WW2 German Minesweepers

Introduction:
The “M-Flotte” of Minensucher (minesweepers) and minelayers (most having this dual role) of the Kriegsmarine took a considerable place numerically and was granted a sizeable attention and finances. These were the most affluent units of all types in the Kriegsmarine (apart U-Boote), with a total of 679 minesweepers (37 WW1 surviving M-Boote, 218 M-Boote between the interwar and WW2 and 424 R-Boote (the coastal ones). To these can be added three dedicated minelayer (Brummer, Brummer II and Grille) and “part-time minelayers”, essentially all 50 destroyers and 36 torpedo boats built and serviced in WW2 had this added capacity as well. Many German destroyers in fact were involved in high profile, risky minelaying missions close to British shores early in the war. And there were converted, auxiliary combat ships called the Sperrbrecher, with a fleet up to 100 former merchant ships.

“real thing” overview of German Minesweepers (and a few minelayers) in the interwar and WW2. The fleet of WW1 M-Boote are shown as they remained in service in the interwar (Reichsmarine) and the smaller MinensRäumboote are also shown, albeit they were seen in another post. This just shows the scale of this fleet, out of proportion of the rest of the Kriegsmarine, showing the Kriegsmarine took mines very seriously in the confines of the Baltic and North sea.

Mine warfare indeed always had been a concern for Germany, as it was for other Scandinavian powers and Russia (and later USSR). This inexpensive weapon could single-handedly sink any kind of warship, from nimble minesweepers to battleships. In the confines of the Baltic, especially in the narrow Gulf of Finland, mines were all the rage before, during and after WW1 and were still a primary concern for the Kriegsmarine in case of war with the Soviet Union. This was also true of the Skagerrak, this time to prevent the allies to “bottle up” Baltic ships sortie, albeit there was the Kiel canal to communicate to the North sea. The Jade bay and Heligoland waters were also traditional “hotspots” of minefields.

It’s a paradox though that of all belligerents, the Germany Navy had the most important fleet of mineswepeers. This type, was indeed more favoured by countries limited to coastal defence and asymetric warfare (Finland, Denmark, Sweden, Norway). Britain, the US, France, Italy, all had far more limited minesweepers/minelayer fleets. In fact the mighty Royal Navy only had until 1939 a single minelayer, HMS Adventure. In WW2 the RN created the Abdiel class. However, as minesweepers went, this was limited to the 14 interwar Halcyon class. In WW2, the effort however was considerable with the Bangor, Bathurst and Algerine class built in series but they all also doubled as ASW patrol ships and escorts.

WWI legacy M-Boote

Prior to these were the M1 class. 26 of these early type minesweepers, called the Type 1914, were built in early WWI at five yards. They were partly derived from large fleet tugs and north sea trawlers. Seaworthy vessels, they could lay or sweep mines at 13 knots and were very active during the war. However 12 were lost in action and the remainder were scrapped in 1921-24.
The next M27 class were a bit larger (480t vs 425t) and slightly faster, but of the overall same design. They could lay 30 mines like their precedessors. 30 were built at various yards and commissioned in 1917 for the last ones. Eight were lost in operations on mines, M55 was wrecked, M27 sunk in collision, M28, M50 were used in the Reichsmlarine, the latter, renamed Brommy was used as S-Boote tender. M48, 51, 52, 53 were sold to Argentina, M42 became a French merchant vessel, was sized by the Kriegsmarine in WW2 and turned into a minelayer, KMS Nymphe. The other were sold for scrap in 1921-22. Only the last class was massively used in WW2.

Author’s rendition, M-Boote Typ 1916

This was by far the most important class still in service with the Reichsmarine and then Kriegsmarine. They were also the base on which was developed the interwar Type 1935. Also called the Type 1916 they were still very close to the Type 1915. In fact some considered these as the “second series” of the M27 class minesweepers ordered in 1916. Thus was a very large serie, from M57 to M162 (105 ships), but only around 30 were effectively completed before the end of the war. Ths type being authorized by the allied commission, many were launched in 1919 and completed for the new Reichsmarine and effectively constituted the bulk of its minesweeper force until the Type 1935 arrived.

The M97 group was similar to M71-96 group but with a better armament. 18 units type 1916 were still used by the Kriegsmarine as auxiliarie, renamed KMS Hecht, Nautilus, Jagd, Frauenlob, Hela, Havel and Taku, all S-Boote tenders. There were also Von der Groeben, Adlers, Nettelbeck and Raule used as R-boot depot ships, Acheron was used as a U-Boat tender/depot ship, Delphin and Fuchs were used as gunnery training ships and Störtebecker, Sundewall, Arcona and Otto Braun were used as test ships given their veneranle age.

