Operation Sonnenblume
The German plans
On May 21th, 1942, a conference chaired by Admiral Raeder decided to send in the Mediterranean 27 submarines. These will be of the new IIE class, derived from the coastal IID design. The small size of this class of boats would allow a relative easy transit through the French Canal du Rhne au Rhin, only solution to avoid the Strait of Gibraltar. Admiral Doenitz strongly advocated for this project, as only viable solution against a greater depletion of the U-boats force in the Atlantic. The following day, he commanded Kkpt Winkler, from the technical staff of the BdU, to study the various aspects of the operation, asking for a first report within two weeks.
On June 4th, Winkler gave his conclusions, which underlined the first difficulties to overcome.
The design of the IIE (length 52.56 m, pressure hull length 38.4 m), and the limitations of the Canal du Rhne au Rhin (locks size 38.5 m x 5.0 m, draught 2 m), will force a relative complex multi phased construction :
1. Manufacturing of the pressure hull, only partial fitting to respect the draught constraint, closing of all the cut-outs (mainly those related to the torpedo tubes and those related to the remaining equipments to install, especially the engines). The selection of what to install in this first phase will be critical: the trim has to be as horizontal as possible, the final assembly must remain easy, and a sufficient stability of the partial-built hull has to be obtained.
2. Making of the external plates, the conning tower and preparation of the remaining parts and equipments
3. Ferrying, either by waterway or railway, according to the components
4. Final assembly : torpedo tubes, engines, remaining equipments, welding or riveting of the external plates
5. Trials
The building, first intended on two sites (Kiel and Bremen), will have to be organized in the following way:
- Manufacturing of the pressure hull and all feasible fitting at Deutsche Werft (Kiel), who has all the needed know-how from the original Type II.
- Subcontracting at AG Weser (Bremen) of the external plates, tanks, ballasts
- Final assembly at the Arsenal of Toulon, by Deutsche Werft specialists. A roughly estimates indicates that about 1000 people are to be transferred from Kiel to Toulon. The same has to be done for all the needed tools, jigs and welding equipment.
The ferrying through the Canal du Rhne au Rhin, the Sane and the Rhne will not be easy, since the raw pressure hulls will have no rudder. A specific floating frame has to be built to give enough stability to the hulls, and this frame must include towing points, a rudder system, and probably other features. This ferrying has to be done in several legs: on barges in Germany, by towing of the hulls in France.
Note: Type IID submarines, smaller, although less capable, are a lot more compatible with the limitations involved by the Canal du Rhne au Rhin. Transit would be possible by only dismounting the conning tower, the torpedo tubes and the bow outer plates. Considerable time could be spared by reducing the difficulties of the transit and shortening the final assembly.
Two days later (June 6th, 1942), Doenitz approved Winklers first conclusions. On June 11th, during a meeting held in Berlin with his technical staff and shipyards representatives, he kept the choice of the Type II E, since, from his point of view, the operational advantages of this design where more important that the time lost in the building process. At the same period, he asked his Chief of Staff (Godt) to find a resourceful ashore, efficient at sea, and smart everywhere flotilla leader, who will be in charge of the transit operation and assume command in Toulon.
On June 11th, 1942, the building schedule was also adjusted:
- In the beginning of August, 1942, start of the the pressure hulls manufacturing, in welded sections, for the whole class of Type IIE submarines (training, Mediterranean, Black Sea).
- End of October, 1942, delivery of the first batch : 9 boats intended for the trials and the training (U-685 to U-693). Those boats will be the only ones built in a single stage. Two other partially built hulls will be delivered: the first one to validate the multi stage manufacturing process, the other one for the trials of the towing system.
- January, 1943 : ferrying of all the needed tools at Toulon
- End of February, 1943 : delivery of the 27 partially built hulls, intended for the Mediterranean (U-694 to U-699, U-810 to U-820, U-830 to U-839). Ferrying of the remaining equipments and plates at Toulon.
- Mid April 1943 : start of the assembly at Toulon
- June 1943 : delivery of the first units and trials at sea
- August 1943 : first operational patrols
The trials of the first units where planned from November, 1942 to January, 1943, with the hope that they will be going well and not be plagued with too much troubles which will put the schedule at risk.
Deutsche Werfts design office was put under high pressure during the following weeks. As told by D-Ingenieur Fleischmann All has to be done fast and without error, while, ordinarily, in less known domains, we took all the needed time to avoid mistakes. And no electronic computer was available! All the computations had to be done by hand. We reused a great number of parts and components of the Type IID, even if they seemed a little undersized, at least for the first batch, and with the perspective to correct the situation for the second one. And for the multi staged building process, what seemed doable on the paper was sometimes a nightmare at the plant level.
