Thursday, Dec 20th, 1917
Routine as usual up to 4 o’clock, then mounted for submarine guard. Should sail 6:30 Friday morning. Hope we do. Addressed by Captain of ship.
Rules / Duties of submarine guard:
1. Never lose sight of object. Point with finger and ring out to next sentry where object is
2. 2,000 yard or thereabout – danger zone
3. Most important time between 4 & 6 early morning
4. Periscope nearly always vertical. Quiet sea 2 ft, High sea 10ft. 4 ft standard
5. Oily patch – look carefully
Frank carefully noted in his diary his duties as a submarine guard. This was no sinecure but a vital role.
The submarine’s biggest advantage was stealth, which meant that it could sneak into high traffic or high density areas, including shipping channels and harbours, and attack multiple targets.
Whilst in the harbour, the HMS Kashmir would be vulnerable to attack. Sonar was only an emerging technology at the time, though by 1918 the Allies had various active prototypes ¹². Therefore spotting a periscope was the only reliable way of detecting the approach of a submarine.
Here is a snippet of film footage of a German submarine in WWI³:
References and further reading:
¹ Science Explorations: Journey into Space: Radar and Sonar at Scholastic.com
² ‘Sonar’ in Wikipedia
³ Fantastic German WWI submarine film 1914-1918 from WWII History & Reenacting