The 67-footer is one of 10 identical yachts designed and built, in steel, in Britain with the amateur crews paying pounds 14,800 to take part in the 28,000-mile round-the-world adventure. After Rio the course takes them against the winds and current round the Horn to Hobart, Tasmania, on to Cape Town and up the Atlantic to Southampton.
Tudor and crew had taken 29 days, two hours and 43 minutes since the start in Southampton on 26 September for an average of 7.2 knots over the leg.
Clare McKernan, a BSC section manager from Swansea, said: 'When I first enrolled on this challenge three years ago I had never been out sailing. Now I have raced over 5,300 miles against nine crews on identical boats and won.
'I was a confirmed, seasickness- suffering, landlubber 29 days ago, wanting to prove to myself that I could overcome my fear of boats,' she said. 'This is a pretty spectacular way of doing it.'
Second-placed Interspray was about 100 miles behind British Steel II and expected to finish last night. Heath Insured, the third boat in the breakaway trio which have held the lead since dodging the Doldrums by going close to the African coast, should be in tonight.
BRITISH STEEL CHALLENGE (First leg to Rio de Janeiro): 1 British Steel II; 2 Interspray (expected last night); (positions with miles to go at 08:00 GMT yesterday): 3 Heath Insured 296; 4 Group 4 525; 5 Hofbrau 539; 6 Pride of Teesside 541; 7 Nuclear Electric 547; 8 Rhone-Poulenc 635; 9 Coopers & Lybrand 645; 10 Commercial Union 1,453.Reuse content