Back in Paris, Doenitz met, on June 16th, the overseer of the occupation Transportation office, to define how the Kriegsmarine could use the Canal du Rhne au Rhin for an operation a lot more complex that the transit of a few MTBs. A compromise was established, for the month of March, 1943, inasmuch as the Kriegsmarine will take all the measures needed to not interrupt the vital flow of merchandises shipped from France to Germany, absolutely needed by the Reich.
On June 21th, Doenitz evaluated the situation with Raeder, and both Admirals agreed upon a name for the whole operation. This would be Sonnenblume (Sunflower), while, like the sunflower, the Kriegsmarine will point now in direction South, to face the threat.
In the beginning of July, Winkler made a short trip to Strasbourg, to check if the harbour installations will be sufficient for the submarines unloading from their barges. He noted that additional cranes would be needed. When he asked about tugs for the journey on the Canal, he was very disappointed by learning that non self-propelled canal boats where hauled from the bank by small electric locomotives running on a narrow-gauge railway between Strasbourg and Mulhouse, and by gasoline or Diesel tractors on tires further on. In Germany, canal boats, a lot bigger, were towed by tugs everywhere. Since the hauling machines were spread along the whole 200 miles of the canal, he understood quickly that not enough of their will be available to haul 27 submarine hulls, until completely stopping the civilian activity or cut the convoy in several smaller ones. And, with the cylindrical shape of these hulls, even stabilized by floats, the hauling from the bank would be calamitous.
At the end of July, a new meeting happened in Berlin. Deutsche Werft informed the participants that the building of the hulls will effectively start in due time, but the two staged process needed still to be refined, mainly due to difficulties in the relocation of tools and manpower in Toulon. To solve the problem of the towing, decision was taken to find the smallest fluvial harbour tug in use, compatible with the small gauge of the Canal du Rhne au Rhin, this type of boat having to be built in sufficient quantity for the needs of the operation. If the case of an unfruitful search, hauling would be the only available choice, and a hard negotiation with the occupation authorities in France should be expected. The general opinion was that all that should be avoided by any means, because the subject will in this case end in Hitler hands, and then, heads will be dangerously on the verge of rolling
In September, the Allied attack against Sicily pulled more and more pressure on the players of the operation. Doenitz had to use all his arguments to convince Raeder that it was not possible to go faster and also that it was out of question to try a new transit of submarines from Atlantic to the Mediterranean. In the hallways and offices of the Admiralty, the operation, which was now known by about everyone, was then nicknamed Sonnenbad (sunbathing)
The German plans
On May 21th, 1942, a conference chaired by Admiral Raeder decided to send in the Mediterranean 27 submarines. These will be of the new IIE class, derived from the coastal IID design. The small size of this class of boats would allow a relative easy transit through the French Canal du Rhne au Rhin, only solution to avoid the Strait of Gibraltar. Admiral Doenitz strongly advocated for this project, as only viable solution against a greater depletion of the U-boats force in the Atlantic. The following day, he commanded Kkpt Winkler, from the technical staff of the BdU, to study the various aspects of the operation, asking for a first report within two weeks.
On June 4th, Winkler gave his conclusions, which underlined the first difficulties to overcome.
The design of the IIE (length 52.56 m, pressure hull length 38.4 m), and the limitations of the Canal du Rhne au Rhin (locks size 38.5 m x 5.0 m, draught 2 m), will force a relative complex multi phased construction :
1. Manufacturing of the pressure hull, only partial fitting to respect the draught constraint, closing of all the cut-outs (mainly those related to the torpedo tubes and those related to the remaining equipments to install, especially the engines). The selection of what to install in this first phase will be critical: the trim has to be as horizontal as possible, the final assembly must remain easy, and a sufficient stability of the partial-built hull has to be obtained.
2. Making of the external plates, the conning tower and preparation of the remaining parts and equipments
3. Ferrying, either by waterway or railway, according to the components
4. Final assembly : torpedo tubes, engines, remaining equipments, welding or riveting of the external plates
5. Trials
The building, first intended on two sites (Kiel and Bremen), will have to be organized in the following way:
- Manufacturing of the pressure hull and all feasible fitting at Deutsche Werft (Kiel), who has all the needed know-how from the original Type II.
- Subcontracting at AG Weser (Bremen) of the external plates, tanks, ballasts
- Final assembly at the Arsenal of Toulon, by Deutsche Werft specialists. A roughly estimates indicates that about 1000 people are to be transferred from Kiel to Toulon. The same has to be done for all the needed tools, jigs and welding equipment.
The ferrying through the Canal du Rhne au Rhin, the Sane and the Rhne will not be easy, since the raw pressure hulls will have no rudder. A specific floating frame has to be built to give enough stability to the hulls, and this frame must include towing points, a rudder system, and probably other features. This ferrying has to be done in several legs: on barges in Germany, by towing of the hulls in France.
