Percy and Simpson were forced to withdraw from the day's races and say they may not sail again at the Perth 2011 ISAF World Championships.
Percy had been suffering back pain after Monday's races but was determined to finish the first race today. However, when he and Simpson received their second yellow flag of the regatta, Percy was brought to shore for treatment.
Simpson said, "We had an okay run, but Iain hurt his back and was in tears. It looks like it's his lower back, but we don't know yet. No way could we have raced the second race.
"It's frustrating; sadly this is the second event in a row Iain has injured himself. He injured his ankle last time," he said. "I guess we won't be sailing, but we never rule anything out."
The paid lead the fleet coming into the third day but their absence has given the leader board a different outlook after six races.
Race 5 was won by Mateusz Kusznierewicz and Dominik Zycki (POL), who led for the entire course. Closely behind in second place were 2008 silver medallists Robert Scheidt and Bruno Prada (BRA) who were well clear of Germany's Johannes Polgar and Markus Koy in third.
In Race 6, Robert Scheidt and Bruno Prada had to settle for another second place after crossing the finish line just two seconds behind Swedish pair Fredrik Loof and Max Salminen. Coming in third and with only one second separating them from the Brazilians were Kusznierewicz and Zycki.
After six races, Scheidt and Prada move into first position overall. Mark Mendelblatt and Brian Fatih (USA) and Kusznierewicz and Zycki trail the Brazilians by only one point.
Star racing is scheduled to continue on Wednesday from 13:00 local time on the Leighton course.
Laser World Champion Tom Slingsby (AUS) has set himself apart from the rest of the fleet with three wins out of four races in competition.
Under ominous skies and on a day where one race went for 10 minutes before it was restarted because of a significant 20-degree wind shift, Slingsby was like lightning, streaking away from the fleet.
"I actually sailed well today. The first race was crazy. There was probably 10 knots difference around the course with pressure and at times I looked really good, then really bad.
"I was lucky coming into the top mark. I might have been 15th but got a nice left-hander right at the top and somehow got into second. Then the leader got a yellow flag on the run and it allowed me to get into the lead."
Slingsby was the only double winner and sits alone on single figures with nine points while his closest rivals are Paul Goodison (GBR), on 18 points and Nick Thompson (GBR) and Germany's Simon Groteluschen who are both on 19 points.
World #2 Thompson had to be content with shadowing Goodison in Race 4 for second place, after finishing eighth in Race 3.
Jean Baptiste Bernaz (FRA), who scored a second and 10th today, sits in equal fifth place with Sweden's Johan Wigforss.
With six races contested in the three fleets today, Slingsby covered the Blue Fleet; Goodison and Australia's Tom Burton dominated Yellow fleet; while Rutger van Schaardenburg (NED) and Pavlos Kontides (GRE) won in the Red fleet.
Laser competition is scheduled to continue on Wednesday from 12:00 local time on the Parmelia course.
After a hard day of dramatically fluctuating winds and more than 10 capsizes in the racing, Denmark's Peter Kruger Andersen and Nicolai Thorsell are the overall leaders at the end of the second day of competition for the 49ers.
Erik Storck and Trevor Moore (USA) finished first in Race 4 for the Yellow fleet, crossing the line 21 seconds ahead of Santiago Silveira and Philipp Umpierre (URU), who placed second.
Race 5 saw Allan Noerregaard and Peter Lang (DEN) win comfortably with David Evans and Edward Powys (GBR) second and Storck and Moore with another good race in third place.
After a complicated start with several boats called back to the line, Kruger Anderson and Thorsell (DEN) took out the sixth race a massive 101 seconds ahead of Jonathan Ladha and Daniel Inkpen (CAN), with Storck and Moore again in third place.
The first race of the day for the Blue fleet saw wind speeds soar to 23 knots in the last leg of the race which allowed Australia's Nathan Outteridge and Iain Jensen to maintain the lead for the entire race, crossing the line 42 seconds ahead of Emil Toft Nielsen and Simon Toft Nielsen (DEN).
Outteridge and Jensen had another win in Race 5, with the Nielsen brothers once again finishing shortly behind by a mere 11 seconds.
Wind changes caused huge dramas in the last race of the day resulting in constant course alterations, but this did little to hinder Stephane Christidis and Emmanuel Dyen (FRA) who crossed the line first, 21 seconds ahead of Will and Sam Phillips (AUS).
Favourites to win the last race, Outteridge and Jensen struggled from the start and ended up finishing 13th.
The 49er competition is scheduled to continue on Wednesday from 12:00 local time on the Owen course.
Israel's Nimrod Mashich heads the leader board after four races in the Men's RS:X event with two top ranked Polish sailors, including the World #1, just four points behind.
The second day of racing began with Dorian van Rijsselberghe (NED) leading a good distance ahead of the Blue fleet in Race 3, until a lapse led him off track. Dropping from a clear first place down to seventh after the third marker, he wound up in 15th position. The race went to Zachary Plavsic (CAN), the first win at the Perth 2011 World Championships for the Canadian RS:X team.
Rijsselberghe redeemed himself in Race 4 however, coming first, 15 seconds ahead of Tom Ashley (NZL). Piotr Myszka (POL) was third.
In the Yellow fleet, Poland's Prezemyslaw Miarczynski won Race 3. However Mashich maintained his overall number one position by placing second and winning Race 4.
The Men's RS:X event is scheduled to continue Wednesday from 12:30 local time on the Centre course.
Spain's Tara Pacheco and Berta Betanzos have wrapped up the second day of competition in Women's 470 with wins in Races 3 and 4 to claim top position overall.
Lisa Westerhof and Lobke Berkhout (NED) held on to their second place overall, while Japan's Ai Kondo and Wakato Tabata slipped from first to third.
After a number of changes at the front of the fleet early in Race 3, it was Pacheco who managed to hold on to the lead as they rounded the sixth and final mark before the finish.
Italy's Giulia Conti and Giovanna Micol (ITA) finished second, Penny Clark and Katrina Hughes (GBR) third, followed by Tina Lutz and Susann Beucke (GER) fourth, while Yuka Yoshisako and Noriko Okuma (JPN) rounded out the top five.
Great Britain's Clark and Hughes led the pack from the start in Race 4, followed closely by Israel's Gil Cohen and Vered Buskila as they rounded the first mark. But as the race ended, it was the Spaniards who again managed to get over the line for the win.
In Race 4, Israel's Cohen and Buskila finished second with Westerhof and Berkhout coming third. They were followed by Ai Kondo and Wakako Tabata (JPN) in fourth and Camille Lecointre and Mathilde Geron (FRA) in fifth.
The Women's 470 competition is scheduled to continue on Wednesday from 14:30 on the Success course.
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