Flying in water - when the hydrofoil went to war

Despite valiant efforts over the past 120 years, engineers haven’t been able to overcome the operational concerns and costs that have tarnished the hydrofoil’s naval career. 

The hydrofoil was developed as a way of achieving high speed for small craft by raising them out of the water on foils and thereby reducing hull resistance. The first designs appeared around 1898/1901, in both England and Italy. Later experimenters included Alexander Graham Bell who began working with hydrofoil in 1908. 

Three decades later, one of the most successful hydrofoil designs was produced in Switzerland after World War II by Supramar, founded by the German engineers, Baron Hans von Schertel (8 January 1902 – 18 April 1985) and Gotthard Sachsenberg (6 December 1891 – 23 August 1961).  

An American military hydrofoil using fully submerged foils (Credit: navsource.org) 

German naval interest 

Hydrofoil speeds of more than 30 knots had been attracting the attention of the German Ministry of Transportation and Finance, the German Navy, and German Air Force and experiments were carried out with various hydrofoil designs from late 1938 onwards.  

Several military hydrofoils of differing sizes, with speeds up to 60 knots, were designed and prototypes acquired during the war, the majority being designs for fast attack craft. 

Range of designs 

During the war, the following experimental hydrofoils were planned or built:  

VS 6: Using Sachsenberg/von Schertel foil system and powered by a 1,400 bhp Avia petrol engine. Built at Gebr Sachsenberg.   

VS 7: 17-tonne hydrofoil built on Dr O Tietjen's s hoop foil system at the Brandenburger Yard. It was constructed with the same displacement and engine as VS 6. VS 7 attained speeds up to 55 knots compared to the 47 knots of VS 6, but the stability and manoeuvrability was much poorer. 

VS 8: Developed by engineer Wankel at Gebr Sachsenberg in 1940, as an invasion tank-landing craft for the army, with an open bay at the stern that would accommodate floating pontoons. To load or unload a small or medium size tank weighing up to 20 tonnes, the bay was flooded and a pontoon, equipped with two 40 hp engines, could be loaded or unloaded. Experiments showed that loading the pontoon took less than two minutes, while unloading less one. It was estimated that this craft would achieve 45 knots on hydrofoils powered by two 2,500 hp engines.  

In 1941 the project was taken over by the German Navy for development as a fast minelayer, capable of laying 15-20 mines, a torpedo boat, or an air-sea rescue boat. However, only two MB 501 (2,000 hp each) were available for trials. To accommodate this reduced horsepower, the propellers were given less pitch, resulting in a top speed of only 37 knots. It was commissioned in March 1943 but in September 1944 was beached after a total engine failure and broke into two during rescue operations. 

VS 9: Sister to VS 8, ordered in 1941, but construction never started 

VS 10: Experimental hydrofoil, with composite hull (wood/steel/light alloys). Ordered March 1942 but destroyed by Allied bombers in 1944 on the stocks  

VS 14: Hydrofoil ordered October 1942 but never laid down  

TS 1-6: Based on the Sachsenberg/von Schertel system and built at Gebr Sachsenberg. Weighing 5 tonnes, these achieved a speed of 40 knots with a Lorrain 380bhp petrol engine, and had an endurance of 36 hours at 25 knots. They were used in the Norwegian fjords.  

Royal Navy’s first and only hydrofoil, HMS Speedy, from Boeing (Credit: Royal Navy) 

Military value doubted 

After the trials of VS 8 and VS 6 in 1944, work was immediately suspend on VS 6, 7, 10, and 14 to save fuel. Also, it was estimated that up to two years would be required to prepare a combat weapon.  

Analysis of the performance of the few craft completed suggests that the hydrofoil was not a practical naval vessel, with its future deemed to be in the commercial world.  

Some of the operational concerns included: 

• Foils sensitive to striking floating objects  
• Large foil draught making berthing and docking limited to certain facilities
• Large turning circle and poor manoeuvrability  
• Very low hullborne speed due to foil drag  
• Slow acceleration from low speed 
• Constant repairs needed because of damage to foils and propellers  
• Severe propeller cavitation due to operating just below water surfaces.  

One of the Italian Navy’s seven Sparviero class units (Credit: DA Donnelly)

A better future? 

It would be many years before military applications of hydrofoils would be attempted again, based on Boeing’s Jetfoil of the 1960s. Equipped with fully submerged foils the design was developed as a passenger vessel and the technology adapted for the US Navy's Pegasus class of six military hydrofoils which operated between 1977 and 1993. The Royal Navy operated a militarised Jetfoil, HMS Speedy, between 1979 and 1986, while Canada built the Bras d'Or as an anti-submarine hydrofoil that served between 1968 and 1971, achieving a speed of 63 knots. The Italian Navy had seven Sparviero class units from 1971, while Japan had three modified versions from 1992 to 2010. All have been decommissioned for multiple reasons including its cost. Despite the many attempts over the years to develop effective and affordable military hydrofoils, it would seem the concept has not proven adaptable to this end.  

John Barnes is a journalist and author and former editor of Marine Engineers Review.