Why Is Carbon Fiber The Best Material To Make Hydrofoils?

Windfoil manufacturers often mention that the carbon they use to make their hydrofoils is “high modulus”. But what does it mean exactly?

Here is a quick explanation:

Carbon fiber is a long, thin fiber (about 0.005-0.010 mm in diameter) composed of at least 90% carbon atoms. The carbon atoms are linked together in microscopic crystals and aligned to make the fiber extremely strong for its size.

Carbon fiber has 2 main properties that make it attractive for hydrofoils – stiffness and strength.

For stiffness, carbon fibers are categorized by performance according to the tensile modulus of the fiber. The most common units of measurements used are “pounds of force per square inch” (psi) or “gigapascals” (GPa).

The tensile modulus enables to differentiate between different types of carbon fiber:

• “Low modulus” or LM carbon fibers have a tensile modulus under 200 GPa (below 34 Million psi).
• “Standard modulus” or HT has a tensile modulus of 200 to 275 GPa (34 to 42 Mpsi)
• “Intermediate modulus” or IM has a tensile modulus of 275 to 345 GPa (42 to 57 Mpsi)
• “High modulus” or HM has a tensile modulus of 345 to 600 GPa (57 to 72.5 Mpsi)
• “Ultrahigh modulus” or UHM: It’s the stiffest option available. The tensile modulus of 600 to 956 GPa (72.5 -145.0 Mpsi).

When it comes to strength, the table below shows that the stiffest carbon fiber is not necessarily the strongest:

• HT has good strength and is not so stiff. It’s also cheaper, around €25 sq. m.
• IM has high strength and moderate stiffness. It’s quite expensive. around €100 sq. m.
• HM and UHM are very stiff and moderate strength. However, it’s very expensive: €200 sq. m.

IM makes very tough components, while UHM makes very stiff components, but is more fragile. So the question: which carbon fiber makes the best windfoils?

It really depends on what you want: UHM is perfect for long and thin race foils, while IM is good for everything else.

As a comparison, you will find under the tensile modulus of carbon fibers versus different metals.

This shows that the highest grade of carbon fiber is much stiffer than aluminium and steel, not to mention that carbon is much lighter than both materials as well (1.5 and 5 times respectively).

Moreover, the fatigue properties of carbon fibers are superior to aluminum and steel, and carbon is one of the most corrosion-resistant materials available.

These are the main reasons why carbon fiber is an optimal material to make foils despite its price.

See also: Which Windfoil Should I Buy, Carbon foil or Aluminum foil?

A big thank to Simon Sanderson, composites engineer and consultant, for his contribution, sharing his expertise and experience in foil-making since 1979 (!)