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Freak wave threat from high speed ferries

MYSTERIOUS WAVES generated by the latest high speed ferries have prompted an urgent industry wide study to avert danger to seafarers, bathers or shoreline walkers.

Fears that lives are being put in danger by the wash from new high speed craft (HSC) have prompted the £1.2M study.

The Ship Wash Impact Management (SWIM) investigation is jointly funded by the Department of Trade & Industry and many others involved in the HSC industry. The work hopes to identify the effects of the freak wave wash and find practical solutions for ship operators.

Tragedy occurred when a man fell from a boat in calm conditions and drowned after being hit suddenly by huge 4m high waves generated from the Stena Discovery HSS vessel over a sandbank in the English Channel off Harwich in July 1999.

A Marine Accident Investigation Branch probe concluded that wash generation from such twin hulled craft was 'complex and is not yet fully understood'.

'Fast catamaran ferries are capable of producing abnormally long period waves - much longer than those which occur naturally in the sea. They are not very high - around 0.4m when out to sea in deep water - but with a long period they move very fast and are very energetic, ' said Queen's University Belfast coastal engineering professor Trevor Whittaker, who is involved in SWIM.

In shallow water, such waves can suddenly reach heights of over 4m, posing a serious risk to unsuspecting water and beach users and small vessels.

'Because the waves are small in deep water, people don't see them coming; then they suddenly rear up and break. It is the lack of warning that is the danger, ' said Whittaker.

The 20,000t HSS fleet pioneered by Stena in 1995 travels at a speed of 40 knots, almost twice the speed of conventional ferries. Their revolutionary design results in high energy displacement of water.

The speed of the vessels can mean the waves they generate may occur some time and distance from the ship. Studies by Whittaker for the Maritime & Coastguard Agency into HSS effects at Belfast Lough revealed that waves capable of swamping small boats can arrive 30 minutes after the first wave is generated.

Dr Jonathon Williams, managing director of consultant Marinetech south which is coordinating the SWIM study, said that finding a solution depended on seabed depth and features, coastline topography, vessel characteristics and water depth, which varies with the tide.

'Slowing down vessels is not a solution and can make matters worse, so blanket speed restrictions are not an option, ' he said.

The study team, which includes consultants Kirk McClure Morton and Posford Duvivier, is also examining the environmental effects of abnormal waves on the coast and its defences.

One outcome of the project may be the development of a mathematical model which ship captains could access by computer, giving them a safe speed and approach operating envelope for given conditions.

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