As decrepit 's advise, I used my spare cell phone with a sim card, downloaded' Windsport Tracker'. Put in water proof bag and on top of my head. Got first tracking as below. Before I get a motion or smart watch. I don't know is it 2 second or how long time period.
Good to monitor. Means I need more practice, at least speed need to get on top of wind speed.
Why I can not go staight? as a line. I always turned with sail side. For example, if sail is at my left side, my board turned left a bit, I have to slow down to adjust direction. That is why I was full powered not long then dropped speed.

Thanks for all surfers' knowledge gave to me here!

well done mate You're ,on the way.
The recording interval should be as short as the phone will go. The gtc preference is 5hz or 10hz. But for where you are now 1sec is fine, and I doubt the phone will do better.
My interpretation of what you are saying, is that you are turning downwind does that make sense?

Basically we have 2 forces, the wind in the sail and the water resistance to that force. So the balance point of the wind's force is called the centre of effort, CoE. the balance point of the board's resistance is the centre of lateral resistance, CLR. So if the CoE is in front of the CLR you will go downwind, if it's behind you will go upwind.
As I mentioned earlier, as you pick up speed the front of the board rises out of the water, shifting the CLR backwards, so you have to rake the sail back to keep CoE and CLR aligned. This is when you can start moving backwards on the board and get into the straps.
If the boom is too low, it's harder to rake the sail back

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decrepit said..
well done mate You're ,on the way.
The recording interval should be as short as the phone will go. The gtc preference is 5hz or 10hz. But for where you are now 1sec is fine, and I doubt the phone will do better.
My interpretation of what you are saying, is that you are turning downwind does that make sense?

Basically we have 2 forces, the wind in the sail and the water resistance to that force. So the balance point of the wind's force is called the centre of effort, CoE. the balance point of the board's resistance is the centre of lateral resistance, CLR. So if the CoE is in front of the CLR you will go downwind, if it's behind you will go upwind.
As I mentioned earlier, as you pick up speed the front of the board rises out of the water, shifting the CLR backwards, so you have to rake the sail back to keep CoE and CLR aligned. This is when you can start moving backwards on the board and get into the straps.
If the boom is too low, it's harder to rake the sail back

Hi decrepit, it really took me a while to learn your answer.
Like a professor talking to a primary student.
I don't think it caused by current or tide, it was my skill, could not cope with the direction well.
As below my hand drawing, I wanted to go the direction called 'target', because I could see something by eyes, and then board nose turned to 'actural' direction. Star is the symbol of me. When I came back, as another drawing in same paper.
If it caused by current or tide, the board should move as below top drawing as my understanding.
Can I ask again please? in this picture, I marked 'A,B,C,D' four direction, which direction should I move/push the mast towards to keep board straight to a target? Not lost direction, like the last picture.





Thanks!

Mots, shortboards will crab sideways to some degree when not planing. In light winds a bigger fin will give you more directional stability and ability to point higher into the wind.
( no walk of shame)
To answer your last Q. If the board wants to bear away as in your diagram. Counter this by raking the sail aft slightly. Its all a balance thing. Speed vs direction. More speed = less side slip.

This is a vector problem, not tide or current. As I said, if the centre of effort of the sail is forward of the centre of lateral resistance of the board you will turn down wind. So you have to get the two efforts lined up to go straight. how you achieve that depends where you are on the board. If you are too far back, you need to move further forward, to get the boards rails to do more work. If you are well forward on the board, as old school says, you have to rake the sail back.
Lets see if I can draw this, back later. OK it's a lousy drawing but I hope it helps to explain what is happening.

Some overnight thoughts.
If your board is pointing to where you want to go, but you don't get there, that's slippage or yaw.
Maybe your sail isn't rigged optimally, if there's not enough downhaul, the sail will have lots of drag, that will increase the downwind forces. and make you travel sideways.
Another effect is your trim of the board. If the windward rail is down, that lessens the board's resistance to going sideways. Putting your weight in the middle of the board, so it sails slightly leeward rail down will give much better sideways resistance.

There's so many interactive elements involves in windsurfing it's hard to tell exactly what's going on without seeing you

yet another thought. Beginners regularly drift down wind and have to walk back up the beach.
It's very easy to loose ground.
While you uphaul you drift downwind.
It's also hard to tell when you are sailing square, it's easy to sail out a bit downwind, then have to sail upwind to get back.
A bigger fin may help you, that will offer more resistance to going sideways

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decrepit said..
yet another thought. Beginners regularly drift down wind and have to walk back up the beach.
It's very easy to loose ground.
While you uphaul you drift downwind.
It's also hard to tell when you are sailing square, it's easy to sail out a bit downwind, then have to sail upwind to get back.
A bigger fin may help you, that will offer more resistance to going sideways

Appreciated for your generous advice!