Buy a Wiggins Lift This Year, Get a Big Deduction!

Sometimes the image of a teeter-totter (see-saw) is used to explain how forklifts work. This analogy is okay as a beginning point about the physics of counterweight. Yet it hardly begins to address what is needed for lifting a boat.

The problem with this analogy is that a teeter-totter does not move, turn, brake, or go downhill like forklifts do.

Moreover, the shape and weights of children on a see-saw are usually not like loads lifted by forklifts—and this is especially true of boats.

Delving into Load Capacity and Load Centers

A better explanation uses the image of a forklift itself. After all, few adults would not be generally aware of what a forklift looks like. So, imagine a forklift.

Suppose that the load we wish to lift (and move!) is a large obelisk—a huge, rectangular weight. This obelisk is 40 feet long, 10 feet wide, and 1 foot thick. (If you have seen Stanley Kubrick’s 2001: A Space Odyssey film, you have the idea.) Our obelisk weighs 5,000 pounds.

Manufacturers let you know the capacity of a lift by telling you what weight (“load”) can be lifted at a certain distance out on the forks (“load center”). Let’s say this forklift has been designed to lift this obelisk, as it sits vertically on the forks, 2 feet out from the carriage:

The manufacturer tells you that this lift can hoist “5,000 pounds at 24-inch load center.” If you put the centerline of the obelisk 2 feet out on the forks, standing vertically, it will lift it. (At the moment, we’ll ignore how high it could lift it—that depends on counterweight, too, because of gravity and other forces). However, if you moved the obelisk one foot farther out on the forks, the lift would become unstable and fall forward.

Forklifts Move (Teeter-totters Do Not)

But there is more to consider. If the lift was designed to lift exactly 5,000 pounds at a 24” load center, and you braked too suddenly while moving it, the lift would also become unstable. Just like when you apply your brakes in your car, the stopping motion produces energy that transfers weight forward, and front of the becomes momentarily "heavier" than the back before it returns to normal. The load did not move forward, but the overall weight distribution did. (This is really an issue of mass, momentum, and the earth's gravity which creates imbalance.) The same would happen if you drove downhill—the vehicle becomes "heavier" in the front.

What we are talking about here is really the “center of gravity.” This is the point or division along a mass ("load") where there is the same weight behind it as in front it. That point is called the center of gravity, and it changed not only from shape to shape, but by increasing or decreasing speed, braking, and traveling up or down slopes.

Therefore, to ensure that our 5,000 pound obelisk can be transported safely while driving, turning, braking, and driving on slopes, it has to be capable of lifting _more_ than 5,000 pounds at 24” inch load center. A salesman might tell you, “Oh, you don’t want to buy that more expensive 5,000-pound @24” lift, when ours will lift 5,000 lbs at 24-inch load center for less money!” It might, but how quickly can you brake in an emergency without creating imbalance? How much of a slope can you traverse without losing the load? Most likely, the cheaper lift has less counterweight and strength in its structure.
If you only plan on lifting a single kind of weight, and never wish to transport it anywhere, then the cheaper lift will probably work fine for you. Otherwise, you may be well on your way to accidents.