Eighteen.

This is coolbert:

As is germane to a previous blog entry, the incorporation of a foreign submarine into the inventory or under the command of the British Royal Navy [RN] not such a wholly unique phenomenon. During WW2 a practice more common than you might think.

"England was terrible[y] short of submarines herself as the British Submarine Force, then still commanded by Admiral Max Kennedy Horton, had lost themselves 31 submarines in operations and accidents since the start of the war"

SHORTAGE OF VESSELS ACUTE!

Foreign submarines under RN command [manned by foreign crews] to include:

• Eight Dutch submarines
• Five Greek submarines
• Three French submarines
• One Polish submarine
• One Norwegian

British Royal Navy [RN] submarine losses during WW2 excessive and primarily incurred during operations in the Mediterranean. Shallow waters with a number of  "choke points" facilitating the task of enemy [Axis] anti-submarine warfare [ASW] forces.

Submarines of foreign origin manned by foreign crews a valuable asset if used judiciously but again, the problem being spare parts.

During wartime and desperate circumstances you have to go with what you got?

coolbert.