Not a great picture but, it appears to my eye that the alternator pulley is to wide and the belt is probably riding on the bottom of the vee, look to see if the bottom is shiny from where the belt is in contact. Many times a lot of these " hi output" offshore alternators come with a new pulley but they aren't sized correctly. I had this exact issue with mine. I ended up at a local rebuilder and bought a stock pulley for something like 8 dollars to match the pitch of the fan vee. problem solved for me.