In 1939-1940 the majority of these vintage M-Boote was reclassified as minesweepers and returned to their former numbers. By the end of 1940 to not create confusion with the new M-Boote their were renumbered, with the 500 series, with a 5 added to their original number.
Originally they were armed like their predecessors with a pair of 8.8 cm KL/45 defensive guns on the forecastle and quartedeck roof. From M75 to M84 they had an additional 8.8 cm/45 guns but all were capable of carrying and laying 30 mines.

It should be added that in WWI the Kaiserliches Marine also operated scores of smaller coastal minesweepers: The FM type built in two groups were sixty-six tiny 170t vessels launched 1918-19, most not completed or seeing service in WWI. The letters FM stands for “Flachgehende Minensucher” lit. shallow draught minesweepers. They looked like austere Type 1916, and were cheaper to built for the same missions, albeit they did not have that dual role and don’t carried mines. Most were sold of BU after the war so they won’t be seen there.

Also the Kaiserliches Marine operated the F class, small wooden hulled 18t motor boats designed in 1914, built 1915-18 (75 made) carried on old battleships, such as SMS Wittlesbach, Schwaben, Prussen and Lothhringen, reconverted as tenders/motherships as they were stored on deck and put at sea with cranes. The idea was to bring them in strategic locations for minelayer missions and support them with these pre-dreaenoughts acting as “naval fortresses”. Most were sold after the war or BU, 12 ended for example in the Belgian Navy.

M57 class in WW2

M28: Trial ships Pelikan 11.29; M528 1940; GMSA* 1945
M50: R-boat tender Brommy 26.11.37; M550 1940, Sunk 15.6.44
M60: Trials ship Hecht 29.8.38, R-boat tender Hille 1939, M560 1940. GMSA 1945.
M61: Mined 26.7.40
M66: Trials ship Stortebecker 26.11.37; M566 1940; GMSA 1945,
M72: M572 1940; GMSA 1945
M75: MS75 1940; Capsized 2.3.45
M81: Trials ship Nautilus 11.3.29; M581 1940; GMSA 1945
M82: Tender Jagd 11.3.29; M582 1940; GMSA 1945
M84: M584 1940; Sunk 30.11.44
M85: Mined 1.10.39
M89: Mined 26.7.40
M96: Hospital ship 1921; Capsized 15.3.22
M98: M598 1940; GMSA 1945
M102: MS02 1940; GMSA 1945
M104: M504 1940; Sunk 9.4.45
M107: R-boat tender Von der Groeben 1939; MS07 1940; Sunk 15.6.44
M108: Gunnery training ship Delphin 2.6.28; M508 1940; To USSR 1945
M109: Trials ship Johann Wittenberg 29.8.38; Sundewall 2.12.38; M509 1940; GMSA 1945
M110: M510 1940; GMSA 1945
M111: M511 1940; Mined 3.11.41
M113: U-boat tender Acheron 20.6.36; M513 1940; GMSA 1945
M115: Trials ship Arkona 1.10.35; M515 1940; Mined 22.5.44
M117: M517 1940 GMSA 1945
M122: M522 1940, Bombed 20.3.45
M126: R-boat tender Alders 1939; M526: 1940 GMSA 1945
M129: Trials ship Owto Braun 29.8.38; M529 1940; Mined 2.12.41
M130: AA TS Fuchs 12.5.28; M530 1940; GMSA 1945
M132: Sunk 13.11.39
M133: Tender Wacht 11.3,28; R-boat tender Raule 24.8.39; M533 1940; Collision 9.5.42
M134: Tender Frauenlob 11.3.28; M534 1940; R-boat tender GMSA 1945
M135: Tender Hela 5.2.23; Fungingen 1938; Gazelle 2.1.39; M535 1940. To USSR 1945
M136: U-boat tender Havel 29.8.38; Mined 26.7.40
M138: Fishery protection vessel Zieten 1923; R-boat tender Nettelbeck 2.5.39; M538 1940; Ran aground 26.1.45
M145: M545 1940; GMSA 1945
M146: Tender Taku 26.8.33; R-boat tender Von der Lippe 1939; M546: 1940; Bombed 17.6.44
M157: M557 1940; Foundered 23.12.41
*GMSA: The postwar German Minesweeping Administration in which worked German minesweepers, tasked of clearing mines on all trade lanes in the Baltic and North sea. See the Bundesmarine for more.

There was a total of 36 of these vintage M57 class boats, with some converted to experimental ships, artillery school ships, fleet tenders or R-boat tenders or survey ship, auxiliaries in the eyse of the allied commission, avoiding scrapping. In 1940, most were re-designated and reconverted as minesweepers.
After the First World War it was incumbent upon the Germans to clear the North Sea and Baltic of all mines, and so the German minesweeper flotillas remained intact, but of course disarmed, after the Armistice.

