Tongue Weight (TW)

The downward pressure placed on the ball by the coupler.

Trailer Loading
One of the main causes of trailer sway is not having a large enough percentage of trailer tongue weight compared to gross trailer weight.  Minimum tongue weight is around 10 percent of the trailer's gross weight.  (some sources say higher wt is desired but I've found 10 percent is a good compromise unless experience indicates more weight is needed).

Too much weight on the hitch essentially lifts the vehicle’s front end, which in turn causes the front tires to lose grip on the road surface and makes steering difficult. In a situation where the tongue weight is too light the trailer will sway side-to-side at all but the slowest speeds.

You should weigh the entire rig to insure that the vehicle is up to the job. You can get the tongue wt at the same time you weigh the total package. There is more about towing and weighing your vehicle at this link: 

Tongue wt can be determined using a bathroom scale but that won't really help unless you know the total wt of the boat and trailer together. Instruction on this method is here:  To help prevent the trailer from swaying back and forth, a few things can be done. Try placing heavier cargo in the front of the trailer, ahead of the trailer's axle.  Also center the cargo left-to-right and use tie-downs to keep the load from sliding.

The best way to verify your tongue weight is to run it across a certified scale. However, you can also use a bathroom scale using the method described below:

Normally the TW under 300lbs can be measured using a common bathroom scale and a box.  On a level surface, place the coupler of the loaded trailer on the scale at normal towing height (not pictured). For heavier tongue weights, use the method diagramed on the right. There is another alternative method described on the Boat US Trailering website.