Blohm & Voss 138 Seedrache (1940)

German Long-Range Reconnaissance flying boat
Maiden flight 1937, service 1940, 297 produced 1938-43.

The Blohm & Voss BV 138 Seedrache (Sea Dragon) was a trimotor flying boat by a famous shipyard, which became in WW2 the primary seaborne long-range maritime patrol and naval reconnaissance aircraft of the Luftwaffe during. Designed prewar by Richard Vogt, under Hamburger Flugzeugbau and known as the Ha 138 it had quite unusual design features for the type: Twin boom tail unit, short hull, trimotor engine configuration. Due to its hull-fuselage it was nicknamed "Der Fliegende Holzschuh" ("Flying Clog") and development was protracted due to multiple redesigns and engine issues.

Notably after the first prototype flew on 15 July 1937, the tail booms had t be redesigned, with the first production model designated BV 138A-1 flying only by April 1940. From December 1940 it proved very adaptable notably when radar-equipped as anti-shipping aircraft, or used as aerial countermeasure to naval mines with the last C-1 introduced by March 1941 and 297 BV 138s built between 1938 and 1943. The BV 138 was found surprisingly resilient, notably off Norway, where early Sea Hurricane were unable to shot it down.

Development

Blohm brother's Company: From shipyard to aircraft manufacturer

In 1933, the shipyard Blohm & Voss in Hamburg suffered due the lack of orders and Great Depression, and started to diversify by creating that year an aviation division, the Hamburger Flugzeugbau. The company started to be subcontracted manufacture of Junkers Ju 52 subassemblies to gain experience. Later the company was untrusted to produce famous desigs from Dornier, Focke-Wulf, Heinkel, Junkers, and Messerschmitt. Under the trio formed by Dipl-Ing Max Andreae, experienced aviator Robert Schröck and designer Reinhold Mewes from Heinkel, the company soon presented it own first product, the HA 135, a conventional biplane trainer powered by a BMW-Bramo Sh 14A.

6 were made, used by the German Air sports association. The Blohm brothers were unhappy with Mewes and obtained to recruit Richard Vogt, working at Kawasaki at the time. In May 1934 Hans Amtmann from Junkers joined his team, bringing his knowledge of seaplanes. Together they designed the ambitious Ha 139 long-range seaplane mail carrier, with four engines and two floats. More so, it was designed to be flown from a giant catapult mounted on Friesenland, in order to depart with passenger's mail during its transatlantic trips, the first of such services worldwide. Three were used by the Deutsche LuftHansa and 4 HA 142 as its land variant.

Next, the company was renamed Abteilung Flugzeugbau der Schiffswerft Blohm & Voss to avoid confusion, RLM code BV. With orders piling up in 1939 another site was built on the shores of the river Elbe, at Finkenwerder, in addition to the Wenzendorf Aircraft Factory.

The BV 138 was thus originally designated as Ha 138 and became the company's first flying boat, a natural fit to apply knowledge in maritime environment and light boats construction practices. For trials, they were situated on the Elbe, ideal for seaplanes. Blohm & Voss would have many aircraft models manufactured over the years very diverse and sometimes quite remarkable, such as the BV 222 Wiking and BV 238 prototype, world's largest aircraft or the famous asymmetric BV 141 tactical reconnaissance aircraft which saw a small production.
The irony is that the BV-138 would soon become the company's best seller, with intention to had it produced at Finkenwerder but due to design delays, the latter was soon absorbed instead to subcontract manufacture and the BV 138 ended subcontracted out... to Weser Flugzeugbau. Finkenwerder survived the massive Allied bombing of Hamburg...

Later, Vogt had to recruit Hermann Pohlmann (designer of the Junkers Ju 87) to assist him as deputy. The company went for radical designs in 1945 such as the jet-powered P 215 night fighter weeks before the was ended.

From the P.12 to the BV.138

The first prototype trials on the Elbe, in 1937.

Design studies went on in 1934 on various configuration for a naval reconnaissance seaboat, and the RLM ultimately selectioned on paper the P.12 for further development under the long range reconnaissance aircraft programme. It's not that the Luftwaffe lacked such models. There was already the Heinkel 59 torpedo-floatplane that the company's Ha 140 twin-engine torpedo bomber/reconnaissance floatplane of 1937 was to replace, and the Dornier 18 and 24 flying boats among others. The Ha 140 was a German multi-purpose floatplane, that first flew in 1937 but never enter production after three were made and tested. It had more to do with the former HA 135.

