[vari]

Hello, I am doing research on motion characteristics of particular catamarans using Star CCM+. I'm particular interested in Heave and Pitch motions.The results I am getting are in time series; I tried performing FFT to convert in into frequency domain, But my Y axis changes to power spectral density instead of showing me the amount of heave and pitch. Can anyone please help me with the issue? I want my X axis to be frequency and my Y axis to show heave and pitch.

Right now My FFT results look like http://imgur.com/a/W6YaE, I am trying to get a graph which is similar to http://imgur.com/a/eW0qu, I can calculate my encounter frequency once I perform FFT,but To get the transfer function (i.e Y axis) I will need Heave amplitude RMS, But I don't know how to proceed from power spectral density.