The format of Wamit file used in hydrodyn

Dear developer,

Hydrodyn needs wamit.1 .3 and hst file file if potential theory is enabled. I noticed that in 5MW_Baseline\HydroData folder, wamit .1 and .3 files seem to adopt a format using period rather than frequency in the first column of .1 and .3 file.

Although I am new wamit, I found wamit will output in a “period” format by default after reading its user manual (i.e. their first column is period).

But my experience of using HAMS found that HAMS can output Wamit result file using either frequency or period in the first column in .1 and .3.

So I want to ask if Hydrodyn must use a “period” format of Wamit?

Ran Tu

Please allow me to share some experience of using HAMS. It is an open-source software which can be used to calculate hydrodynamic coefficients as an alternative to Wamit. But I found HAMS may assume the wave is e^-iwt (I can not 100% confirm), while Wamit assume wave is e^iwt. When using HAMS, users must be careful, as I found the imaginary part of dimensionless excitation force calculated by HAMS is opposite to that calculated by Wamit. The real part of dimensionless excitation force from both two software can match well.

If my understanding is correct (I HAMS may assume the wave is e^-iwt, while Wamit assume wave is e^iwt). Re(X_Hams) = Re(X_Wamit), and Im(X_Hams) = -Im(X_Wamit). Here, Re denotes Real part of complex value, while Im denotes Imaginary part of complex value. X_Hams denotes dimensionless excitation force from HAMS, while X_Wamit denotes dimensionless excitation force from WAMIT. When calculating excitation force, Re(X_Hams * A e^-iwt) actually equals to Re(X_Wamit * A e^iwt). Here, A is the amplitude of wave.
I think Hydrodyn only allows Wamit results file at present. And I guess when Hydrodyn calculates the excitation force, it also assumes wave is e^iwt.
Therefore, users must be careful if they want to import HAMS results to OpenFAST.


Dear @Ran.Tu,

I can confirm that the HydroDyn module of OpenFAST assumes that the frequency-dependent hydrodynamic coefficients assume a WAMIT file format, in which frequency is stored as period. HydroDyn does not support replacing period with frequency.

Thanks for sharing your feedback on HAMS. I can’t confirm this, but hopefully someone else can comment.

Best regards,

Dear @Ran.Tu,

Apologies for my late reply. Yes, HAMS is assuming a time-harmonic factor e^-iwt. But in the Wamit format output, HAMS automatically generates results assuming e^iwt. Thanks for pointing out the mistake, I have corrected the code on GitHub - YingyiLiu/HAMS: An open-source computer program for the analysis of wave diffraction and radiation of three-dimensional floating or submerged structures. Validation is performed on the UMaine floater against the most recent WAMIT version 7.4. The results can be found here: validation results. You may also find a discussion here: imaginary part of excitation force in HAMS Wamit_format is opposite to that in Wamit · Issue #26 · YingyiLiu/HAMS · GitHub

I use BEMRosetta to visualize the validation:

Hope that helps solving your issue.

Best regards,

1 Like

Thanks, Yingyi. I have tested OC3 spar foundation, and HAMS works well. Now I become more confident to adopt HAMS as a preprocessor for OpenFAST.


@Ran.Tu That’s great!