In a couple of British Navy documents concerning the Battle of Jutland there is a detailed description of dismantled and studied German delay-action base fuzes used in their latest "C/11" type 28 cm and 30.5 cm AP shells (these were gathered from the many duds that hit British ships and remained inside of them).

These base fuzes were a real "Rube Goldberg" design based on major modifications to a prior non-delay AP shell base fuze using picric acid as the booster to set off the main filler charge. The straight tube in the old fuze connecting the detonator -- set off by the firing pin forward motion on impact -- with the picric acid booster sticking like a finger deep into the stacked pre-shaped block-TNT main filler charge (each block covered with a thin paper and felt wrapper and the upper end of the cavity having a wooden cushion to reduce the effects of armor impact shock since TNT is not the most insensitive filler, though much preferable to Lyddite if deep penetration of the target is desired). The detonation blast from the detonator would directly hit the exposed bottom face of the booster and set it off. The entire delay would be about 0.003 seconds, when it worked. TNT was so insensitive, though, that the picric acid booster explosive, though one of the most powerful known at that time that was safe to use in a projectile (and as a main filler in pre-Jutland British APC shells), was rather unreliable at setting off TNT (or most other less-sensitive explosives). That is why so many German shells were duds or low-order explosions.

To add the delay action effect to these fuzes, the German manufacturer had a problem in that the fulminate of mercury detonator was so powerful (to try to make sure that the booster went off properly, though that was not the problem with the shell explosive, it was the inadequate booster used with the given filler) that it destroyed the small black-powder wafer used to give the delay, even though the thin wafer was tightly squeezed between two thin metal protective disks with many tiny holes in them to allow the flames from the detonator to enter the rear-facing side and, after the delay, exit forward into the booster (which, being picric acid, could be reliably set off by the blast of a small black-powder charge -- British pre-Jutland APC shell base fuzes used black powder "boosters" flaming directly into their Lyddite main fillers with no problem with reliability due to this cause).

In an attempt to fix this excessive blast problem that caused the delay not to work reliably, you would think that the fuze makers would simply make a smaller detonator to stop this delay-wafer destruction. However, you would be wrong!! Instead, they left the detonator alone and replaced the direct tube connector between the detonator and the delay wafer, which was pressed tightly against the picric acid explosive at the base of the booster, with a triangular-cross-section spiral that caused the blast to have to move several times the distance that the old straight tube allowed, as well as having the blast have to bounce around the corners of the corkscrew tunnel several times before finally reaching the holed metal plate on its side of the delay wafer. This usually reduced the blast enough to preserve the wafer, but not always, as battle results showed. In fact, battle results also showed that the blast was many times reduced too much so that it did not even have enough heat left to start the wafer burning, which of course resulted in a dud unless the projectile broke up and its filler was sometimes set off partially due to friction. To try to fix that too-weak blast problem, some finely-powdered loose black powder was sprinkled into the space just in front of the wafer so that it would hopefully always be ignited and have enough flame to make sure that the wafer was working, though this too didn't work much of the time and would make the delay much longer even when it worked. All-in-all, a poorly-designed fuze.

The British looked at the wafer used and estimated that, when it worked correctly, the delay was 0.025 second. (In reality, at most half the shells would have a delay anywhere near that number, with the rest being non-delay or, more likely, a much longer delay or a dud.) The British examiners of the German fuze were not impressed and called that triangular spiral tube a “tortuous tunnel”, which I do not think was a compliment.

The British immediately started on their post-Jutland "Greenboy" APC shells, which were much superior to any other AP shells of their time (primarily due to their ability to work at rather large oblique impact angles, which no other shells could at that time), in one measure due to the new British APC-shell base fuze used, the Number 16D. The old one was the Number 16 Base Fuze and was a simple non-delay type, once the safety interlocks were unlocked after the gun fired, the shell merely had to impact the target so its firing pin hit its fulminate of mercury detonator and this sent a small blast into a tube with black powder lining its inside to carry the flame to the small ring of black power pressed up against the Lyddite filler at the fuze tip, and then KABOOM! It did not need anything but that black powder “booster” in its tip to set off the extremely sensitive Lyddite (picric acid) filler very reliably.

To make a new base fuze using the German-style fuze delay, which no British APC shell had ever used before, the British decided to use the old Number 16 APC Base Fuze and modify it into the Number 16D -- for “Delay”, of course -- APC Base Fuze. They decided to use the same delay as the German fuze did (when it worked as designed), giving 0.025 second nominal delay (with some variation from shell to shell being expected). (NOTE: One source says the true final delay was 0.024 second. Close enough.) They also developed a new APC-shell explosive called "Shellite" to replace the too-sensitive Lyddite. This was made from 70% Lyddite mixed thoroughly with a much less sensitive and low-power explosive relative of Lyddite called dinitrophenol (Lyddite is trinitrophenol). This new explosive could remain inert under significant armor impact shock, allowing delay-action fuzes to work, which Lyddite would not by itself.

The Number 16D fuze was more complex than the original fuze, allowing a small black-powder delay pellet to be used. This delay was optional, though the fuze was set up to use the delay during normal loading of the shell. It could have the delay bypassed, returning the shell to the non-delay roughly-0.003-second-inertial-delay operation, if a small round plate was screwed into the shell fuze bottom by hand prior to firing the gun.

Without the plate, the fuze had an internal plunger open at the bottom of the fuze to the blast of the gun propellant and this plunger would move upward in the fuze and cause the path of the detonator blast to be directed to a small black-powder delay element prior to then entering into the hollow tube lined with black powder directly to the narrow black powder fuze-tip ring that acted as the “booster” to set off the main Shellite filler charge. Use of black powder as the final detonator/booster -- with the hollow tube lined with black powder ensuring that the flames that reached the final black-powder ring would always have enough blast to set it off, even if a longer delay occurred. But a black-powder igniter in the Number 16D base fuze could detonate Shellite much of the time, just like it had Lyddite. The plate worked by simply preventing the gun propellant blast from moving that plunger, restoring non-delay operation, as noted.

However, it was found by many tests during the period after the end of WWI that Shellite did have a large number of low-power explosions – from mere “violent explosions” like black powder to “partial detonations” -- when using just black powder in the fuze tip, so eventually, when finally perfected in the late-1920s, a tetryl booster -- termed “Composition Exploding” or “CE” -- was eventually added to the Shellite-filled large-caliber APC shells, which previously was only used by new post-WWI cruiser CPBC (6") and SAPC (8") shells and NELSON and RODNEY 16” light-weight APC shells that were using TNT/beeswax fillers. This increased the reliability of the Shellite filler full-power detonation to the desired 90% test standards.

Note that after WWI, both the US Navy and the German Navy went to longer, 0.033-0.035-second average delays for their WWII base fuzes used in APC shells. But the British did not change theirs, to my knowledge.

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28 October 2021 - New essay