Meawnhile, Vogt's design team managed to perfect the P.12 and made it extremely rugged for north sea operations, with a general configuration showing a relatively short hull, igh-mounted wing to improved observaiton and moreover placed high up (over sea spray) its engines, with two standard radial engines and underwings floats. It's most striking feature was its twin boom tail unit. In early 1935 a mock-up was completed and presented to RLM's cadree, whch approved the design at this stage and passed an initial order for three prototypes.

Their demand was that each of these prototypes would tested a different pair of engines to determine the best match. The only requirement was that the pair produced 1,000hp. Eventually, Blohm and Voss determine that the winner engine was the Junkers Jumo 205, but still, engine difficulties led to redesign the model as a trimotor, the third engine being placed at the centre of the wing. Each of these engines had an output of 650 hp and was not in demand, so currently available, as much low-powered engines at the time. It was liquid cooled and well profile, so less risks to ingest seawater.

However, despite changing this configuration, this stage remained protracted, partly due to original engine difficulties and the extensive redesign imposed by three of them, modifying calculaton, balance and flying caracteristics. On 15 July 1937, the first prototype at last performed its maiden flight on the Elbe. It was successful overall, but many issues were single out. Several changes were made afterwards, notably by modifying and extending the vertical tail surfaces and hull. Indeed the first flights revealed severe hydrodynamic and aerodynamic instability. The longer, reshaped hull was completed bby much bulkier tail booms and from there, the RLM ordered five pre-production aircraft, BV-138A-01 to 06. These were the militarized models, capable of carrying multiple defensive machine guns and expected for delivery at best by late 1938, or early 1939.

But issues piled up whereas Blohm and Voss was taken by a complete reorganization of production. The A-1 was thus delayed into late 1939, then early 1940. It's only by April 1940 that the first production BV 138A-01 had its maiden flight, soon to be pressed into service without further tests. The five ones followed suite. The Kriegsmarine just hard pressed the Luftwaffe for these. By June 1940, deliveries enable to fill KüFlGr 506 and 906, three each.

Still, pilots compained of poor structural behaviour in flight and when landing at sea; Much strenghtening again was required. So the BV 138A-1's production run was limited to 25, before being replaced by the BV 138B-1 by the end of 1940. They adopted notably the better Jumo 205D diesel engine (yest you saw right, a diesel) and armament's expansion.

There were only ten BV 138B-0 as pre-production operational test beds in service by October 1940 and after numerous revisions, 21 BV 138B-1 (service from November 1940) were delivered with the 880 PS (868 hp, 647 kW) Junkers Jumo 205D; which were two-stroke, opposed-piston aircraft diesel engines.

The structure still left to be desired, pilots complaining of the screeching, twisting noises in flights, and the final fuselage awas again buffed up considerablyn without engine change but with a the adoption of a 4-bladed propeller for the upper engine. This was the BV 138C-1 introduced by March 1941. This became the definitive version. Vibrations issues were cured by the 4-baded prop, overheating by a new radiator arrangement, while the remaining two had redesigned, broader three-bladed propellers. This version saw 227 being delivered until 1943.

There was a tailored mine-hunting variant as well, but it forked from the C-1 (see later). Production however was terminated in late 1943 as the model started to be obsolete already, the deteriorating condition of the Kriegsmarine, and new priorities for the company under Albert Speer.

The Minesweeper variant

The BV 138MS saw the removal of all conventional armament whereas a roughly 40 feet (12 m) diameter dural hoop was mounted horizontally under the wings. It generated a powerful electromagnetic field, powered by the internally mount auxiliary motor. This electric field, when flying low enough, detonated magnetic mines the allies laid in several key passage area for Kriegsmarine vessels, notably off the Skagerrak and in the north sea. The principle of magnetic field-generating degaussing equipment needed a large equipment, the ring encircling the hull and wings and anchored via two poles at the nose and tail, struts under the wings, all reinforced by cables. A few were epmployed by the Minensuch-Gruppe 1 from 1942 to 1944.

Design of the Blohm and Voss BV 138

Fuselage design

The BV 138 was a trimotor flying boat, meaning the fuselage doubled as hull and helped it float by its own buoyancy. The advantage was that only smaller floats were needed underwing for stabilization. Being a shipyard, the designer, Richard Vogt count count on a wide in-house experience to create a relatively compact hull combined with an hydrodynamic step beneath and flat sides. Its appearance trigerred rapidly the "Fliegender Holzschuh" (flying clog) nickname. Both the nose and tail had openings for machine gunners. The aft one was open, and the nose had an en enclosed plexiglas turret.