Most of the boats were sold either for scrapping or to foreign navies and private owners in the early 1920s, but some of the remaining boats, inactive for a period, were adapted tor various purposes in the late 1920s and early 1930s. There were differences in hull length, superstructure appearance, armament, etc, but in 1940 all were reclassified as minesweepers, adding the prefix ‘5’ to their former pennant numbers. In the late 1940s they were sold to private owners, most of them being converted for harbour and riverine use. There were also the gunnery tenders Drache and Hay and the oceanographic survey ship Meteor in service in 1922.

⚙ M57 class specifications
Displacement	500-525t standard, 535-590t FL
Dimensions	59.3-59.6moa x 7.30-7.40m x 2.15-2.28m
Propulsion	2 shafts VTE, 2 Marine boilers 1750-1800 hp
Speed	15-16.5 kts
Range	2000 nm /14 kts
Armament	2x 88mm L/45 C/14, 30 mines, mechanical minesweeping gear
Crew	40

Specifics of German Minesweepers

General design and construction

Since the development of magnetic mines dated back from 1938 and degaussing systems were then used, construction practices meant these ships were simply built in steel. The only minesweepers still around in WW2 that were of the WWI M57 class had a simple and functional design that was refined from the M1 class in 1914. A forecastle to make them seaworthy enough, cut at the height of the bridge, composed of an enclosed section and open one on top, mainmast with projector, and the large lower deck bulwarked all along to avoid wave splashing, ending in a semi-transom shape with chutes for the mine rails, long enough for 15 mines on either side. The minesweeping gear was also stored on that long working aft deck. Single, tall funnel.
The M57 boats were a 500t design, but late series went up to 575t when fully loaded.

Propulsion

The M57 boats had a classic two shaft arrangement with VTE (Vertical Triple Expansion) engines, study, reliable, made for reasonable consumption with their two navy coal fuelled boilers, and still providing them with reasonable speed at 16 knots based on 1850 hp. Indeed, unlike interwar M-Boote, these old models ran on coal, which was quite handy especially at the end of WW2 as oil was rare. The only exception would be M138 Nettelbeck which in 1939 received a new Germania steam turbines coupled with a MAN diesels for 1,680hp, 14.1kts, and carrying 90t of diesel oil.

The Type 1935, 1939 and 1940-43 had the same powerplant in design, 2 shaft vertical triple expansion engines engines with exhaust turbines to boost the drive shafts, fed by two oil-fired boilers for 3,200 hp (2,386 kW)* for the M35/39 but the lack of oil meant the Type 1940 and 1943 had instead 2 coal-fired boilers, meaning a decrease of power down to 2,400 hp (1,790 kW). The M1935/39 were capable of 18 knots as a results and the M40/43 were down to 16 knots.
*On average, in reality the M35 were rated for 3500 hp and the very first two had the special Voith-Schneider low rev system. The Typ 1938 were rated for 3200 hop and the Typ 1939 for 3700 hp.

As for the range, it depended again on their oil capacity, 143 tonnes for the 1935 and 1939 and 155 tonnes for the Typ 1938. Endurance was, on average, 5000 nm at 10 knots. For the Type 1940 and their coal-fired plants, some sources gives a different output of 2615 hp, 156t of coal, and 17.2 kts, but 4000 nm at 10 knots, sowithout surprise a range decreased of 1000 nm as coal is far less efficient. For the M1943, all sources agrees on 2700 hp, 14 knots, 136t coal, and the same range as above.

Armament

As said above the WWI vintage M57 class still around in 1939 were logically armed with two or three 8.8 cm guns SKL/45 and 30 mines. It should be noted that M57-66 had two 105mm/42 L/45 C/16 guns, M97 to 105 and M107 to M111, as well as M115, 116, 119, 120 and 121 had two 105mm/42 L/45 C/16 with a modified higher elevation mount.
The 1935/38/39 and mobilization Types typically had two 105mm/42 C/16 main guns, fore and aft, a bit like the WWI boats, but wit the addition of a twin 37mm/80 SK C/30 FLAK, and two single 20mm/65 C/38 FLAK. They had rails for 30 mines, had two depht charge tracks (12 depth charges) and a mechanical minesweeping gear.
The Type 1938 had no 37 mm but three 20mm AA instead and the Type 1939 had a modern twin 37mm/80 SK C/30 and 20 depth charges.

Kriegsmarine’s Minelayers

Technically, there was less emphasis on minelayers in the Kriegsmarine from 1935 than in the Kaiserlichesmarine in 1914. In WWI, Germany built four specialized minelayers cruisers and the RN believed more were on order. They were particularly fast, and a whole new class of cruisers was ordered to counter them. There was also a fleet of civilian vessels transformed as minelayers such as the famous Koningin Luise.

