Design of these submarines started in 1934 when navy issued a specification for the cruiser type submarine that would be able to work with the fleet.  The initial design was confirmed in April 1935 and the project was given number 41 or Series XIV.  Design was complete by the end of next year and first three submarines were laid down in December 1936.  A grand total of 62 submarines of this class were planned but as with many other ships in the Soviet naval program of the time that number was wildly optimistic and it was later cut to 20.  In the end only 12 were laid down between 1936 and 1938 of which 11 were completed.

K class submarines were the best Soviet submarines of the war and their characteristics were comparable to large submarines built abroad.  Their characteristics are as follows:

Displacement: 1500/2117 tons
Dimensions:  97.7 m oa x 7.4 m x 4.04 m
Reserve Buoyancy: 41.4%
Machinery: two 9DKR diesel engines, 8400bhp = 22 knots; one 38K8 diesel generator, 800bhp; two PG-II electric motors, 2400 shp = 10.3 knots
Bunkerage: 46 tons oil normal, 240 tons oil full; surfaced, 2400nm at 22 knots, 16,500nm at 9 knots; submerged, 10.4nm at 10.3 knots, 175nm at 2.9 knots
Endurance: 30 days normal, 50 days max
Diving Depth: 80 meters normal, 100 meters full
Dive Time: 50 seconds surfaced to periscope depth, 30 seconds decks awash to periscope depth
Max Underwater Time: 72 hours
Guns: two 100mm/51 B-24-PL (400 rounds total), two 45mm/46 21-K AA (1100 rounds total), two 7.62mm mgs.
Torpedo Tubes: 533mm, six bow + two stern + two external aft; twenty-four torpedoes
Mines: 20 EhP-36 mines
Sonars: from 1939 Mars-12 hydrophones, from 1942 on Lend-Lease Drakon-129
Complement: 62-65

6 submarines (K-1, K-2, K-3 and K-21, K-22, K-23) were completed before the war, and were transferred to the Northern fleet.  Two more (K-51 and K-52) were completed with emergency measures in September-November 1941, and were given to the fleet without even acceptance trials.  Last four were being slowly completed during the war in blockaded Leningrad.  Three (K-53, K-55 and K-56) were completed in 1942-44 while K-54 was never completed.  The boats completed during the war remained in the Baltic sea throughout the war.

These large boats had only limited success during the war since the nature of the war forced them to operate in the nature they were not designed for - in the coastal areas of the north and in the relatively small Baltic Sea where their large size and relatively slow dive times put them at a disadvantage.  During the war K boats confirmed to have sunk 5 transports and 2 warships while damaging several more, in addition several other transports were claimed to have sunk, but non have been confirmed.  In addition Soviet sources credit them with 11 transports and 3 warships sunk but their mines.  The exact number of the ships sunk by Soviet submarines is still a mystery since there are no comprehensive mistake free sources that allow the numbers to be confirmed.

The most famous (and infamous) attack by these boats was made by K-21 on 5th of July 1942 against German Battleship Tirpitz.  For the next 40 years Soviet literature claimed that K-21 hit it and forced the German battleship to retire back to the harbor.  Unfortunately German sources do not confirm that, more so they didn’t even notice that they were attacked!  What most likely happened is that K-21 did indeed attack Tirpitz, but missed and the explosions that were heard by K-21’s crew were the sound of the propellers of the German ships maneuvering.

During the war 5 of the subs were lost to the Germans (K-1 - lost to causes unknown in Sept 1943, K-2 lost on a minefield in October 1942, K-3 sunk by German Submarine hunters in March 1943, K-22 lost to causes unknown in February 1943, K-23 lost to German Submarine Hunters or Aircraft in May 1942).  After the war these subs were the longest ranging sub in Soviet inventory for several years until new post war designs were built and were left in service until the end of 1950s.  K-21 was partially restored and today can be seen in Severomorsk near Murmansk.

Hulls

For a long time HP models K-21 was the only game in town if you wanted a K-21 in any scale.  Fortunately recently Kombrig came out with their own version of K-21.  Looking at the HP hull first thing you notice is that is it completely smooth with no flood holes.  That is unfortunate since K class subs had a large number of flood holes in its hull.  Kombrig hull does have those in abundance.  Putting the hulls side by side other discrepancies become noticeable.  HP hull is 2mm longer than the Kombrig hull.  After measuring them Kombrig hull is spot on (overall length is 13.9 mm while the real boat’s 97.7 meters divided by 700 comes out to 13.95mm).  It seems that HP used the boat’s overall length as its waterline length, since the HP hull is 13.9mm along its waterline.  Second thing that is immediately seen is the difference in the bow between those two models.  Kombrig hull has the characteristic triangle shape and a slight hump seen on the early K boats, while it is absent on the HP hull.  As a warning, several of the later submarines (probably K-51 and up) were completed with a much larger hump on the bow to improve sea keeping, so refer to the references if you are planning to build any boat other than K-21.  There are several other differences between the hulls.  The HP’s desk is wider and the walkway around the superstructure is loner and more square, while Kombrig’s is a lot more proportionate and seems to better fit the photo evidence.  Overall Kobmrig hull beats HP’s hands down.  In addition to the waterline hull Kombrig kit (like their other submarine kits) offers a choice of a full hull or waterline version.  It is beautifully done and recreated the complex shape of the K boat’s hull.

Here the difference between kits becomes even more pronounced.  HP kit has only 9 parts which are rather poorly cast, while Kombrig has 24 excellently cast parts.  All of the rudders and other things that are necessary to build a full hull version of the sub are included in the Kombrig’s kit.  Artillery is also excellently cast, while HP’s guns are nearly unusable.  HP’s superstructure is good 2mm longer than Kombrig’s one and looking at the photos, it would seem that Kombrig is more accurate.  Also gun turrets are identical in the HP kit while Kombrig includes more streamlined aft turret, which is more accurate.  The only problem that I saw with the Kombrig’s superstructure is the absence of the rounded extensions where the aft 45mm gun would be on the superstructure, but it is a very minor detail.  Overall Kombrig once again wins without a question.

Since both kits are basic the instructions are also very basic and easy to read.  Kombrig’s instructions, as customary for them, includes a 2 view drawing and history of the K-21 along with painting instructions and technical characteristics of the ship in Russian.
 

Kombrig’s kit is an excellent representation of this large Soviet submarine and is a lot better than HP kit.  Since the price of the kits is about the same ~$11 there is no justification for getting an HP kit if Kombrig’s kit is available.