CONTINUOUSWAVE

If your local city or county [municipality has a public scale in a location] that is convenient for you, ask if you can use their scale [to weight your boat and trailer]. They probably won't mind if you're not keeping anyone else waiting.

[Drive the boat on the trailer to the scale.] Weigh your boat and trailer. [Drive the boat and trailer to a launch ramp], launch the boat, [move the boat from the launch ramp to somewhere convenient]. Then [drive] back [to the scale site] with an empty trailer [and weigh the trailer]. [Then drive back to where you left the boat, move the boat to be close to the launch ramp, and leave it temporarily. Go back to wherever the vehicle and trailer were left, and move the trailer to the launch ramp. Load the boat on the trailer. Then drive home.]

The weight of an empty boat trailer can usually be found from the manufacturer’s certificate of origin. For any aftermarket parts that have been added to the trailer such as a spare tire carrier, a spare tire, loading guides, and so on, the weight of the added components can generally be estimated accurately. This eliminates the need to weight the empty boat trailer.

In the procedure proposed above, both a certified scale and a boat launch ramp need to be located conveniently close by.

Also, if going to the bother to take the boat to a scale, using a scale with three platforms, as found at certified truck scales, can yield a great deal more information about the weight distribution when towing. For a complete discussion of how to discover more information about towed weights see this 2008 article which has been available for 18-years on the website:

Boat Weight From Certified Scale
https://continuouswave.com/ubb/Forum1/HTML/016667.html