A towed vehicle capable of constant depth operation and bottom contouring has been designed and built for'pol1uiion monitoring studies in estuarine and marine environments. The principal characteristics are:
Length.... ........54 inches
Wing Span .........39 inches
Body Diameter......9 inches
Operation Depth ... Surface to 100 feet
Operating Modes ... Bottom Contouring Constant Depth
Bottom Contouring..10 to 30 feet above bottom
Weight in Air......110 pounds
Towing Cable Type...1/4 inch polypropylene rope
Towing Cable Length ... 100 - 300 feet
The basic configuration of the vehicle is that of an airplane with fixed forward wings and rear movable elevators. The elevators can assume two positions that correspond to (1) diving and (2) surfacing. The distance from the bottom is measured with an echo sounder and the depth with a pressure gage. The control system is of the, open loop type with a yes - no mode of operation. The most interesting characteristics found in the preliminary study and tests with a full size wooden model were that (1) the lift to drag ratio is relatively independent of the velocity for a constant elevator setting and (2) the response to a weaving force of 10 pounds with a period of × seconds is a displacement from the line of motion of approximately × feet.
Sea trials made in the San Francisco Bay showed that without automatic controls the vehicle reached a certain depth in about 20 seconds and maintained this depth within ± 6 inches. In the bottom contouring mode the vehicle followed the bottom with oscillations contained within 4 - 5 feet. In the constant depth mode, depth was maintained with oscillation contained within 5 - 5 feet.
The most common systems used for gathering environmental data in marine waters can be grouped in two general cateogries.
On station measurements or water sampling along the water column.
Underway measurements using a submerged instrument housing or a submerged pump and inboard instrumentation.
Our particular problem was to measure a large number of environmental parameters in estuarine and coastal waters for pollution assessment. In this particular environment, because of the varying of the physical condition (currents, eddies, long shore drift, etc.) it is difficult to establish a clear picture of the distribution and concentration of pollutants unless a great amount of data is gathered in a relatively short time.
In designing an instrumentation system adequate for this problem, we also stressed low cost and simplicity of operation.
We chose to use a towed underwater vehicle housing the sensors, the data recording system and all the necessary power supplies, and capable of maintaining either a constant depth or a constant distance above the bottom, because a large number of continuously monitoring fixed stations is both very expensive and impractical in a marine environment.