Siebel ferry

The Siebel ferry (Siebelfähre) was a shallow-draft catamaran landing craft operated by Germany's Wehrmacht during World War II. It served a variety of roles (transport, flak ship, gunboat, convoy escort, minelayer) in the Mediterranean, Baltic and Black Seas as well as along the English Channel. They were originally developed for Operation Sea Lion, Germany's planned but never-executed 1940 invasion of England. Siebel ferries continued performing useful service even after the war's end in 1945.

Origins and development
As German Army preparations for Operation Sea Lion got underway in July 1940, frustration soon grew over when and what types of ships the Kriegsmarine would eventually supply for use in the planned Channel crossing. The immense task of converting hundreds of inland river barges and motor coasters into proper landing craft had only just begun, however, and the Kriegsmarine was unable to give the Army a fixed date for their availability.

Anxious to begin conducting landing exercises, Field Marshal Walther von Brauchitsch finally ordered General of Engineers Jacob to undertake construction of the Army’s own sea-going transports, independent of the Kriegsmarine’s efforts. Jacob in turn assigned this task to Pionier-Battalion 47 of 7th Army Corps. The battalion was soon moved to Cateret on the western side of the Cotentin Peninsula and the men of this unit fanned out across the French countryside searching for buoyant materials with which to construct self-propelled rafts and ferries.

Aircraft designer Fritz Siebel (now commissioned a colonel in the Luftwaffe), was putting a captured aircraft plant near Amiens in northeastern France back into production when he was approached by a Lieutenant Colonel from Pionier-Battalion 47. The colonel asked Siebel if he could requisition a large pile of empty gasoline containers lying outside next to the plant. Siebel agreed to part with the cache provided the colonel explained what use he would make of them. The answer piqued Siebel’s interest and left him pondering the question of how to ferry troops and heavy equipment across the English Channel given the limited time and materials available.

Meantime, Pionier-Battalion 47 began experimenting with a host of makeshift materials for constructing powered rafts, including wine-barrels, tree trunks, kapok-filled sacks and ship’s canvas. Early efforts to use open-ended river-crossing pontoons came to naught. The iron beams and bolts used to hold them together failed to stand up to the waves and the pontoons quickly filled with water even in a light sea.

Finally, someone at OKH (Oberkommando des Heeres) thought of using a larger closed-end bridging pontoon developed during WWI by Austrian Colonel of Engineers Hans Herbert. By spanning two connected pontoons with a 10m x 10m deck, guns, vehicles and troops could be transported atop them. This idea would later evolve into the Herbert ferry but, at the time, only sixty-four of these pontoons were available, too few to consider mass-production.

However, another type of closed-end pontoon was available in greater numbers. This was the heavy pontoon bridge (schwere Schiffsbrücke), of which 364 were at hand. Colonel Siebel, who by now had been assigned his own Sonderkommando (special command) for improvising Luftwaffe invasion craft, had a prototype built with two heavy bridging pontoons spaced 6m apart in a catamaran arrangement connected by steel cross-beams. In the center, mounted on a pyramid-shaped truss-work, were two end-to-end surplus aircraft engines. This vessel was dubbed the kleine Fähre (small ferry) and initial testing was conducted on Rangsdorfer See, a lake near Berlin.

General Franz Halder, Chief of the Army General Staff, and a party of other Army officers were invited to witness one of these tests but they were unimpressed by the ferry’s performance. It could only make 4 knots and seemed overly flimsy. Halder noted in his diary, “Nothing new, may not stand up in surf.” Others in the party questioned whether the soldiers to be so transported would even arrive in good fighting condition.

Despite the Army’s misgivings, Siebel continued working on modifying the ferry’s design, reducing the initial 6m spacing between the pontoons to 5.5m and requesting from Krupp-Rheinhausen (with assistance from WasserPrüfung 5) construction of a large steel platform covered with wood planking. This would both serve as the vessel’s cargo deck and provide the necessary longitudinal and lateral strength to withstand operating in the open sea. Indeed, this version would later prove capable of surviving even Force 6 waves.

