Boat Trailer Tires - Keeping Your Boat Moving on Dry Land
Probably the most important thing you can know about your trailer's tires is that adequate tread does not equal adequate performance. Those tires are subjected to stresses and forces that other tires just don't have to put up with. They carry very heavy loads in relation to their size. They're subject to salt water and other types of environmental and weather factors. In fact, your tire's tread may look great, and the tire itself be cracked, split, or weather damaged beyond repair.
If you want to keep your boat trailer tires in good shape, the first thing you can do is keep them inflated properly. The weight these tires have to bear is tremendous; under-inflated or over-inflated tires will warp or crack much more quickly than those kept at the proper air pressure. The sidewall of the tire has the correct p.s.i. on it; use a tire gauge to be sure you've got it right, and check it every time you get ready to go out.
Check the sidewalls of the tires frequently, and be on the lookout for splits or weather damage. Rubber isn't indestructible. Don't try to save yourself money by taking those worn tires on 'just one more trip'. You could damage your trailer, your boat, or yourself. It's just not worth it.
If in doubt, replace those tires. Tires are going to cost anywhere between $40 and $150 apiece, depending on size and quality, but the repairs to your trailer or boat because of an accident will cost you much more than that. Buy the best quality tires you can afford, and keep a close eye on them during their lifetime.
One final word: while there are some folks that will launch their boat into the water trailer and all, don't be one of them. Boat trailer tires aren't meant to be submerged for long periods of time, especially in salt water. Not only will the rubber be very unhappy, you're going to lose a lot of performance from your boat itself.