MythBusters Episode 165: Blow Your Own Sail

A sailboat stranded in calm water can move forward by using an on-board fan to blow air into its own sail.

confirmed

The Build Team set up small-scale tests in the shop using a wheeled cart on a tabletop. Kari demonstrated that a forward-facing model airplane propeller could generate enough thrust to push the cart backward. However, after Grant fitted a sail onto the cart, the prop could not move the cart due to the equal and opposite forces acting within the system.

With smaller sails and higher prop speeds, the cart rolled backward again because air spilled around the edges of the sail. A combination of a large sail and an elevated, high-power fan did result in the cart moving forward; Grant theorized that this was caused by the sail reflecting a portion of the prop’s thrust backward. A larger-scale test with a miniature jet engine mounted on a skateboard gave the same effect.

Finally, the team set up a full-scale test on a lake, using a swamp boat with a 40-horsepower fan. In its normal configuration, the boat reached a speed of 20 miles per hour (32 kph). After the fan was reversed and a sail was hoisted, the boat was able to travel forward at 3 mph with some careful steering, confirming the myth premise.

The movie sound effect of a punch sounds like its real-world counterpart.

busted

Adam and Jamie first hung up a pig carcass and took turns punching it, but they had to subdue their punches in order to keep from injuring their hands. The results did not resemble the movie sound effects, so Adam attached a ballistic-gelatin fist to a baseball bat and swung it at the carcass full force, with similar results. A professional sound designer explained that the movie punch sound was heavily manipulated and built up from various component sounds to increase drama.

The movie sound effect of a rattlesnake’s rattle sounds like its real-world counterpart.

confirmed

A snake expert brought in a rattlesnake and coaxed it to shake its tail, resulting in a sound very close to its movie counterpart.

The movie sound effect of a gun fitted with a silencer (a.k.a. suppressor) sounds like its real-world counterpart.

plausible

Adam and Jamie visited a shooting range and fired .45 caliber and 9mm pistols, both with and without suppressors. With the help of sound expert, they found that the suppressor reduced the sound level considerably, from 161 to 126 decibels for the .45. The movie sound effect was not a perfect match, but did have enough similarity to result in a “plausible” verdict.

The movie sound effect of an explosion sounds like its real-world counterpart.

busted

Jamie blew up a car rigged with primer cord and 2 gallons of gasoline. He, Adam, and the sound expert observed that the movie explosion had a longer duration and covered a wider range of frequencies than the real sound. A second attempt, using 2.2 pounds of C-4, gave a more substantial blast but still did not match the movie sound effect.