I must thank Steven MacIssac for drawing my intention to an area that the site has so far neglected, Impressments.
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When I was last in London, May 2014, I came across ADM 6/417 which listed Americans in the Royal Navy, who declared themselves, and what happened to them - unfortunately the file is 241 mb. but if you suspect your ancestor was impressed then it is worth looking.
The information given was their name and the ship they served on and how they were disposed of - by that it seems to have meant sent to prison. If an ancestor was found on the list it would be then possible to trace his previous career through the muster rolls of the vessels he served on.