The urgent need for a non-rigid airship to carry out anti-submarine patrol having been satisfied for the time with the production of the S.S. B.E. 2C type, the airship designers of the Royal Naval Air Service turned their attention to the production of an airship which would have greater lift and speed than the S.S. type, and, consequently, an augmented radius of action, together with a higher degree of reliability. As the name "Coastal" or "Coast Patrol" implies, this ship was intended to carry out extended sea patrols.

Using a French Astra Torres ungainly looking tri-lobed envelope and based on the French design, a new larger scoting ship was required to have a longer duration and heavier bomb load.Design and advancement on the envelope also progressed with an unusual “Tri-lobe” design. This was seen as a stronger, more aerodynamic. Again, the use of existing technology was used to keep the costs down and production speeds up with demand. The gondola was based on two tail-less Avro shorterned aeroplane fuselages connected end to end, and carried 2 engines in tandem. This produced a four of five seater car. The prototype was delivered in early 1916 with other copies to follow.

The Tri-lobe envelope was 196 feet long and a capacity of 170,000 cft. The ridges in the envelope was connected to fabric curtains inserted to help maintain the shape. Four ballonets were also fitted, two in each of the lower lobes, and a single scoop consisting of a single sheet of aluminium tuble of oval cross section reached down towards the forward propellor. Later versions of the ship had the scoop at the rear propellor. Single skids replaced the normal undercarriage and also protected the propellers. The rigging was also the same type as seen in an existing Astra Torres ship.

The Coastals were fitted with a larger bomb load, a wireless, and 2 machine guns, one of which was on top of the envelope. This was accessed by a tube and rope ladder from the gondola. Sometimes commented as the "ugliest" dirigibles ever made, these ships sometimes were erretic and unstable in flight. The responsiveness in the controls was sluggish and often caused the crew to be airsick. However one of the main advances in airship technology was the very high rate of climb which the coastals managed to provide.

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 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 tapers at either end to a point which is slightly more accentuated aft. Owing to the shape, these ships, in the early days until experience had been gained, were extremely difficult to handle, both on the landing ground and also in the air. They were extremely unstable both in a vertical and horizontal plane, and were slow in answering to their rudders and elevators.

The envelope is composed of rubber-proofed fabric doped to hold the gas and resist the effects of weather. Four ballonets are situated in the envelope, two in each of the lower lobes, air being conveyed to them by means of a fabric air duct, which is parallel to the longitudinal centre line of the envelope, with transverse ducts connecting each pair of ballonets. In earlier types of the Coastal, the air scoop supplying air to the air duct was fitted in the slip stream of the forward engine, but later this was fitted aft of the after engine.

Six valves in all are used, four air valves, one fitted to each ballonet, and two gas valves. These are situated well aft, one to each of the lower lobes, and are fitted on either side of the rudder plane. A top valve is dispensed with because in practice when an Astra-Torres envelope loses shape, the tendency is for the tail to be pulled upwards by the rigging, with the result that the two gas valves always remain operative.

Crabpots and non-return valves are employed in a similar manner to S.S. airships.

The Astra-Torres system of internal rigging must now be described in some detail. The envelope is made up of three longitudinal lobes, one above and two below, which when viewed end on gives it a trefoil appearance. The internal rigging is attached to the ridges formed on either side of the upper lobe, where it meets the two side lobes. From here it forms a V, when viewed cross sectionally, converging at he ridge formed by the two lobes on the underside of the envelope which is known as the lower ridge.

To the whole length of the top ridges are attached the internal rigging girdles and also the lacing girdles to which are secured the top and side curtains. These curtains are composed of ordinary unproofed fabric and their object is to make the envelope keep its trilobe shape. They do not, however, divide the ship into separate gas compartments. The rigging girdle consists of a number of fabric scallops through which run strands of Italian hemp. These strands, of which there are a large number, are led towards the bottom ridge, where they are drawn together and secured to a rigging sector. To these sectors the main external rigging cables are attached. The diagram shows better than any description this rigging system.

Ten main suspensions are incorporated in the Coastal envelope, of which three take the handling guys, the remaining seven support the weight of the car.

The horizontal fins with the elevator flaps, and the vertical fin with the rudder flap, are fixed to the ridges of the envelope.

The car was evolved in the first instance by cutting away the tail portion of two Avro seaplane fuselages and joining the forward portions end on, the resulting car, therefore, had engines at either end with seating accommodation for four. The landing chassis were altered, single skids being substituted for the wider landing chassis employed in the seaplane. The car consists of four longerons with struts vertical and cross, and stiffened with vertical and cross bracing wires. The sides are covered with fabric and the flooring and fairing on the top of the car are composed of three-ply wood. In the later cars five seats were provided to enable a second officer to be carried.

The engines are mounted on bearers at each end of the car, and the petrol and oil tanks were originally placed adjoining the engines in the car. At a later date various methods of carrying the petrol tanks were adopted, in some cases they were slung from the envelope and in others mounted on bearers above the engines.

Wireless telegraphy is fitted as is the case with all airships. In the Coastal a gun is mounted on the top of the envelope, which is reached by a climbing shaft passing through the envelope, another mounting being provided on the car itself.

Bombs are also carried on frames attached to the car. Sunbeam engines originally supplied the motive power, but at a later date a 220 horse-power Renault was fitted aft and a 100 horse-power, Berliet forward. With the greater engine power the ship's capabilities were considerably increased.

Exceedingly long flights were achieved by this type of ship, and those exceeding ten hours are far too numerous to mention. The moot noteworthy of all gave a total of 24 1/4 hours, which, at the time, had only once been surpassed by any British airship.

Towards the end of 1917, these ships, having been in commission for over two years, were in many cases in need of a complete refit. Several were put in order, but it was decided that this policy should not be continued, and that as each ship was no longer fit for flying it should be replaced by the more modern Coastal known as the C Star.

The record of one of these ships so deleted is surely worthy of special mention. She was in commission for 2 years 75 days, and averaged for each day of this period 3 hours 6 minutes flying. During this time she covered upwards of 66,000 miles. From this it will be seen that she did not pass her life by any means in idleness.

The Coastal airship played no small part in the defeat of the submarine, but its task was onerous and the enemy and the elements unfortunately exacted a heavy toll. A German wireless message received in this country testified to the valiant manner in which one of these ships met with destruction.

2 Classes of “Coastal” were desgined, the standard Coastal and the “C*” (C Star) Class. The most successful ship in this class was the C9 which flew from Mullion in Cornwall. The ship had a service flying life of 2,500 hours 11 minutes, some 68,201 miles. The longest single flight recorded by a Coastal ship was 24 hours 15 minutes by C24 on 9th 10th July 1917. Production of this class of ship ceased in 1916.

Only 2 were destroyed through enemy action in the entire war. In total 45 “Coastal” Class ships were built, however 12 were totally destroyed in some manner and only 4 survived to the Armistice.