Guys thanks for taking the time to answer my question!
i guess my question is not relevant to the speed i will probably travel.
the speed on the video is not revealed.
My impression was that this was the first place where cavitation started.
when routing my wings i did the easiiest fastest way to do the profile without change of bit.
but if taken the proper time and chage of bit the transition can be made smooth without Sharp angles!
i guess i have to make my wings and strut connections with more atention!
just wanted to know if it made a big difference!
The foil in the video is attached directly to the bottom of the strut/mast , which is also foil shaped.
the fact that both sections are jointly reducing the pressure in that region is why cavitaion first apears there.
Most of the current kite hydrofoils we've seen ( including yours) have sufficient separation between the strut and front wing that the pressure drop is not additive or combined to cavitate prematurely.
Regardless, sharp corners at any joint , whether they are internal or external tend to increase drag
The use of fillets where ever possible help reduce the formation of vorticity in corners, reduce the total exposed wetted area of the joint ( and its skin friction) and help stabilize the joint structurally.
Every foil shape has a well defined pressure distribution that is easily calculated ( or looked up in any of a number of reference sites). if the pressure distribution lowers the local pressure to the point of cavitating, you can choose a foil profile with less lift coeff and increase area ( and/or aspect ratio) to compensate.
but just some thoiughts.
(edits for spelling)