This is a subject that I've written about extensively over the past three decades. First and foremost, the tongue weight should be 5 to 7.5- percent of the rigs total weight. This includes the boat, motor, trailer, batteries, fuel, cooler chests filled with booze, etc.. The only way to obtain an accurate assessment of this is to take the boat to the nearest truck stop that has scales, or a state weigh station and have it weighed. Most of the time there is no charge for this and you can find the scales in close proximity to where you live.
Once the total weight figure is obtained, calculate the tongue weight using the 5 to 7.5-percent figures. If you have access to an accurate, portable scale that will measure heavier weights, so much the better. If not, you can still measure the weight using an ordinary bathroom scale and a few other items.
The measurements should always be made on flat ground, the trailer's wheels should be firmly chocked and never guess at the weight. I know boaters that have lifted their trailer tongue off the ground and said "Oh, that feels like about 100 pounds!" If you can pick up the trailer tongue by hand, then the tongue weight is likely not sufficient.
The secret to success is to position the tongue of the trailer at the same height of the ball hitch--this is extremely important in determining the correct weight. This can be done by placing a concrete or heavy wooden block on the scale, then using the tongue jack, lowering the hitch slowly on the block until the jack wheel is off the ground. If you must use a bathroom scale the following diagram will be helpful.
To adjust the tongue weight the trailer's axle(s) must be shifted forward or back until the correct weight is achieved. This is not an easy job, and if you don't have access to a floor jack, a heavy-duty socket set and good support blocks for the trailer frame, don't attempt to do this--it's too damned dangerous for amateurs to fool with.
In some instances, you can move the boat a few inches forward or back and achieve the correct tongue weight. Again, this must be done on flat ground, and you must take precautions to secure the boat to prevent if from inadvertently sliding off the trailer. The easiest method is to loosen the trailer's winch post, release the winch brake, slide the winch a couple inches forward, then tighten the winch post and winch the boat forward to the bow stop. While the boat is inching forward, have someone keep an eye on the scale and when the desired tongue weight is reached, stop, mark the spot then loosen the winch post and slide it aft until the bow and the bow-stop meet. Tighten all the bolds down to manufacturers torque specifications and go to the next steps.
Be sure the trailer tires are inflated to the correct, trailering, inflation pressure. If the tires are even a few pounds under-inflated this can result in severe swaying, load shift, the tires will overheat and rapidly wear out. Next, check the wheel bearings and make sure they have been repacked for heavy-duty, wheel bearing grease before making that first trip. Keep in mind that sailboat trailers must be submerged in order get the boat on and off the trailer, therefore, a significant amount of lubricant is washed from the bearings, and a fair amount of dirt is usually sucked into the bearing housings. Wheel bearing failure is among the top reasons for trailer-boating accidents.
Be sure the ball-hitch coupler is properly adjusted. There should be little or no up and down movement when the coupler is latched in place. If there is, this should be adjusted using the coupler's adjustment nut, which is located beneath the coupler latch. The nut us usually held in place with a cotter pin, which must be removed prior to adjustment. Adjust the nut until the latch is locked firmly in place, but not too tight. Also, a squirt of WD-40 on the ball, coupler latch and inside the hitch cup goes a long way in preventing excessive wear. WD-40 should also be squirted on both the trailer light plugs electrical connections before connecting to ensure good electrical continuity.
Finally, be sure to use a trailer hitch that is rated for the weight of the rig you are towing. Bumper hitches on pick-up trucks just don't cut it. Spend a little more and get a hitch that is rated to tow a small building and you can't go wrong. Far too many individuals try to skimp on the capacity of their trailers and hitches, which often results in roadway disasters.
Good Luck, and I sincerely hope the above information is helpful,