M24 wreck detective

Tim Smith, M24 Project Manager, above the M24 wreck site.

The maritime archaeologists in the Heritage Branch, NSW Office of Environment and Heritage, have had a long standing interest in the M24 Japanese midget submarine. They have been conducting research into Japanese midget submarines and undertaking their own surveys for the vessel for over 15 years. The agency is now responsible for the wreck site’s long-term management.

Tim Smith is Deputy Director of the Heritage Branch, the NSW State Maritime Archaeologist, and the M24 Project Manager.

Does the discovery of the M24 wreck site mean we can answer some outstanding questions about what happened to it when it left Sydney Harbour?

The discovery of the M24 has closed an amazing wartime mystery – what happened to the submarine and its brave crew? Now that we have the wreck site we can start to ask questions that you can’t ask of the historical account, namely, are the crew on board? What might they have been going through when they came to rest on the seabed in this location? What was the state of the vessel at that time, was it damaged, was it fouled by lines or did something else impede its progress and cause the vessel to stop at this point?

Unfortunately, many of these questions will remain unanswered because of the nature of archaeological sites. Unlike a normal iron or steel ship that comes to grief on the seabed, submarines are manufactured very soundly for pressure resistance and depth, so they remain virtually intact and in that sense sealed. Many of their secrets are still locked inside. We may never be able to confirm that the crew is still inside the submarine. We know that it is two-thirds filled with sand, and if they are inside, then certainly they will be buried.

We could recover the M24, bring it onshore and cut it open in a laboratory and excavate the sediment. That would confirm if the crew are on board, but then you have a huge artefact to conserve in perpetuity. You may have to spend $50 or $60 million to do that. You always have to balance the cost and the difficulty of the intervention with the potential research outcomes.

Has there been pressure to raise the submarine?

The families were certainly interested in a recovery option, but they realised the complexity of it. They agreed it was best to leave the family remains to be honoured and protected where they now lie.

What archaeological work is being done?

Our main aim with the archaeology is to answer some key questions: what is the condition of the submarine? Are there any clues in the wreckage to the fate of the crew and their activities? And what are the potential threats, for example, anchor damage, diver visitation, fishing damage, natural decay, and the hostile effects of scale and swell scour.

All shipwrecks deteriorate over time. Submarines are very robust, however, and tend to last a very long time, longer than a normal ship. Over time it will evolve in a different form to what it is today, so we are about getting data that will assist us to know how best to manage and protect it for the longer term.

What sort of data are you gathering?

We have done seabed mapping and acoustic side scan mapping of the site, so we’ve got an image of the wreck, its orientation and its position on the seabed. And we’ve done extensive photographic and video documentation of the external structure.

Now we’re in the phase of looking at its environment — the water quality, and the effect that might have on corrosion rates and the amount of dissolved oxygen, water temperature, salinity levels, PH. All the factors which interact with an iron and steel shipwreck and have an effect on the corrosion rate and profile.

We are doing an analysis of the sediment surrounding the wreck, because the type of sediment, the density, they way it inhibits oxygen penetration has an effect on how long the steel hull will survive.

The next stage is to look at doing a marine biological study of the hull – to identify the type of marine plants and algae covering the site, because that barrier on the outer skin of the submarine has an effect on the underlying metal.

One of the priorities is to get quantitative data of the corrosion rate because that will tell us instantly how fast parts of the wreck might be corroding. Shipwrecks generally reach an equilibrium rate of deterioration where they sink, they get some corrosion activity happening and a barrier forming and a colonization of marine life and then they tend to stabilize and have a slow, steady rate of decay. In some cases you can address that mechanically by human intervention by placing anodes onto the hull, which preferentially decay to the heritage fabric.

How long will these studies take?

It’s an ongoing monitoring and management approach. It’s going to take decades — maybe after ten, twenty or fifty years we’ll be able to get an accurate picture of the state of the site. After I’ve moved on other archaeologists will continue the story.

Can you see inside the wreck?

Compared to other submarine wrecks M24 is not too deep for study, and because of corrosion, probably caused by past fishing practices, we can look inside the hull. The key area is behind the conning tower or the fin, in the after battery room. There is a corrosion opening that has formed around the skin of the sub — it’s not very wide, about half a metre in width, but it has allowed us to glimpse inside the hull. We can see the machinery and the structure in the after compartment. We have done some initial probing with cameras, but this is a sensitive area, because it is a likely place for human remains.

Could the crew move around the submarine?

The crew sat inside the control room, next to the periscope. There wasn’t much room to move, but they had to move around the vessel during combat. It was Ashibe’s role to monitor the batteries — he would have been quite active working through the boat.

The only area where they could have experienced some ease, or been able to stretch, would have been in the control room, which was about 6 foot in diameter in height. They could only go through the forward battery room up to the torpedo room bulkhead or through the back of the submarine up to the engine room bulkhead. That was a very tiny passageway and they would have had to crawl. It would have been fairly dark, although there was some light.

