Hydrostatics & Stability > Sign Conventions

For various purposes, Orca3D must know which coordinate axis is longitudinal, which is transverse, and which is vertical, and further, what direction the positive sense of each of these corresponds to. For example, the definition of a "station" is a plane at a constant X value, if X is your longitudinal coordinate. And while the direction (positive X aft or forward) doesn't matter for hydrostatics calculations, it is important for speed/power analysis (the program needs to know which end is the bow). Orca3D defaults to a coordinate system with positive X aft, positive Y to starboard, and positive Z up. However, you can set the coordinates in any of 24 combinations, all of which are right-hand rule coordinate systems.

•	Fwd is: Select the coordinate direction that corresponds to the "Forward" direction in your model.

•	Up is: Select the coordinate direction that corresponds to the "Up" direction in your model.

Note: Changing these values will not rotate your model. You must use the standard Rhino commands (Rotate or ScaleNU) to change the orientation of your model.

Orca3D always maintains a right-handed coordinate system. This implies the following:

If Fwd is set to	…and Up is set to	…then Starboard is	…and positive Trim is	…and positive Heel is to
Positive X	Positive Z	Negative Y	Bow down	Starboard
Positive X	Negative Z	Positive Y	Stern down	Starboard
Positive X	Positive Y	Positive Z	Stern down	Starboard
Positive X	Negative Y	Negative Z	Bow down	Starboard
Negative X	Positive Z	Positive Y	Stern down	Port
Negative X	Negative Z	Negative Y	Bow down	Port
Negative X	Positive Y	Negative Z	Bow down	Port
Negative X	Negative Y	Positive Z	Stern down	Port
Positive Y	Positive Z	Positive X	Stern down	Starboard
Positive Y	Negative Z	Negative X	Bow down	Starboard
Positive Y	Positive X	Negative Z	Bow down	Starboard
Positive Y	Negative X	Positive Z	Stern down	Starboard
Negative Y	Positive Z	Negative X	Bow down	Port
Negative Y	Negative Z	Positive X	Stern down	Port
Negative Y	Positive X	Positive Z	Stern down	Port
Negative Y	Negative X	Negative Z	Bow down	Port
Positive Z	Positive X	Positive Y	Stern down	Starboard
Positive Z	Negative X	Negative Y	Bow down	Starboard
Positive Z	Positive Y	Negative X	Bow down	Starboard
Positive Z	Negative Y	Positive X	Stern down	Starboard
Negative Z	Positive X	Negative Y	Bow down	Port
Negative Z	Negative X	Positive Y	Stern down	Port
Negative Z	Positive Y	Positive X	Stern down	Port
Negative Z	Negative Y	Negative X	Bow down	Port

If you choose a coordinate system with the Up direction set to a Negative direction, you may find that rotating the view in the Perspective viewport is non-intuitive. This is because by default, Rhino does rotations about the World Coordinates. To make it more intuitive, open the Rhino Options dialog (Tools > Options, or right-click on a viewport name and select Display Options). Click on View, and then select "Rotate relative to view."