The Hai (shark)

The Hai (shark) was an advanced model of the Marder-class midget submarines created in Germany during World War II and operated by the K-Verband. Its prototype performed poorly during test runs and therefore no other boats were produced.
The Hai was eleven meters long and its length allowed for larger batteries, giving it the maximum speed of twenty knots under water. In addition, the Hai could remain under water for up to two hours.