Mercury - Serpentine Belt Engine Drive System Rubber Composite 865615Q03. Overview. There's a certain peace that comes from running a boat whose engine feels settled and sure of itself. When you turn the key and that V6 or V8 fires up with a steady hum, you know every piece of the system is doing its part. A good serpentine belt doesn't call attention to itself, but you can feel the difference when it's the right one-clean, smooth power delivery and no surprises while you're out enjoying a long