I have suzuki df5 from the year 2007, about 50 working hours at summer and this is the probelm:

In neutral and when going forward up to half throttle my suzuki is operating normally .
When sailing forward at about half throttle the vibrations starts and a metallic sound appears , like scratching ( as it will mess up ) and the boat can't accelerate any more.
Almost as if it was in neutral and slipping propeller + metal sound . But it is not in neutral and even with that sound boat is going forward but can't accelerate any more.

I have been told that if propeller is slipping that the boat can sail only with a minimum speed and up to about half throttle (before the problem) i have normal speed.
In neutral it can reach full throttle , while going forward a problem occurs at about half throttle .
What could this be? Propeller or something else?

Before this summer i had done the service, changed oil and the gear oil, zincs, impeller and cleaned the carburetor .

I removed the Flywheel to check the magnets on it ( for battery charging ) and copper windings of this assembly .
Than i put it back.
Is it possible that this is the cause of the problem ? That maybe i have Insufficiently tightened flywheel nut ?

It's strange to me that the while on neutral there is not a problem.
So another problem is that if I take my suzuki to the mechanic I won't be able to show him what's going on and what is the problem.
I hope for some useful advice .

Best regards