So I want to know, did the Japanese Navy have the ability to destroy it in a surprise attack, let's say on 1941 Dec 7? I'm asking if the Japanese Navy had the ability. I'm not asking if it was a viable strategy or sound war plan.
Well, you're getting both. :) Evaluating it as a war plan is WAY more interesting.
- It would be fixed in six months.
- Shipping can still go around South America.
- They'd have a hell of a time getting there.
- They'd have an even worse time getting back.
- They'd have to face the wrath of the intact US Pacific Fleet.
- They'd hand the US a golden opportunity to destroy the IJN piecemeal.
- All their fleet carriers are out of the first months of the war.
To keep this away from a "what if" answer, I'll lay out the problem in more detail in comparison to the already audacious Pearl Harbor attack, and its knock-on effects even if successful.
First, let's have a good look at the fleet which attacked Pearl Harbor. It wasn't just a few carriers, it was ALL their fleet carriers. And they needed a large support fleet of battleships, cruisers, destroyers, submarines, tankers, and supply vessels. This will become very important because of the range involved.
The submarines were for long range scouting and screening. The destroyers for short range and anti-submarine work. Their surface protection was paltry against the US fleet.
Tokyo to Pearl Harbor is about a 7500 nmi round trip... in a straight line. The Japanese took a circumlocutious route, avoiding shipping lanes, to avoid detection on the way out and back. All in all it took them 28 days.
Tokyo to Panama is twice the distance and twice the time. As we'll see it's likely 20,000 nmi to avoid detection and over two months in transit. Just getting there will be a serious challenge.
How Do You Get There Undetected?
Twice the time in transit means twice the chance of detection. It means twice the chance of someone screwing up and sending a radio message. It gets worse.
Let's start by looking at the shipping lanes at the time.
On their way to Pearl Harbor, the fleet took a northern route weathering rough winter seas to avoid major shipping lanes. They kept radio silence for two weeks and had a screen of thirty submarines.
But getting from Tokyo to Panama requires crossing sea lanes, there's no way around it. The fleet certainly won't risk traveling between Hawaii and the US West Coast. It must move through the island-dotted Central Pacific where it's at risk of being spotted by any number of merchant ships moving between the Americas and Asia.
I've estimated the distance using the Great Circle Distance going from Tokyo, Japan -> Majuro Atoll, Marshall Islands -> Pape'ete, French Polynesia -> Panama City, Panama at nearly 10,000 nmi one way. So 20,000 nmi round trip. Plus zig-zagging to avoid submarines on the return.
How Do You Fuel All That?
The vessels in that fleet have ranges from 5000 nmi (destroyers) to 10,000 nmi (some of the carriers, and both battleships) to 15,000 nmi (submarines). With a round trip of at least 20,000 nmi due to the Central Pacific route, that means refueling at sea. A lot.
Fortunately the IJN knew how to refuel even in heavy seas, something the US thought only they could do, but it's always a risky proposition. They did it for the Pearl Harbor attack, but to get to Panama means twice the number of refuelings, twice the risk, and twice the number of oilers.
And they had to do it all in radio silence lest they be detected.
Unfortunately Japan was so sorely pressed for oilers they impressed merchant ships for the role. All 8 tankers for the Pearl Harbor fleet were impressed merchant ships. To get to Panama they need to find 8 more or leave half the fleet at home.
Since the canal locks are a smaller target than the sprawling naval base and fleet at Pearl Harbor, it's possible they would leave half the fleet at home. Perhaps the two shorter ranged Zuihō class carriers making them available for other strikes at the start of the war.
Our fleet has, somehow, safely arrived undetected and is now a few hundred miles from the Panama Canal. Now what?
The US was already well aware that the Canal was a target and had significantly beefed up their defenses, even against air attack... but it was incomplete on Dec 7th.
For defense against surface attack, the Canal was defended by heavy coastal defense batteries that would give a Kongō class battlecruiser a seriously hard time. So no surface attack was likely. As at Pearl Harbor, it would be from the air.
