Just a thought since you didn't get the answer you were looking for over in the prop forum.
Fill the gas tank as you would for a 'normal' day boating.
Take the boat & trailer to a truck scale or recycling center (I can drive across the scales free if there's little to no traffic at the recycler's) and weigh them together, dropped from the trailer ball of course.
Take the boat off the trailer: into a slip, moored at a dock w/ someone waiting with it, or on some tires in your yard. Go back & re-weigh just the trailer.
The difference is what your boat actually weighs, NOW, not what the factory intended it to weigh dry. In 20yrs+/- it may have gained a little water weight, and you'll know what it weighs w/ all the I/O fluids & gas. Then you can add for extra people & gear to get an accurate 'in use' weight.
