The most obvious and straight forward method is to put the boat on your bathroom scale, first the bow and then the stern, and add the two numbers. This certainly works, and is easy to understand, but there can be two problems. One is that you need one or two other people to help you lift the boat around, and space to do it in. The other is that the scales may be off (they always read high, don't they?), and summing the two readings sums that error.
Here is another bathroom scale method that minimizes these two problems. The downside is that you have to do a calculation to get the answer.
The boat stays on the trailer the entire time. Put the scale under the tongue near the end. You can even put a block or brick on the scale to lift the tongue so you can see the dial. Record the weight. Now, slide the boat aft on the trailer 12" to 18", which for the Windmill is easily done single handedly (if you loosen the tiedowns). You must measure exactly how many inches you moved it, and record that. With the scale in the same location under the tongue, record the new scale reading. Finally, measure the distance in inches between the trailer axle (wheel centerline) and the point on the tongue where it touches the scale.
You have then four measurements:
S1 = first (heavier) scale reading in lbs S2 = second scale reading in lbs X = the distance you shifted the boat, inches C = distance between trailer axle and scale point, inches
The formula is Boat Weight = C (S1-S2)/X
Example, using John Harris' #5400:
S1= 70 S2= 53 X=12.25" C=142"
Boat weight = 142 (70-53)/12.25 = 197.06 lbs. Close enough? Will Chief Larimer let that pass? Don't bet your entrance fee on it. Go ahead and add that extra fitting.
A couple of notes. The result, like any measurement, is sensitive to the accuracy of the input data. In this case, the most critical is the distance you slide the boat. One inch error out of 12 inches will really make a difference in the result. Slide the boat as far as you can and still have a readable load on the scales. Notice, too, that by subtracting the two scale readings, any constant error in the scale is canceled out.
Note to techies: The formula was derived by taking the sum of the moments around the trailer axle for each case and subtracting the two equations. Things that you don't know, like the trailer's weight and the location of the boat's center of gravity cancel out, leaving just the variables that you can measure, and the boat weight.