The Pacific end of the Canal had an SCR-271 radar set, the same at Pearl Harbor and the Philippines. While it successfully detected Japanese air attacks on Pearl Harbor and the Philippines, it was so poorly understood and poorly integrated into the air defense that the detection was ignored. There's no reason to believe it wouldn't also happen in Panama.
For air defense, Panama had 24 Pursuit planes, mostly P-36 Hawks and P-40 Warhawks. These are the same aircraft as Pearl Harbor, and both inferior to the Japanese Zero. As at Pearl Harbor, they'd probably get caught on the ground. They also had various anti-aircraft guns, definitely the 37mm Gun M1, but it was deemed inadequate.
Only two detectors were installed and in operation in the Panama Canal Department. The harbor defenses had less than one complete manning detail available. The anti-aircraft artillery had insufficient personnel to man the armament being installed in the Canal Zone and only enough ammunition for one minute of fire per gun for the 37-mm. guns. There were no barrage balloons.
There's no reason to believe the Panama Canal defenders would fare any better than Pearl Harbor.
The Damage: Six Months
With surprise, superior aircraft, and in larger quantity the Japanese would have total air superiority. With their large quantity of strike aircraft and skill at dive bombing it's a near certainty that both locks would be hit and disabled. They could even be torpedoed. The planners of the I-400 attack estimated this would put the locks out of operation for six months.
A team of three shipping engineers studied the documents and concluded that the locks at Miraflores on the Pacific side were the most vulnerable to aerial bombing, but the Gatun locks on the Atlantic side offered a chance of causing greater damage, since it would be harder to halt any outflow of water. They estimated the Canal would be unusable for at least six months following a successful attack on the locks.
But this doesn't cut off shipping between the East and West coasts. It does mean they have to go around South America adding thousands of miles and weeks to the trip.
This also doesn't cut off supply between the East and West coasts, the US has an extensive rail system to draw on.
What About Gatun Dam?
27 Million Tonnes of Earth and Concrete Doesn't Care About Your 800 kg bomb.
(Technically that's the entire dam, not just the concrete spillway dam... just forgive me this bit of hyperbole.)
Gatun Dam holds back the Chagres River creating Gatun Lake which ships transit going from the Atlantic to the Pacific. It's a very tempting target to do longer term damage to the canal. Could a Japanese carrier air strike breech the dam?
No. It's not like in the movies... unless that movie is Dambusters. Damaging a dam is really hard. Bombing a dam is even harder. Why? Looks easy. Just look at that huge target!
The problem is if you just drop a bomb on a hard surface, like the armor of a ship or a concrete dam or the ground, most of the explosion simply reflects into the air. The Allies found this much to their annoyance trying to damage the much smaller U-Boat Submarine Pens.
To do more than chip at it you have two options. First is to penetrate deeply before exploding. Then the force of the explosion is fully transmitted to the surrounding concrete. The Allies developed bombs like the 5400 kg, 6.4 meter long Tallboy. Cast from a single piece of high tensile steel, it had to be dropped from 5.5 km to allow it to reach terminal velocity and penetrate 5 m of concrete delivering 2400 kg of extremely potent Torpex explosive.
The Japanese had nothing like this in Dec 1941, nor could a carrier bomber carry it. The closest bomb they had was the 800 kg Number 80 designed to take out small bunkers. It was capable of penetrating 0.4 m of concrete, not even enough to bury the 2.8 m long bomb, and delivered 380 kg of Shimose Powder. A B5N torpedo bomber could carry one.
But a dam wall is practically vertical, so the bomb would hit it at a high angle and just skip off.
The second way is to press the bomb against the dam underwater and then explode it. Water is incompressible and so the explosive shock will translate to the concrete. The trick is to get it right up against the dam, otherwise that same incompressible water will shield the dam from the blast. This is why depth charges had a "kill range" measured in meters. This is further complicated by the presence of torpedo nets (I'm not 100% sure the dam was covered with torpedo nets, but the locks were) which will also stop a bomb or torpedo moving through the water towards the dam. WWII bombing techniques, even dive bombing stationary target, was not accurate enough.
