Thomas joined the Royal Navy in April 1903 and was a Portsmouth rating. He was recorded as 5'4" with dark brown hair, hazel eyes and a dark complexion when he joined, but supposedly grey eyes and a pale complexion when he left the service. He had a shamrock tattoo on his left arm when he joined and tattoos on both arms when he left the navy.

The ships and shore establishments he served on are listed below:
HMS Northampton 26 Apr 1903 to 15 Aug 1913 (Cruiser/Training Ship)
HMS Calliope 16 Aug 1913 to 7 Oct 1913 (Cruiser/Training Ship)
HMS Victory 8 Oct 1913 to 2 Nov 1913 (Portsmouth - Shore Base)
HMS Centurion 3 Nov 1903 to 25 Aug 1905 (Battleship) ^[1]
HMS Vernon 26 Aug 1905 to 20 Jan 1906 (Training Ship)
HMS Victory I 21 Jan 1906 to 22 Feb 1906 )Portsmouth - Shore Base)
HMS Furious 23 Feb 1906 to 14 Jul 1906 (Cruiser)
HMS Excellent 15 Jul 1906 to 30 Sep 1906 (Portsmouth - Shore Base)
HMS Victory I 1 Oct 1906 to 10 Nov 1906 (Portsmouth - Shore Base)
HMS Mercury 11 Nov 1906 to 2 Sep 1907 (Submarine Depot Ship) ^[2]
HMS Edgar 3 Sep 1907 to 9 Oct 1907 (Cruiser)
HMS Powerful 10 Oct 1907 to 12 Jan 1910 (Armoured Cruiser) ^[3]
HMS Terrible 13 Jan 1910 to 24 Feb 1910 (Armoured Cruiser)
HMS Victory I 25 Feb 1910 to 2 Apr 1910 (Portsmouth - Shore Base)
HMS Vernon 3 Apr 1910 to 15 Jun 1910 (Portsmouth - Shore Base)
HMS Excellent 16 Jun 1910 to 3 Sep 1910 (Portsmouth - Shore Base)
HMS Mercury 4 Sep 1910 to 25 Jun 1911 (Submarine Depot Ship)
HMS Bonaventure 26 Jun 1911 to 14 Oct 1912 (Cruiser/Submarine Depot Ship)
HMS Maidstone 15 Oct 1912 to 12 Dec 1915 (Submarine Depot Ship) ^[4]
HMS Dolphin (E5) 13 Dec 1915 to 6 Aug 1916 (Portsmouth - Shore Base/Submarine E5)
HMS Titania (Sub J2) 7 Aug 1916 to 9 Jan 1917 (Submarine Depot Ship/Submarine J2)
HMS Dolphin 10 Jan 1917 to 30 Apr 1917 (Portsmouth - Shore Base)
HMS Fearless (K1) 1 May 1917 to 18 Nov 1917 (Cruiser/Submarine Depot Ship/Submarine) ^[5]
HMS Dolphin 19 Nov 1917 to 28 Mar 1918 (Portsmouth - Shore Base)
HMS Fearless (K15) 29 Mar 1918 to11 Feb 1919 (Cruiser/Submarine Depot Ship/Submarine)
HMS Royal Arthur (K15) 5 Feb 1919 to 16 Oct 1919 (Submarine Depot Ship/Submarine)
HMS Dolphin 12 Feb 1919 to 18 Mar 1919 (Portsmouth - Shore Base)
HMS Victory 14 Jul 1920 to to 15 Aug 1920 (Portsmouth - Shore Base)
HMS Inconstant 26 Aug 1920 to 16 Feb 1922 (Cruiser)
HMS Conquest 17 Feb 1922 to 30 Aug 1923
HMS Victory I 31 Aug 1923 to 31 Mar 1925
HMS Vernon 1 Apr 1925 to 3 Apr 1925
HMS Dido 4 Apr 1925 to 31 Jan 1926
HMS Victory II 1 Feb 19926 to 25 Sep 1926 (Shore Pensioned)
I note that the period between 16 October 1918 and 12 Feb 1919 isn't recorded on his file.
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^[1] Centurion underwent a reconstruction at Portsmouth, including a partial rearmament, between September 1901 and November 1903. On 3 November 1903 (the day that Thomas Joined her) she commissioned there for another period of service on the China Station. Departing Portsmouth on 10 November 1903, she called at Malta on 17 November 1903, at Port Said on 25 November 1903, at Aden on 6 December 1903, at Colombo, Ceylon, on 15 December 1903, and at Singapore on 27 December 1903 before arriving at Hong Kong on 31 December 1903.
In 1905, the United Kingdom and Japan ratified a treaty of alliance, reducing the need for a large Royal Navy presence on the China Station. The British squadron there was reduced, and all its battleships were withdrawn. Accordingly, on 7 June 1905, Centurion, in company with battleship HMS Ocean, departed Hong Kong. At Singapore, they rendezvoused with battleships HMS Albion and HMS Vengeance. The four battleships departed Singapore on 20 June 1905 and steamed in company to Plymouth, where they arrived on 2 August 1905. Centurion paid off at Portsmouth on 25 August 1905.
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^[2] I believe Thomas transferred into submarines in September 1910 when he was first assigned to the submarine depot ship, HMS Mercury.
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^[3] I believe that during Thomas' time aboard the Powerful, it spent it's time on the Australian Station.
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^[4] In Dec 1915, Thomas was drafted to the submarine depot ship HMS Maidstone and served for much of that time on the submarine E5. In June 1913, an explosion occurred inside E5's Main Engines, resulting in the death of one officer and two ratings. In addition, nine men were badly burned. Thomas left the E5 in Dec 1915. In Mar 1916, the E5 failed to return from patrol and was presumed to have been sunk and possibly hit by a mine. Thomas clearly survived beyond that time so I Imagine that was one patrol that he didn't go on.
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^[5] Thomas' next submarine was the K1 which he joined in May 1917. During an operation off the Danish coast, K1 had been patrolling on the surface as part of a flotilla of submarines operating in line ahead. The flotilla was led by the light cruiser HMS Blonde followed by submarines K1, K3, K4 and K7 at the rear. Blonde turned sharply to avoid three cruisers which crossed her bows, which caused confusion amongst the following submarines and in the maneuvering, seawater entered the boiler room through the funnels and put the boiler fires out (a common problem in K class submarines). The loss of steam pressure caused the K1 to slow down and although K3 just missed her, K4 struck a blow alongside the conning tower which pierced the hull. Seawater poured into the control room and reached the batteries under the floor causing chlorine gas to be produced. The 56 crew members were transferred from K1 to the cruiser and it was decided that K1 could not be saved, so she was sunk by gunfire from the cruiser.
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He became an Ordinary Seaman on 26 Oct 1903 and was promoted from Ordinary Seaman to Able Seaman on 8 Dec 1904. We can also see from his naval record that Thomas qualified as a navy diver on 22 Sep 1906.

