".... the boat with the experimental aerial is the D-class submarine HMS/M D1. There
is another shot of her taken from ahead with a King Edward class pre-Dreadnought immediately astern.

The D-class were very distinctive. Besides being the first British class with diesels and and external as well as internal main ballast tanks, they also featured a stern torpedo tube (you can just make out the operating gear in your photo) and an upper and lower rudder. The upper rudder was not successful and was prone to damage from wave action on the surface so it was removed at an early date, but the pintle post is still there."

J. David Perkins is author of: Canada's Submariners, 1914-1923 (1989) and The Canadian Submarine Service in Review (2000)