The only propulsion systems readily available in the quantities needed to mass-produce such hastily improvised invasion craft were diesel truck engines and surplus aircraft engines. In consultation with engineers from Pionier-Battalion 47, Siebel settled on a combination of four 75 hp Ford V8 engines (two each mounted side-by-side in the aft end of the pontoons) linked to standard marine propellers and, for additional motive power, three BMW 6U 750 hp aircraft engines mounted on elevated platforms along the aft edge of the cargo deck.

Early problems with this arrangement included premature engine failure due to insufficient cooling. However, this was eventually solved by linking the water-cooled aircraft engines to the diesel truck engines via piping to the pontoon compartments. Steering the craft by water screws alone also presented difficulties. A telegraph connected the wheelhouse to the two pontoon engine crews. But because of the vessels broad beam, the crews had to react nearly simultaneously to requests for changes in speed or the ferry would begin turning, a movement the rudders could barely counteract.

On 31 August 1940, the newly modified Siebel ferry was tested in the Ems estuary by the Special Command of the Merchant Shipping Division. Using only the ship’s aircraft engines, it attained a maximum speed of 8 kn. In contrast to the truck engines in the pontoons, the aero engines were directly controlled by the helmsman via throttles in the wheelhouse, allowing him to vary each engine’s speed as necessary. This greatly improved maneuverability. The aircraft engines were noisy, however, and often prevented voice communication on deck. They also consumed large amounts of fuel. For Sea Lion, it was decided to use them only for the run-up onto the invasion beaches or as a back-up in the event the water screws were damaged.

On 4 September, two additional versions of the Siebel ferry, one powered by Opel Blitz truck engines and one powered by Ford V8s were again tested on the Ems estuary. Using only water screw propulsion, they achieved a cruising speed of 7 kn, though it was believed this could be raised by installing more efficient propellers.

The Siebel ferry pontoons were flat-bottomed and squared off in front. In combination with the vessel’s wide cargo deck, this made for an exceptionally stable gun platform. The Luftwaffe mounted various-sized flak pieces on the ferries and tested their suitability for engaging both air and surface targets while at sea. The 8.8 cm guns in particular proved well-adapted for this role.

Series production of the Siebel ferry began in September 1940 at Antwerp as a joint Army-Luftwaffe venture with the Army’s Böndel Pionier-Sonderkommando (Engineer Special Command) assembling the pontoons, decking and water propulsion while Col. Siebel’s Luftwaffe-Sonderkommando installed the auxiliary surplus aircraft engines. By late September twenty-five of these craft had been completed.

For Operation Sea Lion, the Luftwaffe organized the Siebel ferries into two flotillas: Flakkorps I (assigned to 9th Army) and Flakkorps II (assigned to 16th Army). They were intended to provide flank defense against air, ground and surface targets for the First Wave tow formations. Each Siebel ferry would transport a complete flak unit consisting of one 8.8 cm gun and two 2 cm guns plus their three prime movers (although 9th Army planned to transport the necessary towing vehicles and support personnel separately via barges). Upon reaching the invasion beaches, the ferries were to land their flak units and then assist with unloading the larger steamers anchored offshore. The Army rather disingenuously referred to the Siebel ferries as “destroyer substitutes”.

Wartime service
With its simplicity of design, sturdy construction, good seakeeping and the ease with which it could be disassembled and shipped via rail to virtually any point on the Continent, the Siebel ferry proved a highly useful and adaptable amphibious vessel for transporting troops, vehicles and supplies across open water wherever needed by German forces. It was also easily configured to serve a variety of special purposes, from minelaying to convoy escort.

Following Sea Lion's indefinite postponement in October 1940, the Luftwaffe had decided in the fall of 1941 to continue producing Siebel ferries apart from the Army under its own Special Ferry Command (Fähre-Sonderkommando). It replaced the diesel truck engines on the original design with BMW and other type aircraft engines, housing four of them in enlarged pontoon ends connected to water screws via a reversing gearbox. Some of these vessels were assigned to the Army and operated with mixed crews but the Army disliked the use of aircraft engines, claiming they were prone to catching fire and consumed excessive amounts of fuel. The Army's own Engineer Ferry Construction Command at Antwerp continued using vehicle engines as the main propulsion unit and considered them a more reliable choice than the Luftwaffe's aero engines.