The Port Laing Minefield

This Admiralty Chart from 1901 shows a Submarine Mining Practice Area on the south shore opposite Port Laing.
The live mine-field was probably deployed from here across the shipping channels to the north shore. [The practice area was normally sited to be part of the main mine field.]

A 400 yard wide friendly channel would be marked with green and red buoys:
Cadell’s “Flowers of the ocean bed, Painted both green and red, Signals of safety or danger malign.”

Mines, Guns and Lights

This chart shows the probable position of the minefield, and the known positions of the gun batteries at Carlingnose, the Coastguard Station, Inchgarvie, Dalmeny and Hound Point, all pointing squarely at the friendly channel, as are the electric lights the Coastguard station, Inchgarvie and Dalmeny.

All three elements – mines, guns and lights – combined to provide a protective screen against an enemy attack.

Deploying the minefield

The red band is the 400 yard wide friendly channel, protected on either side by contact mines in the pink zones. The pink zones run from about the one fathom mark (2 meters) on each shore, on the basis that a ship would run aground closer to shore. The total width of both pink zones is about 6 1/2 cables or about 4000 feet. So about 160 contact mines are needed to provide coverage of one mine for every 25 feet of frontage. The friendly zone needs a further nine pairs of observation mines. So the whole minefield involves the laying of about 180 mines, and more than forty cables from the shore station to submerged junction boxes. [This is an upper limit of estimation, the pink zones probably started in slightly deeper water, especially on the north shore where the water shelves very gently. This could have saved perhaps 40 contact mine bringing the total to around 140 mines.]

Records show that with experienced teams, it was possible for two laying boats and associated parties to deploy a minefield of 100 mines in a 10 hour working day from 7.45 am to 5.45 pm. This performance was considered to be typical.

So the Port Laing minefield could be deployed across the Forth in less than two working days.

That sort of performance required careful planning, detailed logistics, rigorous discipline, and lots of practice for the 16 officers and 243 other ranks who made up the corps in 1904.