Once of the benefits of having HMCS SACKVILLE in for refit is that we are able to access compartments that would normally be inaccessible. Two of those spaces are associated with the ship's original underwater sensor suite (originally knows as ASDIC, now better known as SONAR) as well as the former chain (or cable) locker, which I believe was retrofitted at some point to accommodate an underwater sensor of its own.
It should be noted that while much of the following has been contributed by others, more knowledgeable than I on ASDIC/SONAR, we are hoping to find someone who may have been familiar with the installations within SACKVILLE (or corvettes in general) either during the war or after during SACKVILLE's days as a loop layer and research vessel. In particular, my thanks to (in no particular order) Jerry Proc, Clive Kidd, Koji Kamori, and Norrie Millen for their observations and help in interpreting the fittings that remain in SACKVILLE.
In the section below, the ASDIC compartment is labelled as "NO. 1 STORE ROOM" just aft of the cable locker.
Viewed from the exterior of the hull, you can just see the through-hull penetrations from the ASDIC compartment on the left of the following image, with another through-hull penetration from the cable locker on the right.
All of these openings have been plated over to prevent leakage. Viewed from underneath, the ASDIC compartment penetrations can be seen clearly (aft is to the left, the bow is towards the right of this image):
And the opening from the cable locker:
ASDIC was originally developed during the First World War, but was still fairly primitive at the start of the Second World War. SACKVILLE was originally fitted with a Type 123 ASDIC, which was developed in the 1930s. From my writeup on the Haze Gray and Underway website:
"Due to the primitive electrical systems on the early corvettes, these ships had to rely on magnetic compasses, rather than more advanced and reliable gyro compasses. The reliance on magnetic compasses limited these ships to the older Type 123A sonar, as newer sonars required gyro compasses. The use of magnetic compasses alone was a limiting factor, as they were sensitive to the steel construction of the ships on which they were mounted, and tended to bounce around in heavy seas and when the corvettes were thrown about after dropping depth charges, which made it difficult to plot an accurate course.
These "primitive but reliable" sonar sets used an oscillator located under the hull which could be rotated through 360 degrees (by hand!), but was fixed at about 6 degrees below the horizontal. As a result, conditions under which a submarine could be located, and contact maintained, were limited. As the oscillator was fixed in elevation, a corvette was forced to lose contact with a targeted U-boat during the actual attack phase where the corvette passed over the U-boat to drop depth charges, and one tactic early in the war was for one corvette to stand off and hold the contact while another corvette went in for the attack. The development of the Hedgehog, and other "stand-off" weapons later in the war, also helped to overcome this shortcoming.
The improved 123D set came about merely by adding a second magnetic compass and bearing indicator on the bridge, and a loudspeaker system for the sonar operator to communicate directly with bridge personnel. The upgrade appears to have occurred sometime between 1942 and 1943 for some ships, but was never fully carried out in favour of other more improved sonar sets."
The underwater portion, or "hull outfit", of the Type 123 ASDIC unit looked very similar to the following diagram, which I have borrowed from Jerry Proc's excellent website, which he in turn borrowed from another website, and which probably originates from a book of some sort:
On the inside of the ship, only the top protrusion would be visible. The following photo is reproduced from page 102 of Thomas Lynch's book "Canada's Flowers", and shows the ASDIC compartment of HMCS ALBERNI in 1941. The photo is looking to forward and to port, with the forward bulkhead (common with the chain locker) to the right and the interior of the ship's hull in the background. The winch to the left was for raising and lowering the oscillator (transducer) unit.
The shelf in the corner (middle of the photo, with the boxes on it) appears at the bottom of the ladder in this photo taken this past week inside of SACKVILLE's ASDIC compartment (this photo looks in the opposite direction, to aft and to starboard):
Just to the left of the orange ventilation hose are the two openings (one with a gate valve installed on it, the other with a blind flange installed). In addition, aft of the ASDIC openings, there are two cylinders (with covers) on either side of the ship's keel which we have not yet identified. One suggestion (probably the most likely) is that they were transducer pockets for an echo sounder. (Note: This suggestion has been confirmed by several people - the pots would have been filled with an oil to keep air out of the cylinders.)
Generally, the Flower-class corvettes were later upgraded with the Type 127 ASDIC as a stepping stone to the later, more advanced, Type 144. Again, stolen from my writeup on the Hazegray site:
"Macpherson and Milner indicated that the Type 127D was an interim upgrade to early Flower class corvettes between the fitting of Type 123 and the later, much improved, Type 144 set. The Type 127D introduced motorized rotation of the oscillator, and the oscillator was stabilized to maintain a constant attitude regardless of the movement of the ship. The Type 127D also required a gyro compass, and required that the ships be rewired with a low power system. In addition, the Type 127D could be used in conjunction with the new Hedgehog ASW mortar, although without complete integration between the two."
Some corvettes would also have received the Type 144 ASDIC, which would have looked similar to the following diagram (borrowed from the
Weapons and Warfare website).
A labelled version follows:
The interior of the Type 144 appeared as follows (courtesy of the Australian War Memorial website):
From my writeup on the Hazegray site:
"An improved sonar set with a fixed gyro-stabilized oscillator that used a gyro compass for bearing indication. The Type 144 was fully integrated with the Hedgehog or Squid ASW mortars. The Type 144Q was a Type 144 sonar with a "Q" attachment, which was a second oscillator that trained with the main oscillator, but was elevated down further for maintaining contact with the target at close range (possibly around 400 yards).
See Jerry Proc's website (link below) for an excellent description of these sonars, as well as a diagram showing how the Type 144, 144Q, and 147F worked together.
According to Norrie Millen (retired Royal Navy):
"I think under ideal conditions a range of 2800 – 3000 yards was possible, all depending on sea state, temperature of water etc. Q and Q2 [attachments] were used for close range and the switch was made just before instant echoes [were] reached to ensure target was not lost. I do not remember switching to Q at 1200 yards. I had 144 on my 3rd ship (1961-63)."
SACKVILLE's forward boiler was damaged during the war, and not repaired. Because of this, SACKVILE was retasked as a training vessel, and then a loop layer. After the war, she was modified to become a research vessel. Because of this, it isn't immediately clear whether she received the Type 144 set. To complicate matters further, we don't know what sensors she received during her time as a research vessel, and how the original fittings were modified to accommodate those sensors. Presumably as part of her research work, her cable locker was also fitted to accommodate some sort of sensor - the following photo shows two (open-topped) cylinders (or pots) on either side of the keel that look similar to the covered cylinders in the ASDIC compartment, in addition to the gate valve fitted to the through-hull penetration (on the centreline running between the two pots, and towards the top left of the image).
This image only shows one of the two pots, but shows the gate valve on the through-hull penetration a bit better.
In addition, the cable locker was fitted with a voice pipe, for purposes unknown. Another view of the cable locker, looking forward:
One of the two pad eyes still has the remains of a shackle attached to it - I'm guessing that the ends of the ship's two anchor chains might originally have been shackled to the pad eyes to prevent them from going overboard when dropping the anchor.
And if you're wondering - yes, that is the interior of the ship's hull plating on either side of the photo. When the ship is afloat, there is water on the other side of that steel plate. As a museum ship, SACKVILLE is ballasted down to compensate for a lack of fuel and ammunition, and all sorts of other kit that has been removed over the years. If you look closely on both the left and right of the photo, you can see the scum line left by ballast water, as this compartment would normally be full of water when she is afloat.
If anyone has comments or information that might be useful to add to this post, please leave it in a comment, and I will see what I can do to incorporate it.
