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River Science: Mechanical Advantage

Mouse over to sweat the anchor.
Jose and Mr. Van Grondelle use pulley to lift the anchor.

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.

On this voyage, the research team of Alex, Jon, and Tajanae studied the principles of mechanical advantage for their presentation project.

We use simple machines all over the Half Moon in a wide variety of forms and uses. For example, our capstan is a massive winch used for tasks such as weighing anchor. The anchor alone weighs over 400 lbs., 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.

Mouse over to walk the capstan.
The crew walks the capstan under heavy load.

Despite this, with the capstan's help, just eight students can raise our anchor without a problem (though certainly not without effort!).

Matt looks up at the sail while manning the whipstaff.

In the helm hutch, we use a whipstaff 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 back and forth.

The port fore course clew.

Above the ship, we use dozens of blocks (pulleys, such as the one pictured here) in the rigging to handle the sails. The lateen sail alone, our simplest sail, uses more than twenty!

Mr. Van Grondelle, Jon, and Tajanae rig a block-and-pulley experiment.

For our experiments in mechanical advantage, we rig a block-and-tackle system designed to lift a lead ballast ingot (safely wrapped in canvas) a few inches off the deck, using from one to four pulleys to distribute the load. As students haul on the line, a spring scale measures how many pounds of force must be exerted to lift the weight. As an interesting side wrinkle, our spring scale can only measure up to 50 pounds, and the ingot weighs considerably more. Thus, students must extrapolate the ingot's true weight from their findings.

In theory, each down-pulling pulley doubles this simple machine's mechanical advantage. 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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