We just bought our first sailboat a few weeks ago. Launch was okay, stepping the mast was okay too. However, there's a lot of slack in the stern line and I can't seem to get it tightened. The rigging was mostly in place from the previous owner, including the stern line. There is what seems like a tensioner, flat plate with 5-wheels. Either it's worn or the line isn't fed through correctly. Anyone have any ideas what my issue is here? Or what this piece is called?