Cutaway model of a 6cm high explosive shell L3/4, as used in the 6 cm/21 (2.4") Bts KL/21 (Boots kanone / Boat gun), the 6 cm/21 (2.4") S-Bts KL/21 (Schnell-boots kanone), and the 6 cm/21 (2.4") Casemat gun C/98 as used in the captured Belgian coastal defence fortresses during WW1.
The gun was designed by Krupp in 1898 and taken in use in 1900.
As the gun was originally meant as a light gun on destroyers, the breech block is bronze made to be better resistant to sea water (I do not know if the barrel is also bronze made).
Later, the gun was also used on landing boats where a loose gun carriage was also carried on board so the gun could be placed on the carriage for land usage by infantery after the landing.
In WW2 the gun was still used on coastal defence ships and submarine hunters (mostly confisquated and armed fishing vessels or coasters).
The 6cm calibre with seperate shellcase is quite uncommon for a naval gun (quite light), and is more commonly found in mountain guns.
The shell body is steel made and fuzed with the “Haubegranatenzunder (HbgrZ) 17”, a pyrotechnic time and impact fuze.
The explosive charge is placed in a cartboard container, glued into the projectile body. The (Kl. Zdl C/98) booster is placed in a cartboard container, placed in the explosive charge. The space between the explosive charge and the fuze is filled with glazeboard discs.
Functioning of the HbgrZ.17 :
The fuze consists of a steel body. Inside -on the centerline- two firing caps are placed, both facing the two mirrored firing pins, pointing up and down, placed between the two firing caps. Two brass rings containing the pressed powder pyrotechnic delay rings are placed over the steel fuze body. Only the lower brass ring can be rotated to set the desired activation distance in hectometers (100 mtr intervals). 44 hectometer (4400 meters) is the maximum range that can be set. On top of the fuze a steel cap is screwed to lock up both rings, in the base of the fuze body, a brass cap is screwed, closing the powder magazine.
There are two ways the fuze can be used:
As an impact fuze (as in picture) :
The brass setting ring is not rotated, therefore the only part of the ring that has no pyrotechnic powder train remains over the flash channel leading to the magazine, effectvely locking it down ( left side lower ring as in picture). Upon firing the shell, inertia swings the upper firing cap downward into the upper firing pin, activating it and igniting the pyrotechnic powder trains in the upper brass ring, but to no avail.
Upon impact, inertia will swing the lower firing cap forward –riding the spring- into the lower firing pin, exploding the firing cap. The flame of the firing cap travels down, straight into the magazine. The flame of the magazine will ignite the Kl. Zdl C/98 booster, igniting the main charge in the shell.
As a time fuze:
The lower brass ring is rotated to the desired distance setting in hectometers. This places the pyrotechnic delay train in direct connection with the flash channel that leads to the black powder magazine. Upon firing the shell, inertia swings the upper firing cap downward into the upper firing pin, activating it and igniting the pyrotechnic powder train in the upper brass ring, which ignites the pyrotechnic delay train in the lower brass ring. The reason two rings are used is that in the upper ring the flame burns against the rotation direction of the shell, in the lower ring it burns with the rotation direction of the shell. This will even out the deviation in time/distance caused by the flame travelling after the powder or being pressed into the powder by rotation of the shell.
As the flame in the lower ring has burned up to above the flash channel, the flame ignites the magazine. The flame of the magazine will ignite the Kl. Zdl C/98 booster, igniting the main charge in the shell.
If the projectile hits an object before the pyrotechnic fuze has expired, the impact fuze is activated, instantaniously exploding the shell
The brass shellcase is 64mm long and is filled with a linnen bag containing 0,25 Lb (0,144 kg) blade powder 4x4x1,5mm thick. The bag with powder is locked up in the shellcase by means of a pressed in cartboard cup. In contrast to most seperate shellcases the charge cannot be adjusted.
Projectile weight : 6,0 Lb (2,71 Kg).
Explosive charge : 0,32 Lb Picric acid (0,144 Kg).
