Submerged Acoustic Retrieval
Fishing productivity with the implementation of a submerged acoustic retrieval system is critically dependent upon the surfacing buoy. MATLAB was used to provide mathematical models of each buoy’s response time. Different float configurations were simulated to determine the theoretical time it would take for the system to surface from various depths. The full scale simulation was based off deepest offshore fishing occurring roughly 425 meters below the ocean’s surface. After determining the theoretical surfacing time of a 14 in. trawl float was within reasonable limits, testing was performed to verify the theoretical results. The MATLAB model was modified to produce theoretical response times at a depth of 20 feet. This is the depth at which each float configuration would be tested.
The different configurations were tested against the theoretical times in the Chase Engineering Tank. This gave insight into the accuracy of the depth step model. Three different buoys were tested with multiple configurations. The surfacing times were obtained using a hand timer and a pressure sensor located at the rear of the buoy. The sensor was placed directly behind the buoy configurations to minimize its effect on the overall drag and provide greater insight into each buoy’s response. The systems testing included a 9 in. lug eye buoy, an 11 in. center hole buoy, two 11 in. buoys set in parallel, two 11 in. buoys set in series, and a 14 in. center hole buoy. Both the hand time and the pressure sensor clocked the 14 in. buoy with the fastest surfacing time. It surfaced half a second faster than the next best configuration. When submerged to much greater depths, the small difference between the configuration’s response times will be amplified and prove to be significant. For the same 14 in. buoy, the MATLAB analysis provided a theoretical time very close to the experimental testing time. This confirms the accuracy of the MATLAB code, meaning that accurate estimates can be made at greater working depths.
The pressure vessel housing the system’s electronics was modeled in SolidWorks to understand its behavior under pressures caused by deep ocean submersions. The vessel is subject to a pressure of 445 psi at a maximum fishing depth of 425 m. To maximize durability while keeping the pressure vessel light, Aluminum Alloy 661 was chosen as the wall material. In order to fit the necessary electrical components, the inner diameter of the pressure vessel was determined to be 5.5 inches. The step was 0.1 inches and the vessel was designed with a safety factor of 1.5. The safety factor value was chosen due to the fact that the pressure at the maximum fishing depth was used during the design. The optimization tests concluded that the optimal outer diameter is 6.2 inches with a wall thickness of 0.7 inches.
The remote communication between the fishing vessel and the submerged lobster trap will be aided by the use of hydrophones. One transmitted hydrophone will be housed on the fishing vessel while the receiving hydrophone will be attached to the submerged trap. A PSOC 3 microcontroller located in the waterproof pressure vessel will process the incoming signal received by the hydrophone. The PSOC employs a frequency-shift key (FSK) to decompose the incoming signal into discrete frequency changes of a carrier wave. This allows for digital information to be transmitted through different codes which prevents trap theft amongst fishermen. Once the correct signal is processed, the PSOC sends the command to open the mechanical claw through actuation of a servo motor. Due to the considerable force applied on the claw from the upward force of the buoy, two steel outriggers were attached to the arms. One element is fixed to hold the buoy when closed while the other moves on a pivot to swing out and release the buoy upon opening.
Available from the National Sea Grant Library (use NHU number to search) or NH Sea Grant
- Submerged acoustic retrieval (2014). Kris Hopkins, Brian Regan and Jeffrey Davies. Advisor: Kenneth Baldwin.