A Thames Barge
Like June 1943, July that year was a quiet month in terms of shipping losses, with only two recorded ships lost – the Davaar, which pops up in both July and September 1943 (see September 1943 post) and the Thames barge J B W.
The J B W was built at Millwall in 1907 and is clearly recorded as a spritsail vessel, 72 tons, in early documentation.  At that time Millwall was a busy shipbuilding area, and in Historic England’s records we know of at least one other spritsail barge built at Millwall in 1888, the Lizzie, which was hulked and subsequently broken up, 1945-6  but Millwall also saw the construction of much more substantial and famous vessels . . .
From J B W‘s place of build on the Thames, her tonnage, and the spritsail description, we can be confident that she is a Thames barge – by definition they are spritsail vessels. That would have been immediately obvious to a contemporary observer of the Thames, but is the kind of knowledge that has largely passed into heritage or maritime spheres of knowledge. The spritsail is a fore-and-aft sail rigged onto both a standing mast and a diagonal mast known as the sprit. This model of a Thames barge under full sail in the National Maritime Museum clearly shows the sprit behind the mainsail and illustrates how the sail plan functioned.
We know that by 1910 she was owned by the Associated Portland Cement Manufacturers Ltd., Dixon House, Fenchurch Street, London, in whose ownership she remained at least ten years, but towards 1930 or so the vessel came into the hands of successive private operators. 
The location of loss can be an important part of confirming the characterisation of a vessel, which is certainly the case here: north of the NE Maplin Buoy in the Thames Estuary, or in a position described in contemporary sources as 51˚ 37’ 6” N 001˚ 05’ 9”E, which places her in the channel between the Maplin Sands to the west and the Barrow Sand to the east on the Essex coast off Foulness Island. 
This area of the Thames Estuary is full of sandbanks which appear on charts as if someone has decided to paint diagonal yellow stripes right across the estuary, creating shoals and narrow channels, impossible for larger craft to negotiate. For that reason, the region is very well-buoyed and marked throughout for the safe navigation of smaller vessels: the shallow and flat-bottomed Thames barge was built to work this difficult area of shifting sandbanks and shoals. These vessels could come off much more easily if they ran aground than large ships or even smaller vessels with much sharper hull profiles. They worked the coast up to the farmlands of Essex and Suffolk and beyond, and serviced the industrial works on the Kent side, or crossed to the Netherlands, where there are similar coastal conditions.
Thus the place of loss confirms that our identification of vessel type is correct, and is part of a holistic picture of assessment.
A wooden sailing vessel such as this one had no chance against a mine, and both her crew – they typically operated with a couple of crew or a few hands at most – were killed. We can see that there were five other vessels lost to mines off the Essex coast in 1943: three were much larger cargo vessels lost on the edge of the War Channel on the approaches to the Thames, while the patrol HMS Ocean Retriever, a patrol vessel was another War Channel loss. This would be the characteristic pattern of shipping losses to mines – it was the principal shipping routes which were targeted and needed to be swept constantly, and it was just those vessels finding themselves in and around the War Channel – the cargo vessels and the patrol vessels and the minesweepers which tried to keep them safe – which were the victims. 
The sandbanks were also, generally, a danger to minelaying submarines (see post from October 1939 about a U-boat on the Goodwin Sands), and the War Channels offered richer pickings so this was where enemy efforts were concentrated.
Yet mines could strike on the sandbanks closer inshore, forming a slightly different distribution pattern largely relating to the Thames barge – as J B W demonstrates. The Thames barge was able to operate in and among sandbanks and shoals inaccessible to the average cargo steamer and to serve smaller ports.
We see this distribution pattern with other mined barges off the Essex coast: the Rosmé off the Maplin Sands, the Golden Grain on the Maplin Edge, and the Blue Mermaid off the Whitaker Channel, all in 1941, Unique off the Maplin Edge buoy in 1942, and the Ailsa just off the Whitaker Spit in 1943. On that occasion both crew survived ‘despite being blasted into the air with the sand’. (See post for January 1943). 
How did the mines get there when it was the principal shipping routes further out to sea which were the prime target? One plausible explanation is that those mines among the sandbanks were strays, either naval mines which had come adrift or parachute mines dropped by aircraft earlier in the war, of types which may occasionally still be dislodged and come to light even today.
 Historic England Research Records, Lizzie
 Lloyd’s War Losses: The Second World War 3 September 1939-14 August 1945, Vol.1, p695; British Vessels Lost at Sea 1914-18 and 1939-45, Section IV, p49
 Historic England Research Records, data for 1943