Do tires have a negative environmental impact when used as dock bumpers?
No. The chemical-release rate from tires is too slow. Newly discarded tires—a set pulled straight off a car—leach a slightly larger amount of chemicals than those that have been sitting around for a while, but that difference is very small, and not something to be concerned about. Tires will harm water quality when many are packed into a small volume of water.
But just how effective are tires as bumpers, and how will they hold up in the water, battered by your boat? Water and sun exposure does not make them ineffective. The part of the tire that’s useful as a bumper is actually the inner portion—the steel and polyester—which is surrounded by rubber. Even if that rubber eventually cracks, the tire will still be springy. The down side is that extremely worn-out castoffs, with very little tread left, might not be rigid enough to stand up to your boat’s repeated bumping, and could collapse. Plus, tires will almost certainly scuff a fibreglass hull.