Some years ago while operating as flight mech I noticed that the captain smelled of alcohol and appeared slightly detached - his F/O and F/E did not seem to be concerned so I felt that I should keep quiet but was not happy with the situation. It was a long haul 747 passenger flight and we completed the flight without any problems as far as I could see.Shortly after this I was departing the same flight when our customers duty operations officer prevented the same captain from boarding and that was the last I saw of him.Speaking to some of the cabin crew it was common knowledge that he seemed to have a problem.I have always felt that I should have spoken out but with the attitude of the rest of the crew I think it would have fallen on deaf ears.