This is coolbert:
Here is the final word on rocket artillery as employed from submarines during World War Two [WW2].
"The U-Boat Rocket Program"
"America's entry into WW2 prompted a discussion among German strategists seeking ways to strike the American mainland."
"Various methods were available at that time" [American entry into WW2]. Methods to include:
* "saboteurs by U-Boat"
German saboteurs were landed and betrayed almost immediately. Seems the group consisted of men who were not highly trained, poorly motivated and would have probably NOT had much success even if not for betrayal and capture. During World War One [WW1] German saboteurs WERE highly effective. The Black Tom explosions caused massive damage to New York City harbor facilities, the perpetrators NEVER being caught.
* "fly bombers to New York City, and ditch them near a waiting U-Boat in the Atlantic before the fuel ran out."
German long-range bombers on ONE-WAY attack missions against American targets. The planes NOT able to make a round-trip and return safely. Would need to ditch the plane in mid-ocean, the crew of the aircraft being plucked out of the water by a waiting U-boat. A U-boat presumably emitting a beacon of some sort. The results would have been so meagre for the cost that the idea is not even worth considering.
"Yet another method became apparent during a conversation between Dr. Ernst Steinhoff, an engineer at the Penemunde rocket development facility, and his brother, Korvettenkapitan Fritz Steinhoff, Kommandant of U-511."
* "fire an artillery rocket from the deck of a submerged submarine."
"Tests were conducted in May/June of 1942 using a standard army issue Wurfgerat 41 launcher and rockets from 21 to 30cm. The tests proved that it was not only feasible, but that the rockets could be fired from depths up to 15 meters below the surface, without effecting the normal flight path."
[the Nebelwerfer was in the original version a 15 cm. rocket "projector". Rockets to be launched from submarines were bigger by far!]
German Type II submarines were fitted with rocket launchers and did use same against Russian targets on the eastern front during WW2. Firing rockets at land targets while submerged and doing so effectively.
"The Type II U-boat was designed by Germany as a coastal submarine . . . It was too small to undertake sustained operations far away from the home support facilities."
"Six [Type IIs] were stripped down to just a hull, transported by river and truck to Linz (on the Danube), and reassembled for use in the Black Sea against Soviet Union."
Here from KTB "Sharkhunters":
"U-24 and . . . U-9 were ordered into dry dock at Constanta in Romania. The boats were then hidden from view by covering the dock with nets and tarpaulins . . . the majority of the crew were given leave . . . when they returned the found the boats back in the water without any unusual additions . . . A framework for holding the rockets had been attached and none of it showed above the water, even when the boat was fully surfaced . . . Each rocket was about 25 - 35 cm. in diameter and well over 1 meter long. Following this U-18 probably became the first boat to try the rockets operationally by bombarding Russian positions on land and it appears as if a number of sorties were undertaken throughout 1944."
"the following remarks at the end of the log of . . . U-18":
"The close approach to the harbor at Poti made the bombardment with the new projectiles [rockets] a total success."
The American submarine Barb, under the command of that most able commander, Gene Fluckey, too was able to use rocket artillery, in the Pacific theatre, against Japanese targets, right at the end of the war:
"Barb was sent to the U.S. for a yard overhaul and alterations, which included the installation of five-inch (127 mm) rocket launchers . . . For the first time [not exactly if German accounts are true!] in submarine warfare, Barb successfully employed rockets against the towns of Shari on Hokkaidō, and Shikuka, Kashiho, and Shiritoru on Karafuto."
Using rocket artillery from submarines, even while submerged, was a sound concept. But not embraced by either side until the war was nearly over. American east coast cities would have been vulnerable to attack - - but - - NOT exactly as Doenitz had in mind [NYC reduced to a sea of flame and rubble]. A diversion more than anything else. A valid concept but not contributing in a major way to the war effort.