Laid down 1934 by Deschimag, Bremen. Resembled contemporary German tenders in general layout. She was sunk on 14.4.40 during Operation Weserubung, the Norwegian campaign. She was a gunnery training ship/minelayer launched on 21.12 1932. After being commissioned in 1936 Brummer she was attached to the Naval Air Defense and Artillery School (Marineflugabwehr und Küstenartillerieschule) in Swinemünde spring of 1937 and she made two visits to Odde, Göteborg, and Helsingborg. In 1939 she took part in the invasion of Poland, laying mines off the Polish coast and by January 1940 used as a commerce raider in the Baltic Sea, then in April 1940, took part in Operation Weserübung as command ship. On April 14 she was torpedoed by the HMS Sterlet off Jutland but stayed afloat for nine hours so the crew could evacuate.

Specs

Displacement: 1596t standard; 1860t deep load
Dimensions: 308ft 8in wl, 318ft 3in oa x 37ft 9in x 11ft 10 in (94.00, 97.00 x 11.50 x 3.60, 4.00m)
Machinery: 2 shafts, Laval geared tubines, 3x watertube boilers, 2x 4-stroke Sulzer diesels, 6000shp for 21kts
Armament: 4x 127mm/45 C34 (4×1), Twin 37mm/83 C38, 280 mines m/83 C33 420mm
Complement: 168

The second KMS BRUMMER was a gunnery training ship usable as minelayer in wartime launched on 29.5.35. She was the Ex-Norwegian Olav Tryggvason, laid down at Horten N Yd in 1931 captured in 1940 to replace the previous Brummer after she was lost in Norway. Olav Tryggvason was taken, renamed Albatros II on 11 April, and on 16 April Brummer after she had been torpedoed in the Kattegat on 14 April 1940. After capture she was rearmed, her former four 12 cm main guns ended on the Atlantikwall at Hundvåg in Vestlandet by May 1945 and she was used as minelayer off Netherlands and Belgium for the planned invasion of England.
Next she was sent in the Baltic Sea for Operation Barbarossa, laying mines in the Gulf of Finland at Utö with KMS Tannenberg and Hansestadt Danzig on 14 June 1941 in preparation for the invasion.
On 22 June Mine Group Nord operated west of Hanko (Apolda) until attacked by two Soviet Beriev MBR-2 flying boats. The minefield claimed a Soviet destroyer, Gnevny and a submarine, M-81. Precious, Brummer was the only purpose-built minelayer in the Kriegsmarine and multiplied missions, with Operation Nordwind in September 1941, and in 1942-1944 mainly operated in the North Sea and off Norway. On 9 April 1943 the she was torpedeod but missled by Soviet submarine K-21 off Båtsfjord, Finnmark. In 1944 she took part in Operation Hannibal, used for mining until the spring of 1945 and started evacuating German troops and civilians from east prussia. On 3 April 1945 RAF bombers bombed her in drydock inKiel. Her wreck was scrapped in 1945-1948.

Specs

Displacement: 2410t standard; 3010t deep load
Dimensions: 354ft 4in wl, 370ft 9in oa x 44ft 3in x 10ft 6in, 13ft 9in max (108.00, 113.00 x 13.50 x 3.20, 4.20m)
Machinery: 2-shaft Wagner geared turbines, 2 Wagner boilers, 10,000shp = 20kts
Armament: 2×2 105mm/65 C33, 2x 88mm/45 C35, 2×2 37mm/83 C30, 450 mines
Complement: 182 (480 as TS).

Grille was an Admiralty yacht (launched 15.12.34) and first ship in the Knegsmarine to test the high-pressure steam machinery that was scheduled for use in German destroyers. Laid down in March 1934 at the Blohm & Voss yard in Hamburg, she commissioned on 19.5.35 and served in peacetime as an Admiralty yacht during fleet reviews and visits by VIPs. During the war she was used as a minelayer up to February 1942. She was decommissioned in July 1942 and was used asa staff ship at Narvik for the C in C and ComSub Arctic. After the war Grille was transferred to Great Britain and then in 1947 to the USA, where she was sold to private owners. She was broken up in 1951.

Specs

Displacement: 2560 tons standard; 3430t deep load
Dimensions: 377ft 3in wide 442ft 1in oa (not including bowsprit) x 44ft4in x 11ft 2in, 13ft 9in max (115.00, 138.00 x 13.50 x 3.40, 4.20m)
Machinery: 2x Blohm & Voss geared turbines, 2 Benson boilers, 22,000shp, 26kts
Armament: 3x 127mm/45 C34, 2×2 37mm/83 C33, 4x 20mm C30
Complement: 248