The booms of the twin tail unit recalled the Focke-Wulf Fw 189 as they extended horizontally from the outer engine nacelles and ended with apair or rounded tails joint by a massive, compensated rudder. Hydrodynamic dictated the "turn-down" or "beak" at the stern. Of the first prototype had a gull wing, it was not enough to generate lift, and later a third engine was added while wings stayed straight. The underbelly had a hardpoint mid-way at the break for catapult launches, from seaplane tenders that were eventually never built (at laest for the Kriegsmarine).

The wings had a rounded ends, equally angled forward and aft sections. The nacelles were well profiled thanks to the inline choice of engines. The forward cockpit had tandem seats and a well glassed sliding canopy. In addition the roomy hull offered much additional space for passengers (there were up to eight portholes) with folding seats (up to ten passengers could be carried). Forward close to the nose was the position of the radio/mechanic.

Engines

In its final 138 A-1 configuration, the BV 138 was powered by three piston engines. The central one was mounted high above the wing, and many aerodynamic refinements were necessary to optimize its placement and the fairing associated wit its radiator, buried into the wings's forward central lip. The wing engines were lower and had prominent radiator fairing underneath. They were more susceptible to ingest seawater spray and needed fresh water cleaning between missions.

Pre-production prototypes and BV 138A series tested various junkers Jumo variants from 485 to 746 kW (650–1,000 hp). The BV 138B-1 opted for 880 PS (868 hp, 647 kW) Jumo 205D. These were two-stroke opposed-piston aircraft diesels, quite a unique choice in the world of aviation, but perfect to gave extra range compared to regular engines. The cowlings were peculiar due to the vertical six-cylinder opposed-pistons, and recalled 4/6-cyl. inverted inline engines on small utility aircraft, notably those fitted with the Jumo 205 with its propshaft emerging forward at the uppermost front.

This allowed to mount a small prop hub. The choice of diesel (which made sense in naval matters) had an additional benefit, to be refuelled at sea from U-boats, especially the tailored Type XIV Milkshkuhe, using larger diesels. For this, the BV 138 had to be fitted with a fuel filter to avoid condensation during transfer.

But this gave good average performances, not nearly enough to flee fighters (that was not the point) but perfect to bring sufficient radius of action and the needed long range. Top speed was only 285 km/h (177 mph, 154 kn) at sea level when loaded at 14 tons (31,000 lb) and at sea level for a not much slower cruise speed of 235 km/h (146 mph, 127 kn) down to 1,000 m (3,300 ft). The Range was 1,220 km (760 mi, 660 nmi) at 195 km/h (121 mph; 105 knots), even 4,300 km (2,700 mi, 2,300 nmi) with max fuel in ferry range (point to point), over 18h.

Normal mission time was 5 hours on average. This was more than enough to criss-cross the atlantic, this meant reaching the azores from Britanny (and meet a tender), the northnmost point of Norway from Denmark, reach Leningrad and back, reach any moint around Iceland from Trondheim, and criss-cross the Mediterranean. Endurance in all case was five hours and 30 minutes in standard configuration), but neither speed nor altitude, 5,000 m/16,000 ft when loaded at 14,500 kg (32,000 lb), was sufficient to escape fighters.

The BV-138 still can climb (albeit at a sluggish pace) up to 12,800 m (9,200 ft) when loaded at 17,650 kg (38,910 lb) at a rate below 3.67 m/s (722 ft/min) and took 24 min. to reach 3,170 m (10,400 ft). Wing loading was heft at 114.2 kg/m2 (23.4 lb/sq ft) and power/mass 0.106 kW/kg (0.064 hp/lb) so the payload was limited (see below), albeit the general ruggedness and defenses of the Seedrache were its selling point:

Armament

Defensive guns

The BV 138 was well defended against roghe aircraft met at sea with three gun positions:
-The enclosed powered gun turret in the nose (single MG 151/20 autocannon)
-Stern powered glassed turret (single MG 151/20 autocannon)
-Upper gun position in the tail of the central engine nacelle, light 7.92 mm MG 15.
The hull tail gunner indeed had an obstructed arc of fire by the tail and horizontal stabilizer.
Still, this two-tier rear armament was impressive for the compacity of the design. The worst position for the crew was the one of the nacelle MG-gunner alone in a fully open Scarff with ring-like emplacement mount, alone in the cold for the duration of the flight. The nacelle in 1944 generally was upgraded to a 13 mm MG 131 heavy machine gun.
The lower gun position was left generally open (no glassed turret) in the first production models, including the early C-1, however later-built model had the enclosed powered turret with 20 mm gun.

