RN Certificates Of Service
These can be obtained if you know the name or service number of your ancestor and they served in the Royal Navy between about 1840 and 1900. The NA site suggests the date range to be 1873 to 1923 but the dates returned by searches don't seem to tally with this.
The information that can be obtained from a Certificate of service (form S61 prior to 1896, form S536 after 1896, form S459 after Aug 1939, or a copy would be on Form S1213) is very useful and includes DoB, place of birth, trade before joining, height and other distinguishing features, ships served on, courses attended, conduct, promotions etc etc.
An example of such a record downloaded from the NA is shown below:
image copyright the National Archives