Experimental Investigation of the Frequency Domain Seakeeping Behavior of a Surfaced Underwater Vehicle

This paper proposes a study of the seakeeping behavior of a surfaced Underwater Vehicle (UV) in regular head seas. This investigation exposed a welldefined double peak pitch behavior, that also showed a coupling to the heave peak motion. This specific behavior was a key difference to the seakeeping of conventional surface vessels.
A novel experimental rig was developed to provide enhanced testing capabilities in high sea states. The captured data was essential in illustrating the hull’s specific motion. The work presented here focuses on the motion’s general trends in the frequency domain, as a way to primarily elucidate the double peak pitch phenomenon and its coupling. As such, the analysis focused on the resonant response of the UV through the peak Response Amplitude Operators (RAO). Several factors were investigated, such as the travel speed, the wave parameters (height and steepness), and the influence of the Aft Control Surfaces (ACS).
Results showed a stronger influence of the Froude number on the motion response, compared to the wave height. Also, larger non-dimensional motions occurred at intermediate Froude numbers, as well as in smaller waves. This indicated a very complex and highly non-linear behavior. Part of the behavior resulted from a damping effect of the ACS, dependent on the Froude number more than on the wave height or steepness. The damping affected the second pitch peak the strongest. A definitive feature of the heave-pitch coupling was a linear relationship between the two motions for wave frequencies far from the peak RAO frequencies. The cause of the double pitch peak behavior remains uncertain, but several hypotheses are offered.
This work brings a new understanding in the seakeeping behavior of surfaced UV. It provides a first insight, able to support further time domain analysis of the motions and added resistance in waves.