To measure a boat, keep it on the trailer and put a scale underneath the tongue at the end. Put a block around the scale in order to lift the tongue to better see the dial, and then record the weight.
After doing this, slide the boat aft to the trailer 12 to 18 inches, and record exactly how many inches you moved the boat, as well as the new scale reading. The final measurement is the distance between the trailer's axle and the point where the tongue touched the scale. You now have four measurements: the first scale reading, the second scale reading, the distance you moved your boat, and the distance between the trailer and scale point.
W1 equals the first, or heavier, scale reading in pounds; W2 equals the second scale reading in pounds; X equals the distance you shifted the boat in inches; and C equals the distance between the trailer axle and scale point in inches.
To find out the boat weight, multiply the distance between the trailer axle and scale point by the difference between the first and second scale weights. Divide this number by the distance you moved the boat on the trailer. The more accurate your measurements, the more accurate the resulting weight.