The Blohm & Voss BV 138 Seedrache (Sea Dragon), affectionately known as Der Fliegende Holzschuh, was a trimotor flying boat employed by the Luftwaffe during World War II for extensive maritime patrols and naval reconnaissance. A total of 297 BV 138s were manufactured between 1938 and 1943. Originally designed under the name Hamburger Flugzeugbau, it was initially referred to as the Ha 138. The BV 138 featured a distinctive appearance marked by a combination of unique design elements, including a twin boom tail unit, a compact fuselage, and a trimotor engine setup. The aircraft's compact hull, with its hydrodynamic step beneath and flat sides, earned it the moniker "Fliegender Holzschuh" (the flying clog). The twin tail booms extended horizontally from the rear of the outer engine nacelles, resembling the configuration of the smaller Focke-Wulf Fw 189 twin-engined reconnaissance monoplane. To optimize hydrodynamics, the hull incorporated a noticeable "turn-down" or "beak" at the stern.
The initial prototype of the aircraft incorporated a gull wing design; however, it became evident during the inaugural flight that this configuration failed to generate sufficient lift. Consequently, this concept was discarded for the second prototype. These airplanes also featured a hardpoint intended for catapult launches from seaplane tenders. The first standardized model, known as the BV 138 B-1, was equipped with three Junkers Jumo 205D engines, each delivering 880 PS (868 hp, 647 kW). The engine cowlings sported an unconventional appearance, primarily due to the distinct vertical orientation of the six-cylinder opposed-piston Jumo 205 diesel engines. These cowlings bore a resemblance to those found on smaller civilian and utility aircraft with 4 or 6-cylinder inverted inline engines. This design stemmed from the placement of the Jumo 205's prop shafts, emerging forward at the uppermost front end of the powerplant.
The decision to employ diesel engines had a significant advantage, as it allowed the aircraft to refuel at sea from U-boats, which also utilized diesel engines. However, when conducting in-flight refueling at sea, it was necessary to install a fuel filter, given that ship-borne diesel fuel contains a degree of condensation.
The aircraft featured three gun positions for defensive purposes. The first one was positioned at the bow and included an enclosed, powered gun turret housing a single MG 151/20 autocannon. At the stern, the fields of fire were somewhat obstructed due to the presence of the tail and horizontal stabilizer. To address this limitation, there were two additional gun positions on the fuselage. One was located lower on the fuselage, while the second was situated higher, just behind the central top engine.
The gun position positioned behind the central engine, which afforded a clear view over the horizontal stabilizer, featured a fully open emplacement resembling a Scarff ring. This emplacement could accommodate a 7.92 mm MG 15 machine gun; however, most aircraft were equipped with a 13 mm MG 131 heavy machine gun. The lower gun position at the rear of the fuselage was aimed below the horizontal stabilizer. Initially, it was an open arrangement housing a machine gun, but in later versions, most aircraft were fitted with an enclosed powered turret similar to the one at the bow.
The BV 138 Seedrache stands out as one of the most intriguing aircraft designs that not only made it into production but also served in significant numbers within the Luftwaffe. Various versions of this aircraft were adapted for distinct roles. Some were repurposed for minesweeping duties and equipped with magnetic field-generating degaussing equipment. Others found their niche within the specialist KG200 unit, where they were utilized to transport ten fully armed infantry troops instead of carrying bombs.
Today, complete BV 138 aircraft are no longer in existence. However, a sunken wreck of one such aircraft, which met its fate after the war during a British air show, was successfully salvaged from the seabed in the Øresund Sound in 2000. This historic relic is now proudly displayed at the Danish Technical Museum in Helsingør.