After leaving the Royal Navy, he signed on for the Fleet Reserve on 26 Sep 1926 until 28 Sep 1938. Throughout his career his conduct was identified as "VG Superior".
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We see Thomas in the 1911 census living at 70 Kilmiston Street, Landport, Portsmouth, Hampshire, England:
Thomas Knibbs Head Married M 25 Able Seaman RN Old Windsor, Berks
Mabel Knibbs Wife Married (4 years) F 26 Portsea Hants
Frederick Knibbs Son M 3 Portsea Hants

We see Flora's parents and two older siblings living at "Fort Brockhurst" With Married Quarters, Alverstoke, Hampshire, England:
Joseph ALLEN Bombardier (Head) M Male 40 Cornwall, England Bombardier Roy Arty
Charlotte ALLEN Wife M Female 28 West Meon, Hampshire, England
Earnest ALLEN Son U Male 5 Hambledon, Hampshire, England Scholar
Louisa Maud ALLEN Daughter U Female 1 Portsmouth, Hampshire, England
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We can see Flora in 1891 living at 11 Stoke St., Portsea, Portsmouth, Hampshire with her parents and siblings:
Joseph Allen Head 48 Car Driver Cornwall Truro
Charlotte Allen Wife 35 West Meon Sussex
Louisa M Allen Daur 11 Scholar Hants Portsmouth
Grace F Allen Daur 8 Scholar Hants Portsmouth
Ethel M C Allen Daur 6 Scholar Hants Portsmouth
Joseph H Allen Son 2 Hants Portsmouth
Flora B Allen Daur mnths Hants Portsmouth