S-BOAT INNOVATION

High-speed boats are typically built on planing hulls. But the designers of the S-boats knew a planing hull would never work in the boisterous conditions of the North Sea and English Channel – they would have to work with a round-bottomed displacement hull. That demanded some creative thinking.

They turned to the designs of Lürssen – one of the most famous German naval architecture companies of the period – and one renowned for its high-speed race vessels.

For the S-boats, lightweight construction and three V20 diesel engines developing 6,000hp helped to give the round-bottomed hulls their superior speed. But a key innovation was the arrangement of the triple rudders – a main rudder flanked by two smaller rudders.

These could be angled outboard to 30o. At high speed the smaller rudders generated what became known as the ‘Lürssen Effekt’ – in essence creating an air pocket slightly behind the three props to increase their efficiency.

This effect also reduced the wake and helped to keep the boat’s attitude nearly horizontal – not only improving performance but also aiding with the aiming and firing of the torpedoes.

Another innovation was the development of an underwater ‘wedge’ at the stern that prevented the bow from rising as the boat accelerated, allowing the guns to be fired more accurately. This technology is still used on many naval vessels today.