Heave Natural Frequency - AQWA

Im new user in AQWA. To find out the natural frequency in heave do I use the RAO generated by the hydronamic response or hydrodynamic diffraction? If it is through hydrodinamic response, do I select irregular waves and do a frequency statistical analysis?
Photo 1 is from the hydrodynamic diffraction. Why is the curve like this? The geometry is attached in photo 2 and at the end there is a joint of the hinged type allowing only rotation

 

First, don't use Hz, use omega (rads/sec), it will make everything easier.
For response use the RAO's for response, and then handle it like any other spectral input problem. See Dynamics of Marine Vehicles by Bhattacharyya for a full walk through of the process.
Why is the hydrodynamic diffraction curve like that? Think about it for a minute....what does the curve mean? What would hydrodynamic diffraction look like for omega = 0.0000007 (0.000046 Hz) or omega = 1273 (8 kHz)?

 

Interesting. I wouldn't have thought that heaven would have a natural frequency.... oh wait. Now I see it. Hydrodynamics is so cool.