Those three areas: the two passageways and the central control room were the only places for human habitation, so they are the three sensitive areas.

What work will take place over the next year?

We will continue the non-disturbance documentation. In 2009, the Navy Clearance divers will continue that work for us. They will be getting a sense of the internal compartments, and mapping or documenting the physical remains of the dials and the levers and the machinery of the submarine.

What danger do the unexploded charges pose?

There is fairly limited research available on the nature of the scuttling charges on the midget submarines, and there was a change in the way the Japanese naval authorities configured the vessels.

We know that the Pearl Harbour submarines, six months before the Sydney attack, had a different form of demolition charge from the M24. They only had a single demolition charge of greater force and this was located in the after section of the submarine. It is unclear whether it was inside the boat or underneath the boat.

The Sydney midget submarines were different. They had a design modification — there were two demolition charges of smaller capacity, inside the boats. We are starting to get a bit more of a handle on the location of those demolition charges through our research, and their potential size in terms of explosive power.

We have hypothesized that one lies in the after section of the sub, which is a nicely sanded over compartment. It will be buried by sediment and out of sight, so we cannot monitor its physical state. The charge that will be in the forward part of the submarine is concealed in the forward torpedo room, which we cannot see.

We are embarking on a study of the nature of Japanese wartime explosives so we can gauge the potential risk to the submarine and to any future human intervention.

There have been international studies of wartime wrecks, and certainly some battleship shells and the like can still have a residual explosive force. The Australian WWI submarine AE2 in Turkey has an unexploded torpedo and the expert consideration was that there was still a 15% chance of its detonation.

We know that WWII depth charges still exploded on ranges in the 1970s, so if the demolition charges were very well sealed in their canisters, and in an anaerobic environment the canister could have some watertight capacity. The question is: how much has the explosive broken down over that time and what sort of risk might that pose? It’s unlikely, but it’s a risk we have to manage.

Will the public be able to visit the site?

There’s only one submarine that is really accessible to the diving public and that’s a midget submarine off New Guinea, which is in 20 metres of water. There are no archaeological controls covering access to that site, and material has been removed from it by divers.

The other key midget submarine in Pearl Harbour is too deep for public visitation at 400 metres deep. The others that have been found are in a fairly degraded state so the M24 is the first where you could control public access as an education activity.

There are a number of factors to consider such as: the interests of the Japanese Government and whether they would support a tourism type activity on such a sensitive site. We have to remember that the two sailors are still likely to be on board and they are not just sailors to the Japanese people. Emperor Hirohito gave them Hero God status and they are revered as such.

Admiral Yamamoto, the Japanese naval commander also awarded them posthumous naval honours — raising them by two ranks.

The other factor is how you would instigate safe access because of the depth of the wreck. Access would be restricted to those divers with the technical qualifications —mixed gas diving or rebreather trained.

There would have to be a mooring at the site and this has the potential to cause damage.

It’s also quite a hostile part of the coast with swell and wave activity, so there are normal diving safety factors.

We are certainly considering the issue of public visitation. It would be a unique experience for divers to be able to visit a WW II Japanese midget submarine in its original wartime setting. It could be of enormous interest, and potentially a great tourist and economic attraction for the NSW community.

Who owns the wreck?

The parent state generally retains ownership of vessels that went down in combat or in conflict. It is certainly the case that the vessel was lost in combat — it wasn’t surrendered. If it was surrendered then it becomes the property of the victor. Under maritime law the Japanese Government is likely to be the owner of the wreck site and the remains.

The wreck lies within Australia’s jurisdiction, however, so management of the site becomes our responsibility.

The site is protected under two pieces of legislation: one is the Commonwealth Historic Shipwreck Act, but certain functions under that legislation have been delegated to the Heritage Branch in the NSW Department of Planning. We manage the site on behalf of the Commonwealth Government. The site is also protected by the NSW Heritage Act, because it lies within three nautical miles of this state’s borders. The site is on the NSW State Heritage Register, so we also have a direct management role under that legislation.

What is the Heritage Branch’s role with the wreck?

The Heritage Branch has had a longstanding interest in the discovery of this wreck site and has been doing its own surveys and research into midget submarines for over 15 years. Unfortunately, we were unsuccessful in discovering it, but following its discovery in 2006 we used our knowledge of Type A midget submarines, and of historical wreck sites, and how they present on the seabed, to rapidly implement systems to protect and manage the site. The first action was to ensure that legislation was enforced around the site to provide legal protection.

Based on our in-depth knowledge of the type of craft and the sort of issues that we will be dealing with, vis human remains, war dead issues, unexploded ordnance risks, diver attitudes and interaction with the site, and previous wreck site management, we were able to introduce a highly effective management system.

It’s been heralded internationally as a world class example of how to deal with a very complex and challenging heritage site. Certainly feedback from the Japanese Ambassador has confirmed that the Japanese Government is very appreciative of the steps taken by the Australian government to protect the site.