The Benlomond’s situation — unescorted, carrying ballast, on a long ocean crossing — was the exact profile of vulnerability that U-boat strategy was designed to exploit. The ship was not, in the language of wartime logistics, a high-value target: it carried no cargo, and its loss did not remove any strategic material from the Allied supply chain. But it represented a merchant vessel and a crew, and its sinking contributed to the broader campaign of attrition that German submarine strategy was designed to prosecute.
The specific position of the sinking — roughly 750 miles east of the Amazon mouth, near the equator — placed it in a region where Allied search and rescue capabilities were limited. The South Atlantic in late 1942 was not yet covered by the dense web of air patrols that eventually turned the tide of the Battle of the Atlantic. American expansion of anti-submarine patrol operations in the South Atlantic was underway but incomplete. The gap between where the Benlomond went down and where reliable rescue coverage could be expected was real and substantial.
This is why Poon Lim’s isolation on the raft was as complete as it was. It was not that rescue was never attempted or never available; the American patrol aircraft that spotted him and dropped the buoy demonstrated that coverage existed. It was that the coverage was imperfect, the ocean was vast, and a small wooden raft drifting in equatorial Atlantic currents was an extraordinarily difficult object to locate and maintain location on. The storm that moved the raft away from the buoy’s position was not an unusual weather event in that region — the Intertropical Convergence Zone generates persistent unsettled conditions — it was part of the normal meteorological reality of that latitude.