Credit to unknown poster.
You can weigh the boat with a regular household scale using simple math and physics. ...
Too easy.
Except for the "scoot the boat back" part. Even with a roller trailer there will be a lot of friction. You will likely need to jack up the hull and move the trailer forward, or find a fixed object to attach the boat to while pulling the trailer forward with a vehicle. I would do that part first, as getting the boat back on the trailer using the winch is much easier, and can be done without needing to move the scale . Also, don't bother with the exactly 10 inches part - the further you move the boat, the more accurate this method is. Try to get the tongue weight close to zero, take weight and position measurements, then winch the boat back and take new weight and position measurements.
Just did this myself, and getting just over 2200 lb for a 16 foot outboard has me thinking about the possibility of wet foam too.