RE: Problems going upwind
There are a number of factors that may be preventing you from untapping the full lite wind upwind potential of your F-type 158.
What brand and model 8.5 m2 rig do you have...?
What is the windspeed ....? Is it marginal for planing, or can you easily get your board up to full speed on a beam reach?
Where do you have the mast foot positioned... near the back...near the middle... or near the front... of the mast slot?
Are your harness lines truly balanced with your sail raked all the way back and the foot down on the deck....?
Answer these questions and we can figure out the issues that are keeping you from getting the full upwind potential from your board.
Also could you please describe what you think you are doing with your feet to make the board turn upwind.
Getting the board on a full plane before your try to go upwind?
Lifting with your front foot slightly and pushing across the top of the fin parallel with the surface of the water (i.e. not pushing down toward the water)?
Also, how long are your harness lines?
How high is your boom? (i.e. shoulder high...chin hgh..nose high...up above your eyes?
How do you have your sail tuned? Maximum draft (i.e. minimum downhaul and outhaul for max. power); medium power i.e. normal dowhaul and outhaul for the best combination of range and top speed)
tuned for overpowered (i.e. max. downhaul and outhaul to flatten the foil and give the most twist at the top for good overpowered stability)
Are you simply heading too high for the conditions?
"But when I after that try to steer upwind by using my feet the board loose the plane or tends to bear away ." suggests there may be more than one problem here.
If your board falls off plane (from a fully wound up beam reach plane) then you may be going up too high for the conditions, or turning upwind too quickly.
If your board "bears off the wind" tht would mean you are tipping the board too much to leeward and actually almost "initiating a jibe".
Where are your footstraps positioned... in the farthest back and outboard positions, or somewhere else?
Lots of factors here, and I'm sure we can solvew your issues with answers to these questions.
Hope this helps,