How to Invent a DD Tank

How one of D-Day’s most iconic tanks was designed, by the inventor, Nicholas Straussler.

One of The Tank Museum’s most unique vehicles is the Sherman Duplex Drive – the only example with an in-tact canvas screen. How did an inventor from Hungary come to design a vehicle that was instrumental to the invasion of Normandy?

Black and white photograph of a man in a suit
The inventor of the Duplex Drive, Nicholas Straussler

Nicholas Straussler, inventor of the Duplex Drive amphibious tank, was born in Hungary in 1891.

He made his name as a designer of revolutionary vehicles and in the years leading up to World War II established a relationship with Alvis in Coventry who built armoured cars, to his design, for the Royal Air Force.

This is his account of the design of the Sherman DD. Nicholas Straussler died in 1966, having become a naturalised Briton in the meantime.

Straussler's Account

My research led me to the conclusion that fighting machines could only be floated by the provision of temporary additional floatation. This problem was attacked in 1927/1928 by me designing outside floats which collapsed for economical transport. At about the year 1934 I abandoned this line of development and attacked the problem from an entirely new angle.

Apart from the basic idea the solution was extremely difficult from the technical and design point of view as it had to contend with the existing design of the tank and all details and arrangements had to be so designed that they would not be costly and would not necessitate lengthy or substantial alterations to the structure of the hull.

The design was commenced by putting a continuous rigid deck right around the tank at approximately mudguard level, that is to say just above the top of the tracks. A number of tubular frames were made, the contour of which generally corresponded to the outer rim of this decking.

Drawing of a tank with a framework of pipes around it.
The framework of a Sherman DD
A black and white photograph of a tank with a canvas screen round it
A DD with its skirt raised

A very heavy rubberised canvas screen was constructed to connect these tubular frames to the decking. The tubular frames were spaced at suitable intervals so that when the canvas attachment was made the structure formed a hull, flexible in an up and down direction in a concertina manner.

A number of compressed air tubes were placed on the deck and attached to the screen. Inflation of these tubes raised the screen. Since each tube exerts an upward pressure of some 600 lbs when the screen is raised it is held upright by the total upward pressure of some 12 tons.

The next problem was the provision of efficient propulsion. It was quite impossible to break into the tank in order to obtain a power drive from the engine for driving the two propellers. Hence it was decided to use the idler roller as the motive power.

A crown wheel gear was attached to the idler and the mating pinion drove the propeller.

A drawing of a propeller linked to the sprocket of a tank
A diagram showing how the propeller is linked to the rear sprocket
A black and white photograph of two propellers behind a tank on a beach
The propellers of a Duplex-Drive Sherman

Thus, each re-designed idler drove a propeller of some 28 inch diameter capable of absorbing a total of some 350 horsepower.

An entirely new system was designed which permitted the propellers to swing from side to side around a vertical pivot for steering, and lifting about a horizontal axis right out of the way.

This system of floatation resulted therefore in enormous economy of manufacturing speed as witnessed by the conversion of some 2,000 tanks in this country alone, and of some 300 in the U. S. in the course of a very short period of time.

– Nicholas Straussler

Find out more about the Sherman Duplex Drive HERE

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