I suggest a 10-15% success rate.
As it is now they don't achieve anything. In real life they did achieve something;
1. Pearl Harbour: Photographic analysis conducted by the
United States Naval Institute in 1999 indicates one of the five
Ko-hyoteki-class submarines managed to enter Pearl Harbor, and successfully fired a torpedo into the
USS West Virginia
2. Diego Suarez Harbor: On 29 May 1942, two midget submarines, one of which managed to enter the Diego Suarez harbor and fired two torpedoes. One torpedo seriously damaged
battleship HMS Ramillies, while the second sank the oil tanker British Loyalty (6,993 tons).
3. Sydney Harbour: On the night of 31 May – 1 June, three
Ko-hyoteki class midget submarines, each with a two-member crew, entered Sydney Harbour, avoided the partially constructed
Sydney Harbour anti-submarine boom net, and attempted to sink Allied warships. After being detected and attacked, the crews of two of the midget submarines scuttled their boats and committed suicide without successfully engaging Allied vessels. The third submarine attempted to torpedo the heavy cruiser
USS Chicago, but instead sank the converted ferry
HMAS Kuttabul, killing 21 sailors.
These raids actually sank 3 vessels and damaged 2 battleships.
5 torpedo hits made by 10 midgets isn't a poor achievement if you ask me.
- In AE we can only dream about more than a lucky hit (and as I said, after ca 34 attempts I have none).