Question:

I have defined 2 cases of tow coordinates; XYZ and YXZ coordinate system and found the summation of forces generated are exactly the same. How do I tell if the axis has been modified according to intended when the summation of force are the same? Also, the 3 reference axis, what are they corresponding to? 

Answer:

Refer to Tow manual Section 2.2.3 Tow Local Coordinate System, 

1) In towinp., the tow coordinate system as below:-

For YXZ and XYZ, you will note that the coordinate systems is switched so Sway swaps from the X structural coordinate to the Y structural coordinate depending on the Tow coordinate system used. 

2) How do I tell if the axis has been modified accordingly when the summation of force the same?

The TOW coordinate system is reported in Surge, Sway, and Heave coordinates. This follows the documentation in Tow 2.2.3.  Also note the Dynamic Loading Summation reports position, acceleration, force, etc. in terms of both TOW coordinate system and structural coordinate system. 

For example, if you see load case 301 in the Dynamic Loading Summation output, irrespective of what we define as the co-ordinate system, by default we will get a force along global Y direction. This is what SACS is reporting in the "Force and Moment Summary" output. Now, this Fy force will be Sway or Surge, that depends upon what we define as our tow coordinate system. So, the Fy force might be reported either in Surge column or Sway column.

The reported dynamic loading summation is correct for both the XYZ & YXZ system eventhough the value are the same. The alternation of the values are as per the defined axis. There are no alternation of the loads in the "Force and Moment Summary" between both cases.