retained solely for experimental and training purposes.
Approximately thirty of these airships were constructed during
the year 1916, and were allocated to the various stations for
patrol duties.

The work carried out by these ships during the two and a half
years in which they were in commission, is worthy of the highest
commendation. Before the advent of later and more reliable
ships, the bulk of anti-submarine patrol on the east coast and
south-west coast of England was maintained by the Coastal. On
the east coast, with the prevailing westerly and south-westerly
winds, these airships had many long and arduous voyages on their
return from patrol, and in the bitterness of winter their
difficulties were increased ten-fold. To the whole-hearted
efforts of Coastal pilots and crews is due, to a great extent,
the recognition which somewhat tardily was granted to the Airship
Service.

The envelope of the Coastal airship has been shown to be of
170,000 cubic feet capacity. It is trilobe in section to employ
the Astra-Torres system of internal and external rigging. The
great feature of this principle is that it enables the car to be
slung much closer to the envelope than would be possible with the
tangential system on an envelope of this size. As a natural
consequence there is far less head resistance, owing to the much
shorter rigging, between the envelope and the car.

The shape of the envelope is not all that could have been
desired, for it is by no means a true streamline, but has the
same cross section for the greater part of its length, which