Final Float Design

Fully Assembled Float

One of the most important aspects of the velocity float was its ability to travel as smoothly as possible through the water and to carry the provided pressure sensors. Consequently, the exercise of developing the shape of the new design had two aims: to move with the lowest practical drag and to carry the pressure sensors. This should be possible without compromising the other functions and operations of the float and components. The final design consisted of PVC pipes and a PVC sheet that were fabricated and glued together to achieve a torpedo-shaped body and arrow-shaped fins. This shape, shown in Figure 1, significantly reduced the hydrodynamic drag on the float when submerged and satisfied the design requirements.

Figure 1. Final float design with sensor housing

In order for the Sea Dragon Velocity Float to be as stable as possible when at rest in its initial position and when pulled down by the drive float, a bail was added to the body so that the float’s direction would change from horizontal to vertical when enacted upon by the hydrodynamic forces of downward motion. The location of the bail was determined such that the float would be statically trimmed horizontally, and thus would need minimal current flow in order to orient itself in the direction of a minimal frontal drag profile. This position was determined from analysis that calculated the center of gravity, center of buoyancy, and center of pressure of the float for different length configurations, as shown in Figure 2. The results of this analysis determined the float’s static and dynamic characteristics at different lengths, and resulted in the final design dimensions.

Figure 2. Location of key points in the float design to achieve the desired static and dynamic behaviors

Sensor Housing

The sensor housing is designed to hold two Star Oddi Centi sensors. In order for the sensors to properly operate, the membrane on the rear of the sensor must be exposed to the fluid. The assembly must be easy to use in order to allow a diver to retrieve the sensors and bring them to the boat. Once at the boat, the sensors can be removed and inserted into the sensor reader. The final design of the sensor allows for a tool-less operation. The sensor housing is tethered to the bail using locking carabiners. The housing is held together using two cotter pins; terminated by hairpin cotter pins. The multiple Star Oddi Centi sensors allow for redundancy in case one sensor malfunctions. The upper and lower housings were 3-D printed using ABS plastic. 

Velocity Algorithm

The final algorithm will find a cubic curve, fit to the position data, and take its derivative to find the velocity profile. This algorithm meets the sponsor’s stated accuracy requirement within the working velocity range, and is easy to implement. Below are plots of simulated velocity and velocity obtained through the curve fitting algorithm for minimally and maximally buoyant drive floats:

Figure 3. Derivative of cubic curve fit against simulation velocity

The derivative of the cubic curve fit is quadratic in shape. While this matches the maximally buoyant velocity extremely well, the minimally buoyant situation is only matched relatively well. This is due to the piece-wise nature of the system. It is clear from the plot that the worst case error occurs at the maximum velocity. This error is measured to be 0.076 m/s. This error, while it appears significant on the plot, is well within the ± 0.15 m/s required of the algorithm. As the buoyancy of the drive float increases, this error decreases because the velocity curve becomes more quadratic and the curve fit derivative can more accurately match its shape.