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River Science: Mechanical Advantage

Mouse over to turn the winch!
The crew walks the capstan.

Mechanical advantage is the ratio of output force divided by input force in a simple machine, such as a pulley, lever, or winch. The greater a machine's mechanical advantage, the greater its output force compared to its input force. In other words, simple machines with positive mechanical advantage make it easier to lift or move heavy objects.

We use simple machines all over the Half Moon in a wide variety of forms and uses. For example, our capstan (pictured above) is a massive winch used for tasks such as weighing anchor. The anchor alone weighs 300 lbs., plus the weight of its chain, while the total load on the capstan (including the anchor, chain, line, and the strain of pulling the ship against the current) while weighing anchor can approach a ton. Despite this, with the capstan's help, just six or eight students can raise our anchor without a problem (though certainly not without effort!).

Above the ship, we use dozens of blocks (pulleys, shown to the left) in the rigging to handle the sails. Our simplest sail, the lateen, uses more than twenty all by itself!

Mouse over to steer right one half!
Emily pushes the whipstaff to half-right.

In the helm hutch, we use a whipstaff (pictured, right) to steer the ship; the original Half Moon predates wide use of the ship's wheel by about a century. The whipstaff is a simple lever, allowing a single crewmember to easily push the ship's massive rudder and tiller to back and forth.

On this voyage, the research team of Emily, Job, Raven, and Ymkje studied mechanical advantage for their presentation project. To conduct their experiment, they rigged a block-and-tackle system designed to lift a fixed load — in this case a lead weight assembled just for this rig by Mr. Lyke and Ms. Niehaus — a few inches off the deck, using from one to five pulleys to distribute the load. As the students haul on the line, a spring scale measures how many pounds of force they must exert to lift the weight. In addition, the students measure the length of the line they need to pull through the simple machine in order to raise the weight the same distance while using different pulley configurations.

In theory, each moveable (as opposed to fixed) pulley doubles the simple machine's mechanical advantage (the required force decreases as the line distance required increases). In practice, however, the friction of the lines and pulleys counteract this advantage. The effect of friction is negligable at first, but it gradually increases as more pulleys are added, ultimately canceling out any mechanical advantage gained — thus placing limits on the machine's effectiveness, no matter how many additional pulleys are added.

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