Assuming your boat weighs 1500 lbs. = 700kg. One liter of water =1 kg. From the internet, a 16 inch tractor tire has 3.4 cubic feet of air. A liter = .035 CF. So 700kg x.035/ 3.43=7.14 tubes. Add a safety factor of 20% and round up to even number, 9 inner tubes. Maybe Old Glory will weigh in and see if the math works in the real world.