Best thing for towing a kayak with your bike is a homemade PVC cart that uses a 5/8th threaded rod axle and 10-inch inflatable dolly wheels that have 5/8th (lawnmower size) bearings. There are several videos online showing how to make these carts - it's pretty easy. When you're going bicycling speed, you need something with bearings, you want pneumatic tires, and you want a cart that won't collapse because it isn't designed to collapse.