General Principles for Defended Ports

The General Principles were in summary:

• It was vital that commerce and trade be maintained during a time of war and thus harbours etc must remain open and traffic controlled.
• Fixed Defences should be used to keep hostile vessels away. Fixed defences should protect Royal Navy bases and provide confidence to civilian traffic.
• The Royal Navy could not provide warships to defend all harbours and ports. They could, however, probably deal with any major hostile attack by enemy capital ships.
• The primary danger was therefore from motor torpedo boats (MTBS), cruisers and other smaller hostile vessels. The threat of MTB attacks were increased if the enemy had a harbour within 200 miles.
• Both outer and inner defence lines must be considered.
• Night attack by capital ships was not likely, but MTBs etc could attack at night or in poor weather.

Detailed Instruction were laid out in this policy document, including:

• Creation of Examination Anchorages which would be covered by the Examination Battery which was usually two 6-inch guns. An example would be Devils Gap Battery in Gibraltar.  The Examination Anchorage would be located outside the main minefield.
• All traffic in and out and manoeuvring in the port or harbour would be controlled and this would be supervised by the Port War Signal Station (PWSS).
• In respect of Minefields these should be concentrated and covered with Quick Fire guns and electric lights. Electro-contact mines were required to deal with shallow-draught vessels.  Friendly channels should where it was possible, follow existing fairways.
• A breakwater or similar obstruction should be used to create choke points through which MTBS and other small hostile vessels would have to pass. Booms, minefields, Quick Fire guns and electric lights would cover these choke points.  Where breakwaters could not be built, ‘fixed posts’ with Q.F. guns and machine guns were to be built.  The fixed posts could be vessels, under naval command, or in shallow water stone structure under War Department.
• Defence Electric Lights should primarily be fixed divergent beams, used to illuminate defined are of water. Their use needs to be co-ordinated with the fixed defences.

• Quick Fire guns and electric lights were to take preference over heavier guns initially. Land defences were to be left until the sea defences had been completed.  RML guns were to be phased out and replaced by B.L and Q.F. guns.