You didn't get a structural engineer to design this, DID YOU? this structure was doomed to sway from inception.
You don't give limits, can you cross brace between the verticals, that would be easiest and most effective, it could improve the design parameters for the verticals as well but the calculation should be performed by someone who knows what she is doing. To make it a rigid frame would increase the demand for an experienced structural engineer.
Michael.
Timing has a lot to do with the outcome of a rain dance.
If you want the device go rigid with the ship, the support itself needs be rigid enough first, and then the device. If it is slender as it seems, the different modes of vibration can show at specific exciting frequencies either from machinery or sailing/wave action, and displacements become more noticeable. A proper analysis may require to analyze beyond the base itself. The general recommendation is to make everything stiffer and firmly attached to the vessel.
I would have to agree with paddington green in that the frame should have been designed with cross bracing between the two vertical columns, however if there are space constraints associated with doing this perhaps gusset plates on the corners would provide rigid connections and provide a small amount of lateral stability.
We've decided to shorten the A Frame by 6' as well as add more bracing as long as it doesnt interfere with the cables running between the vertical columns