Tongue weight is the force, or weight, your boat trailer's tongue exerts on your tow vehicle’s hitch. The target tongue weight for any boat trailer is between 10 to 15 percent of the gross trailer weight—including boat, engine and gear. If the trailer’s tongue weight is too high, meaning too much of the load is riding ahead of the trailer’s axel, it could affect the tow vehicle’s balance.
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 an extreme case, excessive tongue weight could even cause the tongue, hitch or the tow vehicle’s suspension system to fail.
In a situation where the tongue weight is too light (too much of the load riding behind the
trailer’s axel), the trailer will not track true behind the tow vehicle. Instead, it will sway side-to-side at all but the slowest speeds.
If this happens in a towing situation, the proper course is to hold the wheel steady as you
gradually reduce speed. Don’t break sharply, increase speed or try to steer out of the sway as any of these actions will worsen the problem. Many times you can eliminate sway in a box-type trailer simply by moving some of the cargo forward in the trailer, and securing it so it doesn’t shift.
When you’re dealing with a boat trailer, however, you may have to move the winch stand forward or back on the tongue, and readjust the bunks or rollers, to achieve proper tongue weight.
Determining tongue weight, using a standard bathroom scale, is fairly simple. You first need to know the gross trailer weight, however. Take your fully-loaded boat and trailer to a commercial scale, or search your owner’s manuals for the weights of your boat, motor and trailer. Then, add the estimated total weight of your gear, including tackle, batteries, trolling motor, sonar units, fuel load and anything else you normally carry.
If you own a light, aluminum boat the procedure is very easy. Start by making sure the boat is loaded with the normal gear, then chocking the trailer tires so the trailer won’t roll. For an accurate reading, the scale must be elevated to normal towing height by placing it on a wide, stout block of wood, or something equally stable and sturdy. Rest the coupler on the scale and take the reading. Again, you’re looking for a tongue weight of something between 10 and 15 percent of the gross trailer weight.
Because bathroom scales typically top out at 300 pounds, you may have to assemble a lever scale to measure the tongue weight of a heavy fiberglass boat and trailer. For this you’ll need:
*A 300-pound capacity bathroom scale
*A brick or piece of wood that’s equal in thickness to the scale
*Two lengths of pipe short enough to rest lengthwise on the scale’s platform
*A 6-foot beam that’s stout enough to take the weight of the trailer tongue
Again, the assembly must be elevated to normal towing height, and the trailer chocked. Set the wooden blocks, or whatever standards you use, about four feet apart, on-center. Center the scale on one of them and the brick or wood shim on the other. Center one of the short pieces of pipe on the scale, the other on the shim. Then place the beam across both pipes, as shown in Figure B.*
At this point be sure that the beam is level and at normal towing height, and that the standards are wide enough that they won’t tip forward, back or to either side once the trailer tongue is positioned on the beam. If the scale can be re-zeroed, do it now. If not, note the weight with the pipes and beam in place because you’ll be subtracting it later.
If you are confident the assembly won’t tip or collapse under the weight of the tongue, rest the coupler on the beam two or three feet from the pipe on the scale. Take the reading. If you could not re-zero the scale earlier, subtract the initial weight from the current tongue weight. Now, multiply the final weight by the number of feet between the pipes (by 4, in this case), and you have the trailer’s tongue weight.
*Illustration courtesy of etrailer.com