significance of "7 years on board a King's ship"

[LouisaHills]

So if I'm looking for a HMS, that would just be navy or prison hulks - not a merchant ship - is that right?

His imprisonment 1832 to 1833 was Millbank in London. He lived everywhere from Hull to Westminster to Wrexham, which is impressive in a time before the railways, but makes him hard to track. I only know it's the same person each time because his insolvency records list all these different cities under the same name. Liverpool would have been the nearest port to his birthplace, it was where he was insolvent in 1821 and where he married in 1830, so it seems the best bet for this 'king's ship' clue.

[Rena]

So if I'm looking for a HMS, that would just be navy or prison hulks - not a merchant ship - is that right?

There were two methods of knowing you were looking at a "king's ship".   A King's ship" flew a white ensign flag (white duster) on its  masthead and a merchant ship that transported goods had a red ensign duster (red flag) flying from its masthead.

There's a list of royal navy ships on wikipedia and I see that the first king's ship that carried the name HMS Achilles (named after the Greek hero Achilles) was built in 1747.

Depending on the era, privately owned merchant ships would have SS before the name of the ship.  During the time of ships with sails "SS" meant "Sailing Ship", and Steam Ships with funnels and steam engines would have an SS before their given name.

I think you'd have a better chance of finding him if you knew the year of his birth, because from the details you've given, he obviously learnt a trade at sea when he was a youngster .  You've given his name as Edward Edwin Blythe, but we doln't know which name he chose to use on the crew lists (was it Edwin, Edward, Ted, Eddy?).  Unless you can discover what ship he sailed in I doubt you'll find his able seaman records. 

I notice you list quite a few ports of call, if that was when he was a sailor, you could try a long shot and look in newspapers of the day which always listed all the ships arriving and departing the local port.  The first two names will be the name of the ship and then the name of the ship's captain also given will be the port it left and the next port it will be sailing to..

[a-l]

I thought that phrase meant served a 7 year apprenticeship.