The North Sea Mine Barrage, also known as the Northern Barrage, was a large minefield

Naval mine

A naval mine is a self-contained explosive device placed in water to destroy surface ships or submarines. Unlike depth charges, mines are deposited and left to wait until they are triggered by the approach of, or contact with, an enemy vessel...

 laid by the United States Navy

United States Navy

The United States Navy is the naval warfare service branch of the United States Armed Forces and one of the seven uniformed services of the United States. The U.S. Navy is the largest in the world; its battle fleet tonnage is greater than that of the next 13 largest navies combined. The U.S...

 (assisted by the Royal Navy

Royal Navy

The Royal Navy is the naval warfare service branch of the British Armed Forces. Founded in the 16th century, it is the oldest service branch and is known as the Senior Service...

) between Scotland and Norway during World War I

World War I

World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

. The objective was to inhibit the movement of the German U-boat

U-boat

U-boat is the anglicized version of the German word U-Boot , itself an abbreviation of Unterseeboot , and refers to military submarines operated by Germany, particularly in World War I and World War II...

 fleet.

Origin of the Barrage

The idea of mine barrage across the North Sea

North Sea

In the southwest, beyond the Straits of Dover, the North Sea becomes the English Channel connecting to the Atlantic Ocean. In the east, it connects to the Baltic Sea via the Skagerrak and Kattegat, narrow straits that separate Denmark from Norway and Sweden respectively...

 was first proposed in the summer of 1916 by Admiral Reginald Bacon

Reginald Bacon

Admiral Sir Reginald Hugh Spencer Bacon, KCB, KCVO, DSO was an officer in the Royal Navy noted for his technical abilities who was described by the First Sea Lord, Admiral Sir Jacky Fisher, as the man "acknowledged to be the cleverest officer in the Navy".-Family:Reginald was born at Wiggonholt in...

 and was agreed at the Allied Naval Conference on 5 September 1917. The Royal Navy—and in particular Admiral Beatty

David Beatty, 1st Earl Beatty

Admiral of the Fleet David Richard Beatty, 1st Earl Beatty, GCB, OM, GCVO, DSO was an admiral in the Royal Navy...

 as Commander in Chief of the Grand Fleet—was sceptical about the value of the operation and did not feel it justified the large logistical and manufacturing commitment required.

The United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 was altogether more enthusiastic about the operation as the loss of transatlantic shipping was a major domestic concern and this plan allowed the United States to play an active part in tackling this while playing to their industrial strength and with minimal risk of American casualties.

Objectives

The objective was to prevent U-boats from operating in the North Atlantic and preying on trans-Atlantic shipping. A similar barrage had already been placed across the English Channel

English Channel

The English Channel , often referred to simply as the Channel, is an arm of the Atlantic Ocean that separates southern England from northern France, and joins the North Sea to the Atlantic. It is about long and varies in width from at its widest to in the Strait of Dover...

, which had resulted in U-boats diverting north around Scotland

Scotland

Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...

. The North Sea Mine Barrage was intended to close this alternative route, and it also made it hard for the U-boats to get supplies.

Laying the minefield

In October 1917, the U.S. Navy tendered an order for the 100,000 mines necessary to lay a minefield stretching 230 mi (199.9 nmi; 370.1 km) and dangerous to a depth of 200 ft (61 m). The mines were a version of "antenna" mine that had only been developed in July 1917. Such was the scale of the operation that 80000000 ft (24,384,000 m) of steel wire was required to moor the mines to the seabed. The operation was beset with technical difficulties and delays, with the final mines eventually being laid on 26 October 1918.

The design of the minefield meant there was a theoretical 66% chance of a surfaced U-boat triggering a mine and a 33% chance for a submerged U-boat. In practice the actual odds were assessed at being closer to 20% for a surfaced U-boat and 10% for a submerged one.

Success of the barrage

As the final mines were laid only a matter of days before the end of World War I, it is impossible to assess the success of the plan. It is known three U-boats were sunk on the barrage and a further three are thought to have done so. This represented a return of one U-boat kill for roughly every $13 million spent. Clearing the barrage after the war took 82 ships and 5 months, working around the clock.