'Rhein' (iv) was a U-boat wolfpack operation in the Atlantic against the HX.237 convoy (7/10 May 1943).
Formed from the U-boats of the 'Amsel I' and 'Amsel II' packs, this wolfpack comprised U-103, U-186, U-359, U-403, U-448, U-454, U-466, U-468, U-525, U-569, U-709 and U-752, and neither lost any of its own number nor sank any ship.
The wolfpack was established specifically for the interception of the HX.237 convoy, a movement expected on the basis of B-Dienst intelligence, which reported the convoy’s attempted evasive re-routing to the south. It was at the southern end of the 'Rhein' (iv) wolfpack’s patrol line that on 9 May U-359 located and reported the HX.237 convoy of 46 ships supported by Lieutenant Commander E. H. Chavasse’s Canadian Escort Group C2 (British destroyer Broadway, British frigate Lagan, corvettes Chambly, Drumheller, Morden and British Primrose, and rescue tug Vizalma).
The U-boat was quickly located by HF/DF and forced to dive. The convoy then broke through the patrol line after a Fairey Swordfish anti-submarine aeroplane from the escort carrier Biter, which had reinforced the Escort Group C2 as part of Captain E. M. C. Abel-Smith’s British 5th Support Group (destroyers Inglefield, Obdurate, Opportune and Pathfinder) compelled U-454, which was barring the way, to dive.
U-403 re-established contact on 10 May when shadowing the rescue tug, which had fallen behind. The U-boat was able to drive off a Swordfish with anti-aircraft fire, but was then forced to dive by the arrival of one of the 5th Support Group’s destroyers. As the boats of the 'Rhein' (iv) wolfpack could no longer get ahead of the convoy, the operation against the HX.237 convoy could be prosecuted only with the boats of the 'Drossel' wolfpack arriving from the east.
At this time the 'Rhein' (iv) wolfpack combined with the 'Elbe' wolfpack to form the 'Elbe I' and 'Elbe II' wolfpacks for deployment against the SC.129 convoy.