Toxic paint is used on the bottoms of large ships to prevent fouling, which is when a biofilm layer develops, decreasing the ship’s efficiency moving through the water. To further complicate the matter, the paint must be blasted off and replaced every 5 to 10 years, at which time literally tons of toxic waste is produced and needs to be disposed of.
A ship that operates with a clean underwater surface free from fouling — even thin biofilm layers — will operate so much more efficiently that potential savings can easily reach over five percent in fuel costs alone. Without the concern for fouling, a ship’s underwater coating can be engineered for corrosion protection and longevity rather than its need to eliminate the potential for biofouling.
To eliminate the requirement for toxic paint and its cleaning waste, there needed to be a method to “groom” the biofilm from the underwater portion of a ship. The idea was to create important changes for the ships being built, as well as for the environment. That’s where the HullBUG (Hull Bioinspired Underwater Grooming) concept originated.
Successful promotion of the HullBUG concept by SeaRobotics attracted the attention of the US Navy and specifically the Office of Naval Research (ONR). A successful proposal was submitted to the ONR for funding, and a project team established. The team currently includes SeaRobotics as system designer and integrator, the Naval Surface Warfare Center Carderock Division (NSWCCD) operating as the team management, and Florida Institute of Technology providing a strong knowledge and research base for understanding how the biofilms affect ship efficiencies and what is necessary to combat them.
“The most important feature of the HullBUG is its small size,” according to SeaRobotics Research Engineer Dr. Kenneth Holappa. “It is only about half a meter in length.” This was a necessary design feature to allow the vehicle to maneuver over the curved surface of the hull while continually maintaining close contact with the surface. An additional benefit of the small size was that it allowed a single operator to deploy the vehicle without the use of additional equipment such as a crane.
Because there are hazards associated with operating such a device underwater and in a harbor environment, occasionally a HullBUG might be lost or destroyed during operation. Keeping the size and cost of the system low definitely helped to eliminate damage as a major obstacle to implementation. So, from the very beginning of the project, small size and low cost have been identified as being critical to the satisfactory implementation of the HullBUG project.
Motion Control Components
The selection of the motors to drive the HullBUG involved a number of critical engineering constraints and compromises. SeaRobotics decided to make two basic models, one with wheels and one with tracks, and offer several options for keeping track of the system’s progress.
Sizing of the motors, for example, required a calculated estimate of the power, speed, and torque characteristics of the manufactured devices. Determining factors included the resistance caused from pushing the grooming tool across the surface of the ship, the hydrodynamic resistance of the vehicle itself as it moved through the water, friction losses in the shaft seals that were used to protect the motors from the saltwater, and track or wheel friction, dependent on which version of the unit was used.
“After extensive component research, we chose to use maxon motors and gearheads,” Dr. Holappa said. “Their motors not only provided a very cost effective solution, but they were highly efficient and extremely simple to implement.”
The company used EC Flat motors with planetary gearheads. Two motors were used on the tracked version of the HullBUG (one for each track), while four flat motors were needed for the wheeled version (one for each wheel). An additional EC Flat motor was implemented in the grooming tool. That motor used a simple spur gear for speed reduction. And a final motor, connected directly, was used for the negative pressure attachment device that held the HullBUG in place.