Unlike the converted merchant ships, Ostmark was built for the tender role and arranged a crane and catapult along a flush working deck. At 1,281 gross register tons, with machinery rated at 1,800 horsepower, she was reportedly designed for about 14 knots and could support two seaplanes. She still lacked a hangar, protected aircraft stowage, and substantial covered maintenance facilities. Those limits made her useful as a forward refuelling and light-support base rather than a mobile aviation workshop. Operations off Norway and in the Bay of Biscay ended when HMS Tuna sank her in September 1940.
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Ostmark's purpose-built but minimal aviation support
Ostmark paired a flush deck, crane, and catapult with almost no protected aircraft-maintenance capacity.