Only with tonnage restriction lifted in 1935 with the Anglo-German naval treaty really enabled a true expansion of the Navy. If not, the vintage M57 boats were anyway planned to be replaced by the Type 1935 as a one-for one basis at the end of their service. The first new series were called the Type M-boote Typ 1935 but were ordered from 1936 first and foremost to replace these worn out World War I boats as mentioned.
They were built at Stülcken, Hamburg, Oderwerke, Stettin, Flender-Werft, Lübeck. In all, M1 to M24 were ordered to these yards and constituted the initial 1935 Type. They were laid down from July 1936 to July 1939, completed 1938-1940.
They were larger and as designed, more versatile and seaworthy. The idea from the start was to use them as “jack of all trades” for convoy escort, AA escort, ASW and minelaying on top of minesweeping. In fact they were very well designed to undertake all these roles and Soviet intel even referred to them as “M-type destroyers” (…).
However unlike the WWI boats they proved overly complex and very expensive. In addition, they ran on oil, not coal, meaning they suffered from fuel shortages in 1944 like the rest of the Kriegsmarine. It’s not a surprise that post-1945 GMSA mostly relied on coal-fuelled surviving M57 boats.
A total of 69 ships were built in eight different shipyards and three main sub-classes between 1937 and 1941 so this was a considerable effort that was undertaken here. This was indeed for the size the largest serie of surface combatants in the Kriegsmarine, reflecting their very versatile nature. It’s not surprising that roughly half (34) were lost in action during the war.

M-Boote Typ 1935

The first of these sub-classes represented the bulk of WW2 German minesweeper fleet by far. Only M1 and M2 of the 1935 serie as prototype were equipped to test the Voith-Schneider low-rev vertical-axis propellers for extra manoeuvrability as singular propulsion system, designed to be more agile in a minefield. It was however too complex, costly and unreliable to produce and was not adopted for the entire class however.
They had Wagner or La Mont boilers and Lentz steam engines. 24 were built 1936-40 (1941 for M24).
Losses were heavy: M1, M2, M5, M6, M8, M10, M11, M13-16, M18-20 were lost in action. M22 was scuttled in May 1945, the remainder were surrendered to the allies:
USSR obtained M3, M7, M17, and to France M4, M9, M12, M21, M24. M23 went to UK. The latter was returned to Germany and the GMSA. Both he French and Soviet vessels were considerable postwar service.

⚙ 1935 Type specifications
Displacement	682 lt standard, 874 lt full load
Dimensions	68.1 x 8.7 x 2.12/2.65m
Propulsion	2 shaft VTE+ exhaust turbines, 2 oil-fired boilers, 3,500 hp
Speed	18 knots (33 km/h; 21 mph)
Range
Armament	2× 10.5 cm, 2× 3.7 cm, 2× 2 cm AA, 30 mines, 2 DCT(12)
Crew	84

M-Boote Typ 1938

A total of 12 Type 1938 were built from 1939 to 1942 as M25-M36, between Stülcken, Hamburg, Oderwerke, Stettin, Lübecker Maschinenbau, and Schichau, Elbing.
They displaced more, were longer, beamer from 8.70 to 9.20 meters but same draught. The powerplant changed enough for their outpout to be down to 3,200 hp but they had more oil, 155t compared to the Typ 1935 or 1939, the increased hull dimensions were a test of seaworthiness improvement, and the extra room enabled to install additional generators for magnetic sweeps. Four were sunk in action, the remainder surrendered and M28, 35 went to France, M29, 30, 34 went to USSR, M33 to UK, M32 became a mercantile vessel.

⚙ 1938 Type specifications
Displacement	713t standard, 908t FL
Dimensions	66.6m wl/71m oa x 9.20m x 2.12-2.65m draught
Propulsion	As above but 3200 hp
Armament	Same but no 37 mm, 3x 20mm AA
Crew	97

M-Boote Typ 1939(mob)

The need for more ships and cost concerns signalled a return to a more austere design for mass production. It should be precised that their Wagner or La Mont boilers wotked at 35 atm and 450°C but had, especially the first, the same reliability issues as for other ships in the Kriegsmarine. However the 1939 Type were the most powerful at 3700 hp, they were back to the M1935 in dimensions, except a tad larger, and four tons heavier. They were also repited faster at 18.3 knots.
Armament wise, this was as for th M1935 but with longer depth charge racks, 10 each instead of six DCs. Later during the war, the arament was extended to six or even eight 20 mm anti-aircraft guns. They also had a GHG hydrophone suite.

This was the largest serie by far, 32 ships built from M37 to M256 (missing intermediate numbers for M1940 and M1943 boats). To be exact, M37-39, M81-85, M101-104, M131-133, M151-156, M201-206 and M251-256. In additio to the above yards, they were also built at Neptun, Rostock, Rickmers-Werft, Wesermünde, and Lindenau, Memel. They were laid down in 1940-41, completed 1941-43. 8 were sunk in action, 3 scuttled, 1 lost in collission (M101), and the remainder surrendered to the allies and attributed to UK, France and USSR. Western vessels were most often returned to Germany and assigned to the GMPSA.

