Find my past has 3 merchant navy records for an Andrew Renwick. Unfortunately I no longer have a subscription so can't see if any are captains.
The record in the "Merchant Navy Crew Lists 1861-1913" section is for an Andrew Renwick born in Sunderland c1845. He's a carpenter on the vessel Khalif, however the dates are August to November 1891. Louisa said on the '91 census she was a widow at that point.
In the "Britain, Merchant Seamen, 1835-1857" section, there's a 21 year old Andrew Renwick from Newcastle. The document has boxes for voyages from 1853 to 1857. In the 1853 box it says:
Out
S 10 - 12
British Empress
Shields
1854 is blank, 1855 says:
Out
AB 3 . 7
Newcastle
of N'Castle
Home
AB 21 . 6
Margaret + Ann
N. Shields
1856 says:
Home
AB 10 - 9
Hotspur
N'Castle
J Akyab 14/3
Dis Antwerp 11/8 (this number is written on the dividing line and hard to read)
____
AB 14 - 10
Newcastle
Newcastle
D. Akyab 11/3
The last document is from even earlier, 1845, and just has a list of names and register ticket numbers. There's four Renwicks that have been deliberately grouped together seperate from the rest of the list and headed "Renwick + Renwick"
530147 Renwick Wm Newcastle
558923 Renwick Wm N'castle
374259 Renwick Jas St Helens
352764 Renwick Andw N'castle
Don't know if any of that is of help at all, but there you go.
EDITA ships captain called Andrew Renwick!
SERIOUS CHASE AGAINST A CAPTAIN. Andrew Renwick, master of the Elizabeth and Ann (Ls.), of this port, was charged with stealing a tarpaulin, of the value of £l, the property of the Great Eastern Railway Company. Mr. Oakley prosecuted for the Company B ...
Published: Saturday 18 March 1882
Newspaper: Lowestoft Journal
County: Suffolk, England
Interesting, Lowestoft comes up again.