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Caisson (Phoenix) Breakwater

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Tug Tow of Phoenix Breakwater for Mulberry Harbor

Concrete caisson Phoenix being towed by tugs

Concrete Phoenix caisson being towed by tugs for placement as breakwater for Mulberry harbor. Note Bofors gun mounted on top. WWII.

Two prefabricated harbors, (code named Mulberry), including breakwater, piers and roadways were built in England, towed across the English Channel, and set up off the coast of France shortly after the initial Allied invasion of Normandy.

One of the key components of the Mulberry artificial harbor was the use of large concrete caissons (code name Phoenix), which formed the outer sea wall of the harbor. These concrete caissons were as tall as a five story building and acted as a breakwater for vessels, creating sheltered water for the pier heads and roadways which would be built inside the harbor. U.S. Navy photo above, shows a Phoenix caisson begin towed into position by a tug boats. Note Bofors gun mounted on top for anti-aircraft protection.

The invasion fleet, along with the harbor components was preceded by the mine sweepers which attempted to clear mines in the area. Blockships positioning was begun first, followed by the Phoenixes.

Bofors guns, Phoenix Placement

The tugs towed the caissons at a rate of 4 knots, for over 100 miles from England to the coast of France, and it was expected to take forty-eight hours for a tug to cross the Channel and return. The slow speed made these caissons a target for German aircraft and a 40 mm Bofors gun was placed on top for anti-aircraft protection. The Bofors gun and platform can be seen in the photo immediately above. The Phoenix also carried a gun crew of four, and two sailors on its trip across the Channel.1

Interior of concrete Phoenix caisson as viewed from the top.

Interior of concrete Phoenix caisson as viewed from the top. Note men on walkway.

Once on location, the crew traveling inside would begin the work of aligning and sinking the Phoenixes to form a breakwater. They were placed end to end, and sunk at maximum depth of 5 ½ fathoms (33 feet) which caused them to lie in a curve.

Phoenix Interior

The interior of the caisson had compartments, one of which was used as quarters for transporting the crews which would sink the caissons in place once they had been towed across the Channel. These compartments are visible in the U.S. Navy photograph to the right. Note men standing on walkway.

These sixty-foot high concrete caissons were built in six different sizes, the largest with a displacement of 6,044 tons. Valves in these Phoenixes were opened and it took approximately 22 minutes to sink the largest size.2

Line of bombardon creating an outer breakwater - WWII at Normandy.

Outer ring of Bombardon (outer breakwater) at Mulberry harbor A on Omaha Beach. WWII, Normandy.

Once placed, the Phoenixes accommodated liberty ships or large coasters which discharged their load onto rhino ferries, barges and other landing craft. These crafts would continue on to shore.

Phoenixes Created Calm Harbor Water

The caissons proved effective in creating calm water inside the harbor and were augmented by a line of blockships (Gooseberries) as well as a floating breakwater, called a bombardon, the later consisting of steel cruciform shaped components which were anchored outside the harbor, beyond the Phoenixes.

Once the breakwater elements were in place, the piers and roadways could be built in the calm water which the breakwater had created. A line of Bombardon breakwater at Omaha Beach can be seen in the U.S. Navy photo above, right.


Unfortunately the caissons sustained much damage, along with the other harbor components on the American Mulberry A, during a fierce 3 day storm on June 19-22, 1941, occurring only two weeks after the D-Day invasion.

The outer bombardon failed on both harbors, and is thought to a have lead to more damage as it crashed into harbor components during the storm. The British Mulberry B received much less damage but Mulberry A was eventually abandoned.


1Guy Hartcup. Code Name Mulberry – The Planning – Building & Operation of the Normandy Harbours, South Yorkshire: Pen & Sword Military, 2006, p.114.

2John P. Taylor, The Prefabricated Port of Arromanches – Mulberry B. London: Shipbuilding and Shipping Record, 1945. p.7-8.