⚙ 1939(mob) Type specifications
Displacement	785t standard, 878t FL
Dimensions	66.6 wl-68.4m oa and same as M1935
Propulsion	Same but 3700 hp
Speed	18.3 knots
Armament	same but 20 DCs
Crew	95-199 (1944)
Sensors	GHG hydrophone

http://www.navypedia.org/ships/germany/ger_mine_m551.htm
Although the M1935 was satisfactory in general, it was too complex and coslty to continue production in wartime. Thus, a simplified design was designed, started to be laid down in 1941. The second aspecst was oil consumption, which alreadty started to be rationed. Since efficience imported less as these vessels did not needed to be fast, the Type 40 were given coal-fired boilers. It was a sound decision as oil shortages soon became even more acute. A total of 127 were built between 1941 and 1944, so even more than the 1935/38/69. And on this total, 63 M1940 class ships were sunk, the remainder scuttled, surrendered and attributed to USSR, UK or France, many of which were returned to the German MPSA and later integrated into the Bundesmarine.
The numbering started at M261 and ended at M496. A few ships were completed with TS numbers.
Construction called for the following yards: Atlaswerke (Bremen), Rickmers-Werft (Wesermünde), Lindenau (Memel), Unterweser Schiffbau (Lehe), Oderwerke (Stettin), Lübecker Maschinenbau, Neptun (Rostock), Schichau (Elbing), Elsflether Werft. These were yards accustomed to small vessels and/or civilian ones.

Many were also built by most yards in occupied Netherlands to add to numbers (and numbering complications): The M401-408 series at Rotterdamsche Droogdok, the M411-415 at De Schelde, Vlissingen, the M421-428 at Wilton-Fijenoord, Schiedam, M431-438 aty Nederlandsche Scheepsbow, Amsterdam, the M441-446 at P. Smit, Rotterdam, M451-456 at Gusto/Smulders, Schiedam, M459-463 at Gusto/Smulders, Schiedam, the M459-463 at Nederlandsche Dok, Amsterdam, M467-471 at van der Giessen, Krimpen, M475-76 at J. & K. Smit, Kinderdijk, M483-84 at Boele`s, Bolnes, M486 at Verschure, Amsterdam, M489 at L. Smit, Kinderdijk, and M495-96 at Pot, Bolnes. Of course construction was delayed and sabotaged if possible, but the ships, laid down in 1941, were launched in 1942-43 and completed in 1942-43.

The M1940 ships had a standard displacement of 637 tonnes, full load displacement of 775 tons and measured 62.3 meters overall (57.6 wl), with a beam of 8.5 (8.90m meters in other sources) for a draught of 2.3 meters ro to be precised, 2.34t normal 2.82t deep load.
They were powered as sais above by two coal-fired boilers, driving a two-shaft arrangement with two triple-expansion engine (VTE) still fitted with two exhaust-steam geared turbine units of the Bauer-Wach design. The system works above 13 kts, when the turbines takes over on both shafts between 14 and 17.2 kts (top speed ever recorded). They were indeed a knot slower than the M1935 but this was inevitable due to the use of less efficient coal. This speed degraded as lower quality coal was provided as well. The Naval Type boilers were from Schultz, working at 16.5 atm and 320°C. Being also bulky meant less storage, down to 156 tonnes for 4000 nautical miles a 10 knots. The average output recorded was of 2,400 hp, 17 knots, range 1,043 nautical miles at 17 knots.

Four vessels of this class were also launched for the Romanian Navy in 1943 as the “Democrația class”, built locally at Constanta from German materials, structurally identical but with a different armament.
The Type 1940 Armament was down to a single 105 mm gun forward (105mm/42 C/16 or SK C/32), plus a single 37mm/80 SK C/30 FLAK, and two 20mm/65 C/38 FLAK AA guns.
The standard gear was two small depth charge racks, 6 total, but a more advanced mechanical, magnetic, acoustic minesweeping gear. It seems they were not fitted to lay mines for the ealry series. The next series (M261 onwards) had two 105/42 SK C/32 alternative to a single 105mm/42 SK C/32 but a twin 37mm/80 SK C/30)). The ones with two main guns had also two single 37mm/80 SK C/30, a Flakvierling (quad mount) 20mm/65 C/38, plus twp single 20mm/65 C/38, and a more substantial ASW complement of four DCT, two DCR (42 DCs) and 12 mines.
Some individual ships (M294, 328, 330, 495) with two main guns had between three and four single 37mm/69 FlaK M/42, a Flakvierling 20mm/65 C/38, two single 15mm/84 Maschinengewehr (MG) 151 AA guns, two 86mm R Ag M42/43 rocket launchers, four DCT, two DCR (42 DCs) and also 12 mines.

