BoatsExplained.com - Radar Reflectors & Accidents

On 13 March 1997, the Tasmanianbuilt, wave piercing dtamaran Catlink II, with a maximum top speed of 36 knots and a carrying capacity of 125 cars and 583 passengers, was on her regular service between the Danish ports of Kalundborg and Arhus. 0942 that day, with visibility assessed at between 0.5 to 0.75 of a mile, she collided with, and sank, the 13.4 GT wooden fIshing vessel Lissi. The Skipper, who was the sole occupant of the fishing vessel, survived and was picked up by the catamaran. The catamaran's modus operandi at sea was for Master and Mate to man the bridge control desks with the vessel being steered from the centre seat. 'I\vo radars were fItted. On the day of the accident the Master used a gyro stabilised, true motion, X-band set at 1.5 miles but off-centred so he could detect contacts at a range of 2.5 miles ahead. The Mate had a DGPS stabilised S-band radar set to a range of three miles and off-centred so that it was possible to see fIve miles ahead. The cat's speed was 34.5 knots and no fog signals were being made.
Ahead of them Lissi was moving through the water at about three knots, showing lights and shapes to signify she was trawling. In addition a floodlight was shining astern towards the trawl. The Skipper was in the wheelhouse with the radar switched on as an aid to navigation.
He was unaware of the catamaran's high speed approach from astern. Lissi was fItted with a radar reflector constructed of plates at right angles which had limited reflective properties for X-band radars and virtually none for S-band.
Catlink II's Mate detected two echoes on the radar at a range of four miles and reported them to the Master. The Master became aware of a collision.warning on his radar from one of the two contacts and altered course to port to avoid them. The avoiding action having been taken, the Mate sought the Master's approval to leave the bridge to visit the toilet. The Master, now watchkeeping alone, continued to monitor the very clear computer generated plotting symbols superimposed on the synthetic radar echoes and assessed he was passing clear.Meanwhile he
divided his lookout responsibilities between monitoring both radar displays and keeping a visual lookout. Each time he looked up, he automatically switched on the wipers to remove spray from the windscreen. An independent witness states Lissi became visible at a range of 1000 metres. Neither Master nor Chief Engineer saw it until it had closed to less than 100 metres. Despite taking immediate corrective action, it proved impossible to take the way off, or alter course sufficiently, to avoid a collision. The catamaran's starboard pontoon hit the fIshing vessel.
The investigation showed that the Xband radar's relatively slow rotational speed (24 rpm), together with delays and inaccuracies in the gyro and log inputs, led to large errors in the calculated position and speed of the echoes, so much so that they could not be followed automatically at times. It was found to work more effectively when functioning without the various data inputs.
The performance of the S-band radar was also affected by a number of vertical surfaces in the vicinity of the antenna and the presence of an aluminium box which had been specially installed forward of the antenna. The display suffered from excessive clutter as a result.
Reconstruction of the accident revealed that reliable course and speed data on radar targets had been degraded after course alterations by the catamaran.
The accident investigators drew a number of conclusions - some of which, such as their comments on the wisdom of travelling through fog at 35 knots - are pretty obvious.
They also pointed out that 'It should not be assumed that small boat radar reflector echoes will give adequate warning. Small boat owners and skippers should be made aware of the limitations of many radar reflectors and advised on what is most effective.'
An interesting point is that the Mate - using the S-band radar saw two contacts, while the Master - using the X-band set could only see one. He altered course to avoid the one he could see... and hit the other.

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