Rowing World Cup: The line between stunning success and failure can be a thin one.

Ireland's lightweight four reached new levels of excellence to win their World Cup final in Poznan, Poland on Saturday and the lightweight double scull of Niamh Ni Cheilleachair and Sinéad Jennings took bronze to confirm they, too, have a bright future. But the men's heavyweight four faltered in their A final to finish fifth. They shall not race again.

Ireland coach Harald Jahrling put together all three crews with the aim of finding the best team for the World Championships in August, and while the medal winners will travel on to the final World Cup in Lucerne next month up amongst the favourites, the heavyweight crew goes back into the mix. All six heavyweights will go on, but "we will have a different four," said Jahrling.

The lightweight four of Gearóid Towey, Eugene Coakley, Richard Archibald and Paul Griffin produced one of the great Irish performances at an international event in their final. Egypt and Britain set the early pace, but just after the 500-metre mark Griffin laid down a blistering pace which made the others look like they were in a lesser race. Australia put a good final quarter to finish second and Egypt took the bronze

"That was racing perfection," said Jahrling of Ireland's middle 1,000 metres. "I didn't think the first 250 or 300 metres was so great and the finish was a bit scrambly. But through the middle - that was fantastic racing."

Griffin tentatively agreed it was this crew's best row together. "Yeah, I suppose it was," said the Kerryman. "We had a certain plan and it's always good when you execute (a plan) under pressure. And we executed most of it today. There's a good bit of work to do. It could have been better - there's a good bit of polishing to do. Now is not the time to be going fast, not at our absolute best. It's just August (the World Championships), that's our aim."

Ní Cheilleachair and Jennings exploded into contention for honours in August with their row - although Saturday's race was predominantly about China. Their two boats took gold and silver and both finished inside the old world best time. For much of the 2,000 metres Ireland seemed to be the only other crew in the contest.

Dongxian Xu and Shimin Yan seemed to win as they pleased, but Ní Cheilleachair said the Irish targeted the second Chinese boat, of 15-year-old Xuefei Fan and Haixa Chen, and hoped to take them if they faltered. "We were going to take the opportunity if it arose. We weren't just happy to sit in front of the others," she said.

This was a first medal in an Olympic event at this level for an Irish women's crew, but perhaps more importantly, it is the first time the power and sheer physical courage of Jennings has been harnessed in a crew boat. "It's a brilliant confidence (boost) to know Sinéad's behind you. She can just keep going and going and going," said Ní Cheilleachair.

"Today we raced at a higher rate than we had before, but there was never a doubt that either of us could do it. No matter what rate I set Sinead would pull the same the whole time."

The Chinese mixed an matched their crews in Poznan and still produced the goods, but only one Chinese crew will be allowed at the World Championships in Eton.

Xuefei Fan's emergence at such a young age is remarkable. "It seems a miracle to me that a 15-year-old can row that fast - in a strength (and) endurance sport," said Jahrling. "But, I don't know, that's up to them, you know."

The German had good news for Caroline Ryan, the other big Ireland success story in Poznan. The rising graph which saw her take second in the B Final has booked her a place in Lucerne.

However, the heavyweight four of Cormac Folan, Sean Jacob, Con Collis and Alan Martin will definitely not move on to the next stage. The pair of Sean Casey and Sean O'Neill could only take fifth in their B Final, and now all six heavyweights will be tried in different combinations.

"We will bring that squad together again and look at new combinations. And we will have a different combination together on the water in Lucerne," said Jahrling.

Crews that cannot reflect well on their country at World Championships will not get sent. "You have to be at that level in order to be allowed to compete at it. We don't want to disgrace ourselves," said Jahrling.