ing to remember to use the Dir command is to use the
Rhino backface coloring option. To do this, run the Rhino
Options command. In
Rhino Options
go to
Appearance
>
Advanced Settings
>
Shaded
. In the
Shading settings
set
Backface settings
to
Single Color for all backfaces
and s
the
Single backface color
to a color that you don’t typical
use for you surfaces. Now, when you turn on shading mo
in a Rhino viewport, you can easily see the surface orient
tion because the backfaces (the side opposite that to whi
1) Using the Dir command, it can be seen that the nor-
mals are not pointing into the water. The displacement
will be negative.
2) After running the Flip command, the normal directio
is correct.