Ship-shape

Researchers in the US are developing a flexible surface that will greatly reduce submarine noise and enable ships to travel faster and use less fuel.

Researchers at the University of California in San Diego are working with the US Navy to develop a flexible surface that will greatly reduce submarine noise and enable ships to travel faster and use less fuel.

The compliant coating, known as a tensegrity fabric, is being created by the Flow Control Laboratory at the university’s Jacobs School of Engineering, for the US Navy’s Office of Naval Research (ONR).

When ships and submarines are on the move, particularly when they changedirection, deflection of water from their hull creates water pressure fluctuations, resulting in vortices. These cause both drag and noise.

The coating works by dampening the turbulent flow of water near the vessel’s surface without requiring the use of sensors, computers or electricity, behaving like the skin of a dolphin to reduce the drag and noise. The skin is designed to absorb energy from turbulence by moving with it, and so it reduces the impact of the turbulence on the ship itself.

The UCSD team, headed by Prof Thomas Bewley, said the semi-permeable membrane can be stretched over a very thin, flexible substructure to form a system that passively changes shape in response to pressure and friction created as the water flows past the vessel.

The membrane must be semi-permeable to reduce the load on skin fitted to a submarine when the vessel changes depth.

Once placed over the solid hull of a ship, the compliant coating will reduce drag, allowing for higher speeds and less fuel consumption.

On a submarine, dampening the vortices that collide with the vessel’s surface will significantly reduce noise and improve the vessel’s ability to conceal itself from enemies.

The skin is formed from a number of geometric tensegrity cells, self-supportedpretensioned structures consisting of rods made from a lightweight composite material that are laid across each other and held in place by compression. The rods are interconnected by flexible Kevlar or nickel-titanium tendons held in place by tension.

The cells’ construction mimics spider silk, nature’s strongest material, which is several times stronger than steel when compared to its mass.

By interconnecting the millimetre-scale tensegrity cells, the research group created extensive sheets of material over which a semi-permeable membrane is stretched.

This is done by attaching the membrane to each of the top nodes of the interconnected cells beneath, or laying it across the top layer of tendons to increase the membrane’s support and reduce damage to the skin if the vessel hits an object.

In water the material adjusts its stiffness, mass, damping, and the orientation of its various rods and tendons in response to the continuous fluctuations of the ocean flow past the vessel, weakening the strength of the vortices’ flow.

Using computer modelling, the skin can be fitted to areas most likely to create vortices, such as the hull, propellers, and control surfaces, rather than being placed over the entire vessel.

This compliant skin can also be retrofitted over existing vehicles without costly redesign.

The UCSD team believes that the skin could be mass-produced using a modern textile machine with relatively simple modifications.