VENT 7 - MOBILITY

PROPULSION
We realize that propulsion systems for AUV's and ROV's is somewhat different as their needs vary as well as the power available at a given time. Our current design, therefore, includes the following considerations:

AUV's
They will be equipped with two horizontal thrusters that will provide heading control. Two vertical thrusters for vertical control might be desirable, although we realize that lack of power might rule out this option. Depth control might be therefore achieved by slight changes in buoyancy instead. It is possible to use a dynamic system that compensates for pressure at any depth through a complex system of valves, ballast tanks and compressed air. Unfortunately, a dynamic system requires a large amount of space and may not be reliable in maintaining proper buoyancy, so this option might have to be ruled out at the end. It is important to point out that our AUV's will not need as much vertical mobility, yet they will need to travel relatively long distances. Thus, focusing on horizontal propulsion will yield better results.

ROV's
Our ROV's will need great maneuverability and accurate displacement along the vent fields. Therefore, vertical propulsion will be of utmost importance for their control. At the moment, we are considering two horizontal thrusters to provide heading control and four Nektor (TM) propellers for vertical maneuverability. The Nektor propulsion system was designed to provide maximum underwater agility by using incredibly thin and flexible propellers. At the time, Nektors have been tested in the Pilotfish ROV, and they have provided a response rate up to 20 times faster than regular thrusters, as well as a greater angular acceleration. (see Nekton Research) In theory, four Nektors can provide omni-directional thrust (360 degree propulsion), just enough to suit our goals, but we believe that horizontal thrusters will be better for traveling long distances. The Nektors will be placed at regularly spaced intervals along the hull's perimeter (45, 135, 225, and 315 degrees).

Lately, we have been considering a crawling ROV that will be in charge of drilling the ocean floor to obtain the samples asked by the geological team.

POWER
Nektors -- 10 kW of power (approximately 13.5 hp) are enough to move a 150 kg. vehicle. This will depend, however, on the actual design of our vehicle as well as its weight.
Power needed to displace our vehicle will depend therefore, on the hydrodynamic design (cross-area) and on the actual weight of the vehicle. The heavier and bulkier the vehicle, the more power we will need and the larger the radius of the rotary propellers. We need therefore, to decide on the final dimmensions of our vehicle before we can consider the dimmensions of the thursters and consequently the power they will need to operate.
A safe approximation, however, on currently used thrusters is of 3.6 kW of theoretical peak power, but only of 1.92 kW of practical peak power per thruster. It is evident that there is no need to operate thrusters at full, so actual working power might be of 252 W. This numbers however, are mere estimations and vary according to the weight and cross-area of our actual design.

GENERAL CONSIDERATIONS
Our unmanned underwater vehicles (UUV's) will at all times travel at speeds of around 1 to 2 knots (1.8 to 3.6 km/hr). This is intended to minimize power consumption and provide a longer battery life. According to Prof. Kimball of the MIT hydrodynamics laboratory, great propeller efficiency at these speeds is not a great concern, although might be desirable. In sum, energy efficiency differentials depending on different kinds of propellers at slow velocities are depreciable, and that is why most AUV teams do not take propeller design into consideration when building their AUVs or ROVs.
Other issues we have to consider when designing and placing the thrusters and nektors is the sea-floor. Turbulence caused by the propellers might damage some geological features as well as sea-life.

Additional information on propulsion with Nektors at great depths

"We haven't tested our Nektors at depth yet, though I'm confident the depths you're talking about are feasible. Our larger motors (3/4 hp) are brushless servo and the housings have been designed for oil compensation, though there isn't an internal pressure compensator. As far as the polyurethane fins go, they'll be fine at that pressure. The motors are 5.5" square x 4.5" tall and the fins have a 12" span 6" cord." -- Brett Hobson -- 10/26/01

RESOURCES
Prof. Kimball, MIT Hydrodynamics Laboratory
Syseng's power design
Nekton Research
Duke-NCState competition AUV's