R/V Rob – The A-frame

Why an A-frame?

To facilitate deploying and recovering the CTD from R/V Rob some sort of crane or slide needed to be fitted to the vessel. It was debated, and finally viewed as a fall-back option, to leave the CTD in the water and tow it from location-to-location. However, the drag from towing an object such as the CTD would greatly reduce the speed and range of Rob. This is a very similar issue that is experienced aboard S/V Quagmire when towing a dinghy such as Swampy. While it works for short distances, the extra drag from the towed object is very noticeable in regards to both speed and maneuverability.

I considered having the CTD deployment off the bow of the boat to help keep it clear of the propellers as well as help the boat wind-vane while deploying the CTD like when a sailboat is at anchor. However, due to most large oceanographic vessels having an A-frame off the stern we decided, in the end, to try starting there.

Design

To keep the implementation and operation as simple as possible, Rob’s A-frame was designed to be completely controlled by the winch. Larger A-frames usually have hydraulic cylinders that move the A-frame inboard and outboard with winch operations being separately controlled. Rob’s instead has torsion springs that push the A-frame into the outboard, or deployed position and rely on the winch to pull it inboard, or retracted position.

RV Rob's A-frame design in SolidWorks

Early SolidWorks A-frame design (credit Yesenia)

It was originally hoped that the CTD would sit on the deck of the boat so that if the line broke while the A-frame was retracted that the CTD would stay with Rob and not be flung to a watery grave. To achieve this, however, would require being able to control the winch’s free-spooling drag (friction). Without being able to increase the drag during deployment the CTD remained on the deck while the A-frame pulled out line; while increasing free-spooling drag after A-frame deployment keeps the CTD from sinking. Attempts to have the A-frame deploy this way and then winch in a little to drag the CTD over were deemed risky due to entanglement issues on the boat.

Installation

The A-frame that the mechanical engineering student designed is built out of carbon fiber tubes, aluminum fittings for these tubes, and ABS rapid prototyped bases. The bases are through-bolted to the deck of the boat in a structurally sound area (no backing plates). The winch is mounted to the hatch top to provide a reasonable angle for the line. Initial testing has indicated from line fraying that during deployment/retraction that an additional block will be necessary to help feed the line into the winch’s level winder at the proper angle.

A variety of torsional springs have been tried. The initial set, two springs (one per side), each 270°, 22lb force fling the CTD with excessive force at the end of the deployment cycle. The second set, two (one per side) 180°, 20lb and third set, two per side, one 270° ~10lb and one 90°, ~10lb appear to do better but further testing is required.

Control

One of the bases has two reed switches embedded in it to sense when the A-frame is fully retracted or deployed. These signals are used by the APM microprocessor to control winch speed. When the forward sensor is tripped, the winch is disabled. When neither sensor is tripped, the winch speed is set to a slow retraction speed. When the aft sensor is tripped, the winch is allowed to go full speed as it pulls the CTD up from the cast depth.

Testing

Initial lab testing with the actual CastAway CTD. Credit Yesenia.

River testing with a rapid prototyped mock-up of the CTD. Shortly before this video was taken the mock-up fell off in shallow water and was recovered. But still it illustrates the necessity to do testing with a mock-up rather than the actual device due to unforeseen problems—in this case a broken zip-tie that was being temporarily used as a small shackle.

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