Bob Semple’s Tank: New Zealand’s Homegrown Tractor-Tanks

In 1941, war hysteria gripped New Zealand and its neighboring country of Australia. The Japanese army was advancing rapidly across South East Asia invading Burma, the Dutch East Indies, New Guinea, the Solomon Islands, Manila, and Kuala Lumpur. After the crushing collapse of British Malaya and the fall of Singapore, the Japanese began making air attacks on northern Australia, beginning with the devastating bombing of the city of Darwin. The Australian Prime Minister John Curtin predicted an invasion of Australia was coming.

Bob Semple’s Tank

Unfortunately, neither Australia nor New Zealand was combat ready. Australian troops were fighting in the Middle East, and some 15,000 Australian soldiers were captured and became prisoners of war during the battle of Singapore. John Curtin pleaded Britain and the United States to send in armored vehicles and tanks. But with Britain fighting for its own survival, there was no likelihood of production being spared for Australia and New Zealand. Rather than wait for supplies to arrive from America, both countries began to seek solutions from within their respective borders.

Robert Semple, the Minister of Public Works of New Zealand, announced that they would be making their own tanks.

If this country is going to be invaded, we need to have equipment as good as that of the other fellow, if not better… we could not buy tanks from outside, but had to act on our own resources. Luckily we had big tractors here, and they were a godsend. They have proved one of the greatest boons the country has ever known, permitting us to build highways, aerodromes, camps, and fortifications in record time in the Dominion. They have proved invaluable for other urgent purposes outside New Zealand.

The tractor Semple was praising about was the medium track-type Caterpillar D8. Though it came in many configurations, it was usually sold as a bulldozer equipped with a detachable large blade and a rear ripper attachment. The Public Works Department had 81 of these and another 19 were available ready to be put into service.

Robert Semple on a caterpillar tractor, between 1935-1940

The PWD proposed they take this fleet and build armored bodies for them. Work began on a prototype at the Temuka PWD Depot in June 1940. The suspension of the D8 was modified and the track assembly was lengthened. Minor modification was made to the driver’s controls, and the original gear box was replaced with an improved 2:1 ratio box.

Because armor plates could not be sourced, corrugated manganese plate was used to build a superstructure over the tractor. Lack of heavy-caliber cannons meant that the tank had to do with BREN machine guns instead. Six of them were set along each primary hull facing—one at each side, a pair facing forward, one to the rear and one fitted to a fixed turret at the top. Power came from the tractor’s 6-cylinder diesel outputting around 127 horsepower which limited speeds to 8 miles per hour and operational range to 100 miles.

The tank was crewed by eight. There was so little space inside that one gunner had to lie on a mattress on top of the engine to fire his Bren gun.

By March 1941, a second tank was finished, and both took part in a parade in Christchurch. One was then sent to Wellington and then on to Auckland to promote the war effort. It was paraded there in May 1941. These public outings were intended to bolster flagging domestic spirit. Instead, it promoted media ridicule. A popular myth has it that the tank was made using corrugated roofing. In reality, the armored structure consisted of 8mm thick armor plate, fully welded, on top of which was an addition of 12.7mm thick manganese rich corrugated steel plate. This arrangement was found to be sufficient to stop enemy anti-tank rifle bullets up to 20 mm caliber. The corrugated shape was thought to help deflect bullets.

Initial trials showed the tank to be top-heavy, causing it to roll badly during off-road movement making firing on the move very difficult. In addition, the vibrations generated by the noisy six-cylinder diesel engine rattled the crew and often caused the tank’s guns to jam. Besides, a faulty transmission system forced the driver to come to a full stop before shifting gears. Another glaring flaw was the lack of a hatch on the turret. In actual combat, the absence of this hatch would have seriously hindered swift escape from the vehicle in the event of a fire due to enemy bombing.

Bob Semple’s Tank is frequently derided as the “worst ever tank”. But despite the shortcomings, Major General Puttick, the Chief of New Zealand General Staff, remarked that it was a very useful weapon for certain styles of fighting. Robert Semple himself was extremely proud of his invention.

That tank was an honest-to-God effort to do something with the material at our disposal when raider were at our back door…instead of sitting down and moaning we felt we ought to do something to manufacture weapons that would help to defend our country and our people.

When a reporter once mocked Robert Semple, the New Zealander retorted: “I had the vision to try and create something while a lot of others were just sniveling.”

At some point, these two tanks were officially handed over to the Army. General Puttick recommended that the vehicles be put for beach defense. Fortunately for New Zealand and Australia, Japan’s defeat in the Pacific front meant that the threat of invasion was over. The tractors were stripped off their armors and returned back to civilian duties.