Donato,

  The rudder is made of two halves so some evidence of a seam may be normal.  Again the Amel rudder is new to me so if an expert like Oliver responds I would be listening to him.  

   Have you tried applying moderate force to turn the rudder each way (perhaps two men pushing on the rudder TE while the rudder shaft is held in some fashion) to see if what you think is a crack opens and closes as the force is applied and released?   

Best,

James Alton

SV Sueno, Maramu #220

Arbatax, Italy