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 Teeter-totter Analogy
Imagine a teeter-totter that is 20 feet long. We want to place a 35-pound child on one side, 5 feet from the end. If want the see-saw to remain balanced, you would place a 35 lb weight on the other end, 5 feet from the end. It “counters” the child’s weight and placement perfectly. Simple, right? This is the concept of “counterweights,” just as on a forklift.
If you moved the child a foot farther out (four feet from the end), his leverage makes the see-saw unstable, and the child hits the ground, crying, because you are not an engineer. The same would happen if you placed a 45-pound child 5 feet from the right end.) Moving the counterweight back a foot, or adding the properly-calculated weight to it, would achieve balance. This is similar to how forklifts work.
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.
Forks, Carriages, and Masts Matter Too!
Furthermore, lift capacity and movement are not the only considerations. A lift might be fully capable of hoisting a load at the capacity you need to the height you want. It might be capable of handling turns, slopes, and braking. But what if the steel of the forks, carriage, or mast is not high grade? What if the axles are at the minimum to handle the stresses put upon it?
The Bottom Line
Wiggins lifts are engineered with these issues in mind, and manufactured using high-quality steel and other components. Some have said that Wiggins forklifts are “overbuilt”—we say they are designed to be tough, dependable, and to get the job done safely. Other manufacturers often scrimp in order to undercut pricing. As the saying goes, “you get what you pay for.” Wiggins lifts are the standard by which all others can be judged.
Lifting boats raises a whole host of other physics and engineering issues—which Wiggins has been addressing for decades. We’ll look at the unique issues of marina lifts in a future post, and discuss how Wiggins solves those engineering challenges, too.