Mercury - Serpentine Belt Engine Drive System Rubber Composite 865615Q04. Overview. There's something reassuring about hearing your engine settle into that familiar, even rumble when you turn the key. On those mornings when the water's glassy and the sun's just clearing the trees, you want every part of your boat working in quiet harmony. A good serpentine belt doesn't draw attention to itself, but you feel its presence every time your alternator charges cleanly, your cooling system holds steady