Displacement: Between 1090 tons, surfaced to 1575 tons submerged
Dimensions: Length - 270ft    Beam - 26.5 ft      Draft - 12ft
Speed:  Surfaced - 16 knots.  Dived - 9 knots
Armaments:  1 x 4" Gun and 6 x 21" torpedo tubes
Complement:  Five officers and 48 men 

    "Thetis" was launched on June 26th 1938 and was 
Commissioned on March 4th 1939.
    After completing dockside and basin trials, she proceeded to the Clyde area for sea and engine trials on April 30th.  However, before the diving trials could be carried out in the Garelock, the fore - planes jammed at hard dive, and "Thetis" had to return to Cammell Laird to have the fault rectified.  The  Admiralty then gave approval for the diving trials to be carried out in Liverpool Bay at a later date.
    During May, torpedo equipment trials were carried out, after which the tubes were given a coat of bitumen paint and enamelled.  Unfortunately, the bitumen solution blocked the test-cock holes were given a coat of bitumen paint and enamelled.  These were then enamelled over, and it was an error which was to have far reaching and tragic consequences.
    Everything was considered to be in order for her first dive when "Thetis" left Birkenhead at 09.40 on June 1st.  As well as her normal crew of 53, she had an extra 50 people on board - some observers and others who would be involved in the trials, plus two caterers and a Mersey Pilot.  She was led out to sea by the tup "Grebecock" which was to act as escort for the day.
    "Thetis" reached her diving position at 13.40, and a signal was sent that she would be diving in position 54 45N 04 00W for three hours. "Grebecock" took up station half a mile away as "Thetis" attempted to dive. Lieut. Coltart on "Grebecock" was a submariner himself and was surprised at the length of time it took for Thetis to go under the surface of the water.  On board the "Thetis" it was quickly    continued