Note: Type IID submarines, smaller, although less capable, are a lot more compatible with the limitations involved by the Canal du Rhne au Rhin. Transit would be possible by only dismounting the conning tower, the torpedo tubes and the bow outer plates. Considerable time could be spared by reducing the difficulties of the transit and shortening the final assembly.
Two days later (June 6th, 1942), Doenitz approved Winklers first conclusions. On June 11th, during a meeting held in Berlin with his technical staff and shipyards representatives, he kept the choice of the Type II E, since, from his point of view, the operational advantages of this design where more important that the time lost in the building process. At the same period, he asked his Chief of Staff (Godt) to find a resourceful ashore, efficient at sea, and smart everywhere flotilla leader, who will be in charge of the transit operation and assume command in Toulon.
On June 11th, 1942, the building schedule was also adjusted:
- In the beginning of August, 1942, start of the the pressure hulls manufacturing, in welded sections, for the whole class of Type IIE submarines (training, Mediterranean, Black Sea).
- End of October, 1942, delivery of the first batch : 9 boats intended for the trials and the training (U-685 to U-693). Those boats will be the only ones built in a single stage. Two other partially built hulls will be delivered: the first one to validate the multi stage manufacturing process, the other one for the trials of the towing system.
- January, 1943 : ferrying of all the needed tools at Toulon
- End of February, 1943 : delivery of the 27 partially built hulls, intended for the Mediterranean (U-694 to U-699, U-810 to U-820, U-830 to U-839). Ferrying of the remaining equipments and plates at Toulon.
- Mid April 1943 : start of the assembly at Toulon
- June 1943 : delivery of the first units and trials at sea
- August 1943 : first operational patrols
The trials of the first units where planned from November, 1942 to January, 1943, with the hope that they will be going well and not be plagued with too much troubles which will put the schedule at risk.
Deutsche Werfts design office was put under high pressure during the following weeks. As told by D-Ingenieur Fleischmann All has to be done fast and without error, while, ordinarily, in less known domains, we took all the needed time to avoid mistakes. And no electronic computer was available! All the computations had to be done by hand. We reused a great number of parts and components of the Type IID, even if they seemed a little undersized, at least for the first batch, and with the perspective to correct the situation for the second one. And for the multi staged building process, what seemed doable on the paper was sometimes a nightmare at the plant level.
Back in Paris, Doenitz met, on June 16th, the overseer of the occupation Transportation office, to define how the Kriegsmarine could use the Canal du Rhne au Rhin for an operation a lot more complex that the transit of a few MTBs. A compromise was established, for the month of March, 1943, inasmuch as the Kriegsmarine will take all the measures needed to not interrupt the vital flow of merchandises shipped from France to Germany, absolutely needed by the Reich.
On June 21th, Doenitz evaluated the situation with Raeder, and both Admirals agreed upon a name for the whole operation. This would be Sonnenblume (Sunflower), while, like the sunflower, the Kriegsmarine will point now in direction South, to face the threat.
In the beginning of July, Winkler made a short trip to Strasbourg, to check if the harbour installations will be sufficient for the submarines unloading from their barges. He noted that additional cranes would be needed. When he asked about tugs for the journey on the Canal, he was very disappointed by learning that non self-propelled canal boats where hauled from the bank by small electric locomotives running on a narrow-gauge railway between Strasbourg and Mulhouse, and by gasoline or Diesel tractors on tires further on. In Germany, canal boats, a lot bigger, were towed by tugs everywhere. Since the hauling machines were spread along the whole 200 miles of the canal, he understood quickly that not enough of their will be available to haul 27 submarine hulls, until completely stopping the civilian activity or cut the convoy in several smaller ones. And, with the cylindrical shape of these hulls, even stabilized by floats, the hauling from the bank would be calamitous.
At the end of July, a new meeting happened in Berlin. Deutsche Werft informed the participants that the building of the hulls will effectively start in due time, but the two staged process needed still to be refined, mainly due to difficulties in the relocation of tools and manpower in Toulon. To solve the problem of the towing, decision was taken to find the smallest fluvial harbour tug in use, compatible with the small gauge of the Canal du Rhne au Rhin, this type of boat having to be built in sufficient quantity for the needs of the operation. If the case of an unfruitful search, hauling would be the only available choice, and a hard negotiation with the occupation authorities in France should be expected. The general opinion was that all that should be avoided by any means, because the subject will in this case end in Hitler hands, and then, heads will be dangerously on the verge of rolling
In September, the Allied attack against Sicily pulled more and more pressure on the players of the operation. Doenitz had to use all his arguments to convince Raeder that it was not possible to go faster and also that it was out of question to try a new transit of submarines from Atlantic to the Mediterranean. In the hallways and offices of the Admiralty, the operation, which was now known by about everyone, was then nicknamed Sonnenbad (sunbathing)
