If you are not familiar with the Immigrant Ships Transcribers Guild, you are in for a treat. This collection of transcribed ship passenger lists is volunteer driven, and may reveal a passenger name that has been alluding you.
You can search by passenger name, or browse by ship name,. In addition you can browse by special ship grouping, port of departure, port of arrival and captain's name.
If you find your ancestor in these transcriptions, then you have the information you need to seek out the original passenger list.