Churchill appointed him to the "Royal Anne" at Portsmouth, and made him a lieutent in the marines. Forbes was midshipman on HMS "St George" in 1704, and was with George Rooke at the Capture of Gibraltar, where he was on shore as aide-de-camp to the Prince of Hesse-Darmstadt and in the Battle of Málaga the same year. Forbes became heir to the earldom on the death of his elder brother, a captain in the Scots royals, from wounds received at the Battle of Blenheim.