V0 : 1434 Fps (437 mtrs/sec).
Max range : 3.732 yards (3.400 mtrs).
Rate of fire : 12 to 14 rnds /minute.
Regards, DJH
The gun was designed by Krupp in 1898 and taken in use in 1900.
As the gun was originally meant as a light gun on destroyers, the breech block is bronze made to be better resistant to sea water (I do not know if the barrel is also bronze made).
Later, the gun was also used on landing boats where a loose gun carriage was also carried on board so the gun could be placed on the carriage for land usage by infantery after the landing.
In WW2 the gun was still used on coastal defence ships and submarine hunters (mostly confisquated and armed fishing vessels or coasters).
The 6cm calibre with seperate shellcase is quite uncommon for a naval gun (quite light), and is more commonly found in mountain guns.
The shell body is steel made and fuzed with the “Haubegranatenzunder (HbgrZ) 17”, a pyrotechnic time and impact fuze.
The explosive charge is placed in a cartboard container, glued into the projectile body. The (Kl. Zdl C/98) booster is placed in a cartboard container, placed in the explosive charge. The space between the explosive charge and the fuze is filled with glazeboard discs.
Functioning of the HbgrZ.17 :
The fuze consists of a steel body. Inside -on the centerline- two firing caps are placed, both facing the two mirrored firing pins, pointing up and down, placed between the two firing caps. Two brass rings containing the pressed powder pyrotechnic delay rings are placed over the steel fuze body. Only the lower brass ring can be rotated to set the desired activation distance in hectometers (100 mtr intervals). 44 hectometer (4400 meters) is the maximum range that can be set. On top of the fuze a steel cap is screwed to lock up both rings, in the base of the fuze body, a brass cap is screwed, closing the powder magazine.
There are two ways the fuze can be used:
As an impact fuze (as in picture) :
The brass setting ring is not rotated, therefore the only part of the ring that has no pyrotechnic powder train remains over the flash channel leading to the magazine, effectvely locking it down ( left side lower ring as in picture). Upon firing the shell, inertia swings the upper firing cap downward into the upper firing pin, activating it and igniting the pyrotechnic powder trains in the upper brass ring, but to no avail.
Upon impact, inertia will swing the lower firing cap forward –riding the spring- into the lower firing pin, exploding the firing cap. The flame of the firing cap travels down, straight into the magazine. The flame of the magazine will ignite the Kl. Zdl C/98 booster, igniting the main charge in the shell.
As a time fuze:
The lower brass ring is rotated to the desired distance setting in hectometers. This places the pyrotechnic delay train in direct connection with the flash channel that leads to the black powder magazine. Upon firing the shell, inertia swings the upper firing cap downward into the upper firing pin, activating it and igniting the pyrotechnic powder train in the upper brass ring, which ignites the pyrotechnic delay train in the lower brass ring. The reason two rings are used is that in the upper ring the flame burns against the rotation direction of the shell, in the lower ring it burns with the rotation direction of the shell. This will even out the deviation in time/distance caused by the flame travelling after the powder or being pressed into the powder by rotation of the shell.
As the flame in the lower ring has burned up to above the flash channel, the flame ignites the magazine. The flame of the magazine will ignite the Kl. Zdl C/98 booster, igniting the main charge in the shell.
If the projectile hits an object before the pyrotechnic fuze has expired, the impact fuze is activated, instantaniously exploding the shell
The brass shellcase is 64mm long and is filled with a linnen bag containing 0,25 Lb (0,144 kg) blade powder 4x4x1,5mm thick. The bag with powder is locked up in the shellcase by means of a pressed in cartboard cup. In contrast to most seperate shellcases the charge cannot be adjusted.
Projectile weight : 6,0 Lb (2,71 Kg).
Explosive charge : 0,32 Lb Picric acid (0,144 Kg).
V0 : 1434 Fps (437 mtrs/sec).
Max range : 3.732 yards (3.400 mtrs).
Rate of fire : 12 to 14 rnds /minute.
Regards, DJH
