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Saturday, 10 February, 2001, 07:13 GMT
Danger from the deep
Submarine
Submarines are a hidden threat to smaller craft
Collisions between submarines and surface vessels are an unfortunate inevitability given the world's increasingly crowded oceans.

Submarines operate in many busy shipping lanes, and with their immensely strong hulls - built to withstand huge underwater pressure - will always come off the better in the event of a collision.

About 25% of all recorded collisions involving British and American submarines took place in coastal waters, either when a submarine was berthing or in the mouth of a harbour.

Even huge supertankers are not safe from the hidden threat.

In 1993, the French submarine Rubin collided with a supertanker in the Mediterranean. The ship was damaged and oil leaked into the sea.

But incidents involving smaller vessels - often fishing boats - are more common and far more likely to involve loss of life.

Deadly catch

In 1989, the USS Houston snagged a towing cable and sank the commercial tugboat Barcona, 10 miles off Long Beach, California. One crewman drowned and two more were rescued.

By the nature of their work, fishing boats run the risk of making a far deadlier catch.

Every year, several instances are recorded of boats being dragged along the surface for miles after ensnaring submarines in their nets.

The results are sometimes fatal - in the Irish Sea from 1980 to 1989, at least 17 trawlers from various countries disappeared without trace claiming 37 lives, often in calm waters.

The levels of secrecy submarines operate under, and the fact that they spend 90% of their time under water, means that many accidents are never properly investigated.

But the US Navy alone receives claims for thousands of dollars of compensation from trawlermen who have had to cut their nets to escape being dragged under.

Sonar

These accidents occur as the result of the way in which submarines operate.

By using active sonar, they are able to accurately identify other vessels around them.

However, active sonar gives away the submarine's position - so they often rely on passive sonar.

This device provides information which is difficult to interpret, especially if the vessel is carrying out a series of manoeuvres.

The use of passive sonar at shallow depths can mean that submariners are as blind to the threat of collision as those on the surface.

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