The TS series were a torpedo boat conversion: They had either two main guns, (or alternative, single 105mm/42 SK C/32, two single 37mm/80 SK C/30), two single 37mm/80 SK C/30, a Flakvierling 20mm/65 C/38, two single 20mm/65 C/38, two 533 TT, 4 DCT, 2 DCR (42), 12 mines and retained a minesweeping gear.
It’s not clear how the TS series tactical gains were perceived. They were essentially coal burning, slow and conspicuous auxiliary torpedo boats, it seems mainly intended as escort roles. These ships also had a better sensor suite: In adition to their GHG hydrophone, same as for the previous 1939(mob) they also mounted presummably the S-Gerät sonar.

⚙ 1940 Type specifications
Displacement	543 lt standard, 775 lt full load
Dimensions	62.3 x 8.5 x 2.8 m (204 ft 5 in x 27 ft 11 in x 9 ft 2 in)
Propulsion	2 shaft VTE+ exhaust turbines, 2 coal-fired boilers 2,400 hp (1,790 kW)
Speed	16 knots (30 km/h; 18 mph)
Range	1000 nm at 10 kts
Armament	1× 105 mm, 1× 37 mm, 6–8 × 20 mm FLAK
Crew	80
Sensors	GHG hydrophone

This was a further simplified and slightly enlarged version of the M1940 type. These ships were designed for pre-fabrication and were produced in four versions:
A Minesweeper, and ASW vessel with extra depth charges, a Torpedo boat, with two 533 mm (21 in) torpedo tubes, a Torpedo training vessel. Production was much simplified with niceties of previous designs omitted and simpler shapes. They were also all armed with two torpedo tubes as the previous TS series of the M1940 type.
They were almost a repeat of the M-Boote Typ 1940 but armed wit the following: Two 105mm/42 SK C/32 main guns, two 37mm/57 FlaK M/43, flakvierling 20mm/65 C/38, two single 20mm/65 C/38 FLAK, 2x 533mm (21-inches) TT, 4 DCT (36), 24 mines, mechanical, magnetic and acoustic minesweeping gear. They were also the first with a FuMo 61 radar, but only installed on some ships. The S-Gerät sonar and GHG hydrophone were the default ASW sensors.

However, on the mass that was laid down, only 18 vessels were completed by the time the war ended. The series comprised M601 to 633 from Neptun, Rostock and its subsidiary yard as well as Atlaswerke in Bremen.
Production was ramped up also at Schichau, Königsberg, with M801 to 816 being laid down. Laying down dates unknwown. They were launched in 1944, about half were completed but almost none entered service. Many were asso scuttled incomplete and the remainder BU incomplete. The ones from Schivahu were the onl ones basically to enter service: M801 was surrendered in May 1945 and was given as war reparation to Italy in June 1949 (as Gazzella), M803 same as Gaino, three more were sunk, two on 11 March and one in April 1945. Two were completed for the Soviet Union, three were completed for the postwar GMSA.

On this chapter, the ships used by the Bundesmarine were completely rearmed, with one 76mm/50 Mk 22 DP gun forward, two twin 40mm/60 Bofors Mk 1, three 20mm/70 Mk 7 Oerlikon AA guns, and the same mechanical, magnetic, acoustic minesweeping gear. They helped through the GMSA to clean up maritime trade lanes from leftover mines, a job that took almost a decade, but mooring mines and forgotten minefields were discovered for several decades and driting mines were still a reality until the 2000s.

⚙ 1943 Type specifications
Displacement	582 lt standard/821 lt full load
Dimensions	67.8 x 9 x 2.27-2.8 m (222 ft 5 in x 29 ft 6 in x 8 ft 10 in)
Propulsion	2 shaft VTE+ exhaust turbines, 2 coal-fired boilers, 2,400 hp (1,790 kW)
Speed	17 knots (32 km/h; 19 mph)
Range	4000 nm/10 kts
Armament	2× 105 mm (4.1 in), 2× 37 mm + 6–8 × 20 mm FLAK
Crew	96-107
Sensors	GHG hydrophone, S-Gerät, FuMo 61 radar

The “junior division” of the German minesweeping force, tailored for coastal operations, are of the course the MinenRäumboote for which a dedicated article was created.
The Räumboote, or “R-Boote” were the mass-produced wooden-built German coastal minesweepers. In time, they also performed other missions, notably escort, ASW patrol and rescue. They were built from 1929 with the following R1, R17, R25, R41, R130, R151, R218, R301 and R401 classes, the latter mostly not completed at end of the war. In total, the Kriegsmarine operated 424 R-Boote from Lürssen, Bremen-Vegesack, Abeking & Rasmussen, Lemwerder at Schlichting. They soldiered in the Baltic, Norway, Channel and German bight, Mediterranean and Black sea, distributed among 19 flotillas. 140 surviving boats of the Räumboots-Flottille were used postwar by the Mine Sweeping Administration (GMSA) predecessor to the Bundesmarine.