This was solved, after much work, for Operation Chastise aka the Dambusters raid. It used a special spinning, bouncing depth charge called Upkeep that could skip across the water, hit the dam, roll down it, and explode at the bottom right up against the dam.
The Japanese had no such bomb, didn't have the aircraft to carry it, and didn't have the expertise to develop it. Upkeep weighed 4000 kg and delivered 3000 kg of Torpex. Japanese carrier based bombers were limited to 800kg. Japanese overlooked anti-submarine warfare, much to their great loss, and their depth charges stank. The available Type 1 Number 25 air dropped depth-charge carried just 140 kg of explosive.
...but a spillway gate might care about a few torpedoes.
Torpedoes were an option, if there were no torpedo nets. Not the dam itself, but the metal gates at the top used to control the water were vulnerable. During the Korean War, Hwacheon Dam was attacked by bombs and torpedoes disabling the gates.
This won't produce a catastrophic failure, but it will make the gates inoperable and might flood the lake causing additional problems and more things to fix.
The IJN kicked the hornet's nest. Now they need to get home.
If they're smart they refueled just before the attack, but now they're over 8000 nmi from home and will need to refuel at least once, probably twice, slowing them down and making them vulnerable.
After Pearl Harbor they left a shattered US Pacific fleet behind them, but still the US Navy searched. After Panama they have the intact and superior US Pacific fleet in front of them. And the US admirals are drooling at the opportunity they've been handed.
When faced with a fleet of roughly equal power to their own, a commander will try to attack it piecemeal. During WWI both Britain and Germany tried to trick the other into splitting their fleet so each piece could be overwhelmed. The Japanese, looking to repeat the success of Tsushima, had Kantai Kessen which would whittle the US fleet down with submarines and air attacks as it raced to defend the Philippines.
Now the Japanese navy had split itself. Its carriers, with an inadequate surface screen, were 8000 miles away from the rest of their fleet and busy kicking off the rest of the Pacific war. They'd be slowed by their merchant oilers and their need to conserve fuel for the long journey home. They'd been at sea for a month possibly needing repair.
The bulk of the US fleet would wait while US submarines, destroyers, and long range maritime patrol aircraft searched for the returning Japanese fleet. And the Japanese would have no choice but to run this gauntlet. Once found, it would be their turn to be whittled down.
The primary target for these attacks would be not the warships, but the oilers. So far away from home, the US knew they would need to refuel. Attacking the slow, vulnerable oilers would leave the Japanese fleet low on fuel. They'd have to slow, reduce their zig-zagging, and take a more direct and predictable route home giving the US more opportunities to attack. The US Pacific Fleet would serve to block their path.
Finally, with the Japanese fleet harried, damaged, low on fuel, and reduced, the intact US Pacific Fleet of 9 battleships, 20 cruisers, 3 aircraft carriers, and dozens of destroyers would attack and destroy the cream of the Japanese Navy.
Was It Worth It?
This is what the Japanese had left ready to fight on Dec 7, 1941.
- 3 light carriers
- 8 WWI vintage battleships (2 each of Kongō, Fusō, Ise, and Nagato)
- 16 Heavy Cruisers
- 19 Light Cruisers
And this is what the US Pacific Fleet had.
- 9 Battleships (some WWI)
- 3 Fleet Carriers
- 12 Heavy Cruisers
- 8 Light Cruisers
With the Canal knocked out it would slow, but not stop, the flood of ships from the Atlantic including 3 battleships, 2 fleet carriers, and any number of cruisers.
With the Japanese Navy taking on both the US and British navies, this would be sufficient to defend the Philippines leaving the US with a base deep in Japanese territory. The Pacific War would go down something more like expected in War Plan Orange.