Looking to air

Since William Froude discovered a ship?s resistance could be divided into frictional resistance and residual resistance much research has focused on reducing the impact of these two elements.

Dr CH Thill, project manager for ships powering at Marin, the Dutch research institute, says computational fluid dynamics (CFD) tools have largely brought the residual resistance, commonly understood as wave-making resistance plus form effects, under control such that CFD can benefit even experienced vessel designers.

If a ship?s performance is to be improved still further other techniques have to be used to reduce the remaining parameter, that is friction or drag, he says (see table). Although the table does not cover all techniques, it does indicate three with promise. These are:

l Air cavity ships

l Micro bubble drag reduction

l Air film and highly water repellent (HWR) coating

The Dutch government is funding research into the first two techniques within a project called Project on Energy-saving air-Lubricated Ships (PELS). There are ongoing discussions to get the third technique included. The project lasts for three years and includes five Dutch shipbuilding companies, two shipbuilding organisations as well as Marin.

Research aims

Recent research has demonstrated the savings under laboratory conditions, but the aim of PELS is to investigate its practical applications to ships, not to deepen the research. This includes investigating the manoeuvring and seakeeping performance for the first time ever, says Thill.

The first year of the project has produced a fully-non-linear potential-flow-based CFD code that can cope with the free surface inside the cavities, under an arbitrary constant pressure. The CFD code allows optimisation of cavity design at an early design stage. “The experience with this code is promising,” says Thill.

PELS also aims to develop airflow control procedures. Currently, first tests in the larger water circulating tunnel at Marin are being conducted, aimed at validation of the basic principles by measurements on simple bodies with Laser Doppler Velocimetry measurements of the boundary layer with and without air lubrication.

As PELS shall not be restricted to one type of ship, a more or less ship-alike design, will be tested on resistance, propulsion and manoeuvring in calm water and free sailing in a seaway, says Thill. In particular, the free sailing tests will assess the controllability of airflow, the maintenance of the cavity or film and the applicability of air lubrication on ships under any operational condition.

Drag reduction techniques and their impact on efficiency (source: Marin)

Technique Possible drag Problems in naval application reduction [%]

Aeronautical techniques <20 Different Reynolds number

Air cavity 80 Air control and stability

Air film and HWR coating 90 Control at vertical side walls

Bionics ! Lack of rotating propulsors

Boundary layer suction 20 Is applied to some extent (eg by the efficiency increase caused by a working propeller)

Compliant coating <7 Maintenance, earnings and ageing

Magneto Hydrodynamics ? Efficiency and complexity

Micro bubbles 80 Control and relaxation

OLD/LEBU 7-40 Bio fouling and slamming

Oscillating hull surface <45 Mechanical complexity

Polymer additive <80 Environment

Riblets 7-10 Bio Fouling

Slipping skin 9 Mechanical complexity

Transition delay 0 Too high Reynolds number of ships

Vortex generation 0 Effect not measurable