This is good reading....I use the 2nd link (in reverse) to determine my air pressure...figure the total weight of loaded boat and divide by 2 (number of tires I have-single axle) to get total weight applied to each tire.....then from the chart for ST tires determine the air pressure....note that in both links that ST tires are not rated over 65 MPH, so that is my maximum when pulling a boat.....

11 Things To Know About Boat Trailer Tires - Trailering - BoatUS Magazine

http://www.goodyearrvtires.com/pdfs/rv_inflation.pdf