R130 specifications
Displacement: 150 tonnes
Dimensions: 41.10 x 5.80 x 1.60m
Propulsion: 2 shafts MAN diesels, 1,800 bhp
Speed/Range: 19 kts, diesel oil 11 tons, 900nm/15 kts
Armament: 1x 37mm/69 M/42 FLAK, 6x 20mm/65 C30 AA, see notes
Crew: 38

While the primary objective of Minesweepers was to detect and disable mines, another measure was more radical in concept. These were the “pathfinder” a rough translatuion of the concept of the Sperrbrecher. A bit like flail-tanks on land, these were vehicles that cleared a path in a mlinefield followed by other vehicles and infantry, a quick shortcut to avoid spending many hours cleaning a minefield.
At this was the same concept. To blockade Germany, the RN laid many minefields and were ready to pounce in the voluntary left open channels. Thus the Kriegsmarine decided to convert a serie of “expandable” ships would role would simply to punch their way through marine minefields, to be followed by notably blockade runners. The technique was not new and a bunch of Kaiserliches Marine’s auxiliaries were already named that way and for this role. Theor origin is quite obvious, as it took 200 times as long to clear a minefield as to lay it. Specialized minesweepers had a very dangerous job, and could be at any moment attack by air or sea during the long hours spent sweeping mines.
In WWI already, 30 Sperrbrecher were used to clear mine paths, of which eight were lost, some (such as Rio Negro, Plauen or Wigbert) also deployed aepolanes as mines were much easier to spot from the air. The idea was resurfaced in WW2 an somewhat expanded.
*Miniature: Sperrbrecher 131.

To create a safe passage through a minefield a ship was modified in order to simply detonate the mines and not only they had to be of robust construction but they were also reinforced for the task. To withstand mine explosions the bow was generally reinforced by additional beams and bulkheads, strengthened hull in general and fillings with a buoyant material in order to to keep the ship afloat even if the bow was mushed by 2-3 mine blasts. In 1940 magnetic mines were often dropped by aircraftby night on both sides, on the path of harbours and naval bases. They were small, deep, and stayed undetected, so the Sperrbrecher converted at the time for this task had a VES-System, a huge magnetic field generator capable detonating magnetic mines at a save distance, 460 metres (500 yd) from the hull. This was soon discovered and British RN intel bureau was soon able to find a way to fuse the mines, laid as traps, desensitised and only activated when the ship was directly above. This work and sank many more Sperrbrecher.

In order to their magnetic systems, mostly used early in the war, and their reinforced bow, they also were heavily armed to face any air attack. The larger the ship war, the heavier was the AA armamment as additional protection, even barrage balloons for some. The official term in the Krisgmarineamt was ‘Special Purpose Merchant Ships’. The British referred them to as “Heavy Flak Ship” due to their heavy armament. In fact they were apt surface combatants as shown by an encounter with HMS Wanderer, an escort destroyer, which limped back to port after receiving from the captain’s own admission “a bloody nose”. Some captains even were decorated such as Karl Palmgreen, receiving the Knight’s Cross of the Iron Cross on 3 August 1941 in command of Sperrbrecher IX and I.

In total over 100 Pathfinders were converted, but no dedicated class was built. This freed resources, man hours, yards occupation, cost and time to convert mainly former merchant ships ranging up to 5000 GRT. They were simply named “Sperrbrecher-” with a number, but not all are known in detail. Many were also (if not most) simply large trawlers. What is certain is that roughly 50% were lost in action and the crew thus had better chances of survival compared to U-Boote crews (later in the war they used to precede U-Boats to their cruise stations). The few surviving ships were reconverted for a merchant career until the 1970s.
The ships for which we have specs are not numerous: 18, 31/131, 32/132, 33, 34/134, 36/136, 38/138; 39/139, 137 and 138.
One example: SS Friedrich Karl (1938) was a small (1360 GRT) 72m cargo which was first vorpostenboot V 108 Friedrich Karl (Multirole escort Flakship), She was redesignated as Sperrbrecher 138 on 26 September 1941, assigned to the 1 Sperrbrecherflotille but on 23 December 1942, she struck a mine and sank off Borkum.

Read More

on deutsches-marinearchiv.de
wlb-stuttgart.de/
usni.org/ german-minesweepers ww2
navypedia.org/ minelayer m57
navypedia.org minelayer 35/39mob
en.wikipedia.org/ M class minesweeper
plimsoll.southampton.gov.uk/
kbismarck.com minesweepers type 35
kbismarck.com minesweepers type 40
kbismarck.com minesweepers type 43
worldnavalships.com german minesweepers
german-navy.de kriegsmarine minehunter mboot 43
german-navy.de kriegsmarine mboot 35
german-navy.de mboot 40
german-navy.de mboot 43
german-navy.de sperrbrecher