Bombs

The "clog" had no bomb bay, as it would be an hydrodynamic non sense, but six hard point in the inner part of the wing root, between the fuselage and engines nacelles. They could not be accessible during flight. They generally mounted up to three 50 kg (110.2 lb) bombs or two 150 kg (330.7 lb) depth charges starboard. With the Umbau set, the same were installed under the port wing so six bombs, four depth charges. The bombs were generally not intended to deal with ships but submarine, with the hope that a near-hit would damage the pressure hull and prevent diving.

The BV-138 could also mix bombs and depth charges, albeit the load reduced range. Umbau sets allowed these modifications in the field, for adding a bomb rack under the port wing as at factory, ony the starboard underwing was equipped. The payload could be doubled and in German nomenclature, these were called BV 138 C-1/U1, "U" standing for the Umbau.

⚙ BV 138 specs.
Light/Gross Weight	11,770 kg (25,948 lb), 14,500 kg (31,967 lb)
Max Takeoff weight	17,650 kg (38,912 lb)
Lenght	19.85 m (65 ft 1 in)
Wingspan	26.94 m (88 ft 5 in)
Height	5.9 m (19 ft 4 in)
Wing Area	112 m2 (1,210 sq ft)
Engines	3× Jumo 205D 6-cyl. LC opposed piston diesel engines 647 kW (868 hp) each
Propellers	3-bladed constant-speed propellers
Top Speed, sea level	285 km/h (177 mph, 154 kn) at sea level at 14,000 kg (31,000 lb)
Cruise Speed	235 km/h (146 mph, 127 kn) at 1,000 m (3,300 ft)
Fuel capacity	3,750 L (990 US gal; 820 imp gal) max. int. fuel
Range	1,220 km (760 mi, 660 nmi) at 195 km/h (121 mph; 105 kn)
Ferry Range	4,300 km (2,700 mi, 2,300 nmi) with max fuel
Autonomy	5 hours and 30 minutes
Time to altitude	3,170 m (10,400 ft) in 24 minutes
Wing loading	114.2 kg/m2 (23.4 lb/sq ft)
Power/mass	0.106 kW/kg (0.064 hp/lb)
Climb Rate	3.67 m/s (722 ft/min)
Ceiling	5,000 m (16,000 ft) at 14,500 kg (32,000 lb)
Armament	2×20 mm (0.787 in) MG 151 nose, 1× 13 mm top rear, 1–3 × 7.92 mm (0.312 in) MG 15
Bombs	3 × 50 kg (110.2 lb) bombs or 2 × 150 kg (330.7 lb) depth charges
Crew	6 (pilot, navigator, radio operator, nose-gunner, rear-gunner, upper-rear gunner), 10 passengers

The BV 138 in action

The BV 138 was deployed to the Küstenfliegergruppe 406, 506, 906 (coastal patrol) and the Seeaufklärungsgruppe (naval recce) 125, 126, 129, 130, and 131. It was also deployed with the Küstenflieger-Ergänzungsstaffel 138 (or Coastal Aviation Supplementary Squadron), and well as the Minensuch-Gruppe 1 der Luftwaffe (see the MS 1 variant) and the Kampfgruppe zur besonderen Verwendung 108 or "Combat group for special use" where to could perform unusual missions.

Operation Weserubung, the invasion of Norway, in April 1940 had two pre-production models used as troop transports. By December it was at last operational in its main reconnaissance role, the first units being based in Western France. Several models were converted to sweep naval mines. Pilots complained however for its insufficient structural strength.

By March 1941, the BV 138C-1 entered service and was soon equipped with the FuG 200 Hohentwiel low-UHF band maritime search radar set for anti-shipping missions in all weather and by night. The crew varied up to six personnel, ten passengers such as the variant KG200 deployed from the modifications experimented at Weserubing. In that case, the Luftwaffe had a model of seaplane that can carry ten fully armed infantry troops.

They also took part in Operation Wunderland in 1943, to assist the U-255 sent to the East coast of Novaja Zemlya. The latter refuelled its recce BV 138 4 times during its flights over the Kara Sea up to the Vilkitsky Strait. Not reporting any prey, the mission, later to be given to Lützow, was cancelled. The BV 138 was robust enough to test the Walter HWK 109-500 Starthilfe RATO jettisonable rocket pod. Two were mounted aft of the hull for shorter takeoff, jettisoned after use but fitted with parachutes to be recovered after landing. Their perimeter of action was restricted to Norway in 1944-45 as in the Mediterranean, allied aviation ruled supreme from 1943 and the fall of Italy did not improved matters.