This week's swim competition in Sydney will be the last chance for Marcus Cheah and Nicholas Tan to qualify for August's Beijing Games.

But the swim for these Olympic hopefuls will be an upstream one, as they will have to produce their best times without their regular coach.

For Jack Simon, 70, who coached them, served his last day as head coach of The Grassroots Aquatic Club on Monday.

In a seemingly desperate move, the Singapore Swimming Association (SSA) appointed 21-year-old national swimmer Leonard Tan to replace Simon late last week.

Simon has 46 years' coaching experience, including 25 with the United States national team.

Tan, who has less than three months' coaching experience as Simon's assistant, will oversee the team at the Telstra Grand Prix 2 event from July 4-6.

Said Nicholas, 18, who is just 0.08 second off the men's 100m butterfly Olympic B qualifying mark (54.71s): 'I'm not as comfortable as I would be if I had an experienced coach going with me.'

Nicholas and Tan were teammates at last year's South-east Asia Games.

Nicholas' mother Eunice said: 'The kids have trained so hard. Nicholas is so close to qualifying. It will be a shame if he doesn't make it because his regular coach is not with him.'

Yesterday morning, led by Tan, the Simon-coached trio of Cheah, Nicholas and Quah Ting Wen, 15, and Russell Ong, 18, left for Sydney.

'Imagine the intensity the boys are going to feel,' said Ting Wen's mother Anne, whose daughter is the only one among the four who has qualified for the Olympics.

Jeffrey Leow, the SSA's president, believes that Tan is the best person to be with the swimmers on the trip.

'He has had the most direct access and familiarity with Marcus, Nicholas and Ting Wen,' said Leow.

'Other than Jack, Leonard knows their past two months of workouts best.'

Temporary help, though, will be available in Sydney in the form of renowned Australian coaches Michael Bohl and Ken Wood.

The SSA, with help from the Singapore Sports Council, has arranged for Bohl, who coaches 200m individual medley world record-holder Stephanie Rice, to help Tan out at the meet.

John Dempsey, the former head coach of the now-defunct SSA Centre of Excellence (COE), has persuaded his friend Wood to assist the swimmers at the meet, after Cheah, Ting Wen and Nicholas, in desperation, called Dempsey for help.

'Leonard is a lovely man and one of my former swimmers, but he's a novice coach,' said Dempsey over the phone from Hong Kong, where he is now a club coach.

'I have a long history with the trio and feel obligated to help them out. Ken Wood has graciously agreed to assist them as much as he can.'

Wood will be in Sydney with his Brisbane club swimmers, including 200m butterfly world record-holder Jessicah Schipper and Australian 10km and 1,500m champion Melissa Gorman.

Dempsey has briefed Wood on the trio's performances and training and racing routines.

Simon's departure marks the second time in three months that the trio have been left in the lurch.

Dempsey left in March, just before the four-year-old COE at Toa Payoh closed.

Some observers felt the Australian had been asked to leave as he was not part of the SSA's plans.

Dempsey felt it was unfortunate that the trio's preparations have been less than perfect.

'But I'm confident they'll do the best they can,' he said.

jwang@sph.com.sg