Could also be that the trailer and boat aren't matched....the boat is too heavy for the trailer causing the the spindle/tires to lean towards the boat, causing the inside wear. Same thing could happen if the trailer was built to handle a heavier boat, but then the wear would be on the outside of tires. You might be able to have the axle bent a little more, but I'd suggest a trailer company do it. If everything is perfect, you should be able to put a level on the tires with the boat sitting on the trailer, and the tires should be either plumb, or slightly tilted away from boat.