edit this was wrote before Gunnar so sorry if something is redundant
To kosta - its not so easy
http://www.performance-composites.com/c ... ties_2.asp
In hydrofoil mast almost all that matters is stiffness. This structure has defined thickness - So what define it stiffness is young modulus of material. Normal woven CF composite has ca. 70 Gpa and significantly less than that in any other than 0 and 90 direction. UD have 135 GPA but only 10 Gpa perpendicular to fibers. So we can assume that mean YM for carbon composite in Foil mast is in 50 Gpa range.
Aluminum alloy have 72 GPA /and this 72 GPA is 72 gpa in all direction/ - and its death easy to made it repeatably good with internal I beams. Usually it has also much thicker wall because alu is much weaker /ultimate strength not stiffness/ So you end up with stiffer alu mast much easier than carbon.
To made carbon as stiff as alu you must use UHD carbon and either solid mast /no weight advantage to alu/ or with carbon internal I beams and very hi end technology /prepreg and autoclave/ So far it is very hard to make carbon mast as stiff as alu.
One cheap method can be use of hardwood or bamboo core but it is quite hard adapt it for mass production.
Now the question is how much stiff carbon mast actually had to be. And how much in torsion and how much in bend. Gunnar position /please sorry to saying for you/ is the stiffer the better and as he rode 10000000x better than me and also is very knowledgeable about gear there is no reason to not believe him.
My own experience with this is fuselage stiffness is absolute critical both in torsion and bend. This is very easy to feel - mast is harder for me to feel as i rode quite gently and not carving etc. so hard. Also water condition is changing so sometime im not sure if i feel turbulent water or wobbly mast
On french pioneer foil diy forums this was thing they solve over and over to death with no clear conclusion.