It depends on the trailer make. You are looking at a loaded boat plus trailer weight of around 5,000 lbs. Take off a 500 lb tongue weight and on a single axle you are looking at a load per tire of 2,250 lbs. Add 20% as a safety measure and you should get a tire rated for at least 2,700 lbs. Get tires with the "ST" designation which are specifically designed for trailers. Understand these tires have a maximum speed rating of 65MPH. They should be filled to maximum PSI to prevent overheating. Now you need to check the size of your rims to see what size of tire you need; it could likely be 14" or 15" X 4.5 or 5.5" but you need to be exact. In 20 years the original rims and axle could have been replaced.