Hi John,
If the book is full of statements like that, go to the store and let them give you your money back.
"Unstable hull" = unadulterated BS. Teething problems are well known, such as flaws in weld lines (it was difficult to run a continuous hot weld once around the boat without interruption for such trivia as air attacks/took abt 20 hrs), frame weakness in the area of the torpedo hatch, flaws and resulting breakdowns in the extensive hydraulic system, and the difficulties resulting from having to shoe horn bigger engines into the boat than designed, since designed engines were unavailable because of lack of production capacity.
All above did not detract from the enormeous influence by this design on future submarine construction. the writer can't be serious.
Type 21 boats were lost in air attacks which I would classify as combat. 2 boats went out on extended patrols. The mock attack refered to was carried out by one of the two (U2511-Schnee)before the capitulation and surrender but after a cease fire order was received from the BdU. Rgds, RB