The D-Day map at Southwick house, showing the state of the amphibious assault on Normandy at 7.25am on 6 June. The big white stripe across the English Channel is a German minefield – ten safe channels were cleared through it by RN and RCN minesweepers just before the landings.
But that wasn't the end of the minesweeper's work. Immediately after the assault, the same flotillas went to work clearing the spaces between the channels and sweeping clear channels between the anchorages off the beaches.
But the Luftwaffe & Kriegsmarine were fighting back, laying new mines in the Bay of Seine almost nightly. In June alone, bombers attacked the anchorages off the beaches every single night except one, dive bombing ships & laying mines. 📷Dive bomb damage on HMS Bulolo, IWM A24001
One of their most dangerous weapons was a new type of mine – one that lay on the seabed and was activated by the change in water pressure caused by a ship passing over it. At first the Allies, who named it the Oyster mine, had no means to sweep it. 📷IWM A30625
In the shallow waters off the beaches, and even inside Mulberry harbour, these mines played havoc with shipping until effective counter measures could be developed. The most effective solution was to slow ships down to reduce their pressure effect. 📷 IWM BU1024
This didn't impact the build up as much as might be supposed as LCT – the bulk carrier for the build up – were not especially fast anyway. Other countermeasures were soon developed & it was found they could be swept under certain sea conditions. Rough weather helped considerably.
But it meant almost continual work for the minesweeping flotillas, some of whom swept during the day and guarded the defensive Trout Line on the eastern side of the assault area at night. The total number of mines swept is imprecise, but was in the region of 1,500 in June & July.
But losses to them were severe. In June alone the Allies lost 11 ships to mines & a further 21 damaged. 📷LST 496 sinks by the stern after striking a mine off Omaha Beach. A rhino is alongside rescuing survivors. Original photo by Brian Carter, from his book Saved by the Bomb.
The minesweepers themselves suffered too, sometimes to mines, sometimes to torpedoes. 7 were lost and 7 damaged in the Battle of Normandy. Perhaps the most tragic loss was on 27 August when RAF Typhoons were ordered to attack a flotilla, with the loss of 3 ships and 78 lives.
Incidentally, I know very little about minesweeping. To get really good insight, follow @Sweepers3945 and @Jack_A_Hunter!

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More from @SeaSpitfires

28 Mar
Over the past week, I’ve done a series of threads to highlight the #BattleofSeineBay, aka ‘the part of the Battle of Normandy that gets no attention’ – the battle at sea. I’ll group them all together in this thread with some thoughts. 📷IWM A23977 Image
Even on D-Day itself, an amphibious operation that the Navy led, they are frequently reduced to a taxi service. These acclaimed books for example, might do a good job of telling the Canadian experience of D-Day, but they do a poor one of relating the naval element. Image
After D-Day, it only gets worse, and save Mulberry, the war at sea is almost completely ignored. It’s been that way for a while – consider maps like this one, or modern book series like Battle Zone Normandy. From these you might assume the Channel was irrelevant. ImageImage
Read 14 tweets
27 Mar
Shells fall around HMS Glasgow during an artillery duel with shore batteries around Cherbourg on 25 June 1944. During the battle of Cherbourg she sustained 2 hits, but only minor damage. 📷IWM A 24306 Image
The Royal and US Navy's involvement with the Battle of Cherbourg is usually reduced to the bombardment carried out to support the US advance into the town. But Cherbourg is a port and, as such, had been of considerable interest to the Allied navies for some time. Image
As part of their effort to clear the Channel in advance of the landings, Coastal Forces regularly intercepted S-boats sailing from the port, and even patrolled within 5 miles of it. As D-Day approached they mined the port's approaches. 📷IWM FL15328 Image
Read 8 tweets
25 Mar
US Patrol-Torpedo Boat PT 509 (left) stands by USS Tide after she struck a mine off Utah Beach on 7 June. Two months later, PT 509 would herself be sunk. 📷Naval History and Heritage Command 80-G-651677
One flotilla of PT boats participated in D-Day & 2 more became operational very soon afterwards. Operating from Portland the 33 boats worked alongside the more numerous Motor Torpedo Boats of the Royal Navy & Royal Canadian Navy in the English Channel. 📷Library & Archives Canada
Portsmouth Command fielded no less than 14 flotillas of Coastal Forces during the Battle for Normandy. Their boats shielded the flanks of the invasion forces on D-Day and guarded the swept channels to the beaches for the weeks afterwards. 📷IWM A24047
Read 10 tweets
24 Mar
A German pilot abandons his Linse explosive motor boat. In theory it would now be radio controlled from a control boat following close behind, who would pick up the pilot. In practice, these boats were virtual suicide weapons used in the Battle of Normandy.
The Linse was one of a number of single manned weapons developed in 1943 and 1944 that were rushed to Normandy in the wake of the D-Day landings. Another was the Neger, a tiny submarine actually made from a torpedo, with an armed torpedo slung underneath it. Image
The Neger couldn’t dive – it’s Perspex dome just broke the surface. A slightly larger version, the Marder, eventually replaced it at Normandy. The stretched upper torpedo had space for a diving tank, allowing it to submerge. 📷BillyHill Image
Read 9 tweets
1 Mar
A WW2 Combined Operations beach assault. This 2 min film has been doing the rounds for a while, described variously as Exercise Fabius (1944), Commando training and even Dieppe (1942). In fact it's none of those - it appears to be @RAF_Regiment training in 1943. 📽️ Unknown
I've tried to find this film in various places, but haven't yet found its original source or context. But there are numerous clues as to its location. Beach hardening mats, and in the background, the unmistakable pier of an embarkation hard.
In the film, several piers/dolphins are visible, and combined with the tree lined hill just inland, this is definitely Stokes Bay in Gosport. But, looking west, there's no evidence of Mulberry Phoenix construction which began in December 1943.
Read 11 tweets
28 Jan
Hugely enjoyable chat with @James1940 and @almurray on @WeHaveWaysPod today. Lots of chat about some of my favourite subjects - we were going to talk about Coastal Forces but almost immediately got side-tracked by landing Craft and D-Day! play.acast.com/s/wehaveways/2…
As some of you know my archaeological speciality is concrete, which sounds easy (or dull) but is really quite fascinating. To listen to two concrete specialists nerds) discussing WWII concrete, check out this @CITiZAN1 discussion with @ckolonko and myself.
The number of vessels involved is phenomenal. Some 7,000 vessels of all types were involved, although the exact number varies according to source (Official History on left, Admiralty Staff History on right). These are assigned numbers: I'm working on the exact number that sailed.
Read 22 tweets

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