ALAIN Prost, driving a Williams-Renault, won his 47th Grand Prix with an emphatic victory in Barcelona. The Frenchman's victory was a formality from the moment teammate Damon Hill of Britain withdrew in mid-race with engine trouble. Brazilian Ayrton Senna finished second in a McLaren, with Germany's Michael Schumacher third in a Benetton. Italy's Riccardo Patrese was fourth in a Benetton, with American Michael Andretti fifth in a McLaren and Austria's Gerhard Berger sixth in a Ferrari. Up until his withdrawal, Hill was the one and only challenger to Prost's supremacy as the two Williams drivers once again demonstrated their cars' superiority. Hill, anxious about the threat of Senna on the grid, got the good start he needed, streaking ahead of Prost. Prost attributed the slow start from the 25th pole position of his career to his automatic gearbox failing to change up from first to second, forcing him to operate manually. He said: ''This cost me some time. But Damon made a good start and drove very fast.'' However, the Englishman was overtaken on the 11th lap by his more experienced teammate, and Prost gradually inched away to build up a four-second cushion. Senna, meanwhile, held a distant third, no thanks to Erik Comas' reluctance to let the Brazilian pass. Traffic on the tight Catalonia course put obstacles in the way of Prost's Williams-Renault, and the Frenchman struggled to lap Derek Warwick (Mugen-Honda), Martin Blundell (Ligier-Renault) and Alessandro Zanardi (Lotus-Ford). The congestion allowed Hill to close in on the lead, getting in behind Prost's slipstream to twice attempt a takeover. The ''professor'' was having none of it, failing to return the junior driver's earlier courtesy in giving up his lead and eventually Hill lost his battle when he was forced to retire on the 14th lap. That left Prost with a 22-second lead over Senna and he was able to win with relative comfort as the Brazilian was affected by visibility problems - he said four or five cars blew engines in front of him - and Schumacher by severe vibrations and gearbox trouble. Prost said: ''I knew I was braking later than Damon so I was able to pass him, but he pushed me very hard and I felt tired. Physically, it has been one of the hardest races I have known. ''I couldn't drive much quicker than that. ''When Damon went I knew I could go through without changing tyres and I just concentrated on bringing the car home on the road.'' His decision not to change his tyres demonstrated his composure once he had shaken off Hill's challenge. ''I'd almost decided that if he'd overtaken me, I would have to stop,'' Prost revealed afterwards. The need for that disappeared as a mystery mechanical fault suddenly robbed Hill of power and he drifted frustratingly into the pits. This enabled Senna to assume second place, a position briefly threatened by Schumacher in a Benetton before the German ran into oil on the last few laps and lost time struggling to remain on the track. Patrese eased the pressure on himself after a disappointing start to the season by finishing fourth. There had been speculation that Nigel Mansell was considering a return from Indycar to join Benetton with Patrese making way. Senna's McLaren teammate Michael Andretti's fifth place gave the American his first points, and first finish, in Formula One.