The hire boat Liberty, the most exclusive on the Broads costing �3000 a week in high season, was damaged in the collision and was drifting towards Haven Bridge when the family of Germans on board - a mother and father and two sons - dialled 999 at about 4am yesterday.

The father, an experienced helmsman, tried to start the engine but the trailing ropes fouled the twin propellors.

PC Paul Bassham, from Norfolk Constabulary's Broads Beat team, described the action of the vandals who untied the ropes at Yarmouth yacht station as 'reckless' and 'life threatening'.

He said: 'They were in deep sleep but the father was woken by a knock which was probably caused by the boat hitting the first road bridge.

'He got up after a second knock and realised what was happening when he saw street lights moving.'

The collision with the lower Vauxhall Bridge, a foot and cycle access from the station, tore down the vessel's radar arch causing damage up to �1000.

He said thankfully the boat grounded on a mud bank otherwise it might have been caught by the treacherous current and crashed into Haven Bridge.

PC Bassham said: 'This was a very frightening experience for the holidaymakers onboard. Fortunately, one of the passengers is an experienced boat handler and was able to take control of the situation and call the coastguard for help.

'To remove mooring ropes from a vessel is completely irresponsible and not only endangers the lives of those on board but the lives of other river users.

'I would urge anyone who may have witnessed people acting suspiciously in the area of Tar Works Road between 3am and 4am to contact police.'

He said they were currently trawling through CCTV for evidence.

Peter Reeve, yard manager at Horning Ferry Marina which bought Liberty, a luxury Mediterranean style cruiser, over the winter, said: 'Once the father realised what was happening he woke up the rest of the family and they all put on their lifejackets.

'We are thankful we spend up to two hours talking hirers through the controls and safety equipment on their boat.'

He said it was a very dangerous part of the Broads and the incident could have turned out worse for less experienced boaters.

One propellor was cleared at Yarmouth and the second put back in action at Acle after a diver was called out.

Mr Reeve said the family, who live in France, were continuing their week-long holiday and were in remarkably good spirits considering their ordeal.

'We offered to put them up in one of our riverside properties but they wanted to carry on and said it was just something that happened,' he said.

Adrian Vernon, the Broads Authority's head of ranger services, said: 'We will be talking to the council and the police to see what further measures we can take to improve safety at the yacht station and we will be discussing it at our boat safety management group next month.

'The police have some suspects and if they are caught we will be doing some educational work with them. We will explain the dangers and implications of their actions, not just to the culprits but to their circle of friends and in the area where they live.'

Anyone with information should contact PC Bassham at Broads Beat on 101 or call Crimestoppers anonymously on 0800 555 111.