Hi Guys.
I tried it again today without the turnfin, and now it can come up to 1/2 throttle safely.

It turns quite bad now at high speed.
But now it has a new problem. The prop is "walking" on the water, making the boat yaw sideways. The faster it goes, the more sideways the bout is....even though its going in a strait line!
Thanks for the tip Jan, I wonder where I can get one....