Hi Slavko, I think you said in one reply that there was no parts or stubs of the screws left even though when you fitted them you had screwed them tightly in. These were not exactly your words but my understanding of what you wrote. Clearly when you are thrusting to port, the propeller is pushing in to the bow thruster, while to starboard it is pulling away. If there are no stubs of screws left they have been pulled bodily out. On the to port thrust the screws must have been in place or the propeller would have spun on the hub. This means when you did the starboard thrust the screws pulled out. they didn't sheer off or the would have been stubs left. Possibilities. 1) the screws are too small for the hole in the hub and so don't hold. 2) you over tightened the screws and stripped the threads of the screws. 3) the threads in the hub are stripped allowing the screws to pull out. 4) the screws are too short and not gripping on enough threads. When you fit it and tighten the screws look to the back of the hub you should be able to see the tips of the screws 4)When you fit the propeller to the hub does it slide all the way on and butt closely in the the back of the hub. Some propellers and hubs are not a good match and it can be hard to push the propeller right home.