The spigot mortar is something of an old favourite in here, and I saw some footage of the Hedgehog anti-submarine weapon in use recently, but of course, there was insufficient detail in the old film to gain any real insight into the workings. However, I happened to be looking up some related info, and there was the beast in detail.

24 spigot mortars firing a box of mortars around the enemy sub, and by all accounts a much more effective attack that the depth charges that are so much loved by the film-makers.

Unlike the depth charge, the hedgehog device are contact devices, and detonate only on contact with an object. Result being they deliver a much more damaging strike to the target - the reports suggest there are no records of a submarine surviving a Hedgehog strike, and the ship's sonar, or ASDIC, was unaffected by repeated failed detonation that accompany depth charge attack since the Hedgehog doesn't go bang unless it meets something.

Anti-Submarine Projector Mks 10 & 11 (Hedgehog)

I would have thought the small props on the top of the missiles might have been some kind of device to detonate the weapons at a certain depth as an alternative to contact explosion.

Er...

These aren't missiles, they're shells fired by the detonation of a propellant charge at their base.

The propellers on the nose are fuzing devices.

Spun as the shell travels towards its destination, it operates a screw that releases the final part of the detonator, which then explodes the shell on contact. This means the shells are "safe" to handle on deck, and (probably) won't explode if dropped or mis-handled. Something the film-makers like to ignore, as it makes a good story if they can have an accident. The fuze-setting propeller was a fairly standard feature, and can be seen on most bombs, their detonation on hitting the ground being preferable to being set off in turbulence. Torpedoes had them too.

The important thing about this device is that it specifically did NOT operate by depth, and therefore did not result in 24 (or a lot more as as there could be more than one launch)  needless explosion that made the ship's sonar blind/deaf until it had subsided, which could take some considerable time, during with the submarine could make its escape completely undetected.

Again, the waritme film-makers liked to show chases and explosion time after time, but the reality is much less exciting.

That makes sense as the props were clearly there for a reason.

Excuse a less than ideal choice or use of words...

I wasn't implying that real sub-hunting wasn't attention-grabbing, just that if it wasn't dramatised for the screen, then watching it might start to approach that other epic - watching paint dry.

Came across a detailed background on the Hedgehog, and which includes a detailed pic showing the nose mounted spinner:

The Hedgehog