Episode 165: Blow Your Own Sail

Air Date: May 11, 2011

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.

25 Comments

  1. DrunkenToast says:

    I did this in sience class in middle school to win a paper car race. I mounted a balloon backwards aimed at a sail. I easily won.

  2. juan says:

    Dear Mythbusters, i think in the part where you test the weapons with surrpressors, can you possibly use different type of weapons like a ruger mk 3. 22 pistol with a gemtech outback 2 surpressor. I GUARANTEE that you will get the movies effect. Thanks.

    • IronMaiden says:

      You are a hittman? How can you GUARANTEE? :p

    • IronMaiden says:

      I just realize that I replied a year after his post. Probably he is already dead or in prison.

      • HeloniLynn says:

        lol – you said what I was thinking in your first post

      • Stephen says:

        He just never replied… maybe someone “silenced” him?

        In reality about silencers… it sounds more like someone spitting, but loudly.

        However, I don’t agree with the Mythbusters result of 161 to 126 decibels… it is much lower than that. It also does depend on sub-sonic to super-sonic bullets, though size of calibre is also a factor.

        So why do we have sound-suppressors for guns? Just for movies? C’mon…

        I’ve seen a 9mm Uzi with a silencer shoot… as I said, sounds like someone spitting… but 10m/30ft away, it is hard to hear unless it is completely silent.

  3. Paul says:

    Hi I would like to know were you got that minijet that was shown on the episode.

    Thanks

  4. Mike Hockin says:

    Sorry but a revolver with a muffler / silencer does not work on a Revolver as the gases escape through the chamber!
    Regards
    Mike Hockin
    Reading Berkshire UK

    • Stephen says:

      Its more funnier when someone approaches you to manufacture a silencer for his revolver!
      My dad, who made guns years ago, had this situation plenty of times:
      Customer: “Please try to make a silencer for a revolver”
      DAD: “Sorry, not possible!”

  5. kelso says:

    any idea what lake they used for that piece?

  6. Simon says:

    Need to know where to get mini jet engine. Thanks

  7. Trevor says:

    Hey guys, I hate to seem like your usual critics, I love this show and that you guys don’t just test myths you take them to another level. But like Juan said above, there are different types of suppressors and I’d like to add, there are many variables in this test. The firearm, the caliber, the suppressor. My suggestion if you guys wanted test it again, the first detachable suppressor I know of was used by the United States Navy Seals in Vietnam, its nickname was the “hush puppy”. Damn near no sound except for the click of the hammer, used to “take out” guard/watch dogs without alerting the nearby soldiers. Please I urge you to have a fun day at the range to test many different varieties and put this one to rest.

    Thank you,
    Trevor

  8. Abbie says:

    I was absolutely disgusted, outraged and highly offended having just watched the episode of Mythbusters where Adam and Jamie hung up a pig carcass and turned it into a punching bag. To have that little respect for a dead animal is totally obscene. I realise that some people will pull this post apart saying I am an over-sensitive animal rights activist etc… I’m not. I’m not vegan, I eat meat and I don’t belong to PETA. I am just someone who has a healthy respect for animals and couldn’t just watch that episode and say nothing.

    • T. Rollins says:

      You know what they do to your burgers before they’re processed? I’m sure a big spike through the head of a live animal is much better than punching some dead carcass…. besides, would you rather them use a dead person?

    • HeloniLynn says:

      You’ve never seen CSI? Pig carcasses are the closest in relation to humans; and they’re already dead. Hello – ever take a science class? They use pigs and frogs. Also, human cadavers that have been donated to science – I suppose you would have preferred to see that broadcasted on TV for everyone to view. I think maybe you went a little hormonal on this one – find a real cause to get excited about. For example, have you ever seen the footage of that rat – a live rat, mind you – with the cloned human ear growing out of his back? True story. Now that’s something to be outraged about. They do all kinds of messed up things to rats. Then again, there are about a hundred billion more important things to care about that are still futile to be “disgusted” and “offended” by. I’m concerned about the starving children in the world – where do I post an angry comment about that? More importantly – what good is it going to do? Now you should go watch this movie Food, Inc and think about that the next time you’re eating your hamburgers, fried chicken and porkchops – the regular process of preparing animals to be eaten is what you might call “obscene”. Not to mention the way they’re treated while they’re still alive. Have a nice life of overreaction!

    • Stephen says:

      Why are you complaining about a dead pig?
      It is dead!
      Dead!
      DEAD!!!
      And you complain about it been a punching bag?

  9. Mark Sheridan says:

    “A sailboat stranded in calm water can move forward by using an on-board fan to blow air into its own sail.” In this synopsis and during the show you are applying the physics wrongly. You cannot jump straight from the fan to the sail when applying the forces. The forces act between the fan and the air and then the air and the sail. The air is not “on” the boat, so it decouples the fan and the sail. The sail dos not know where the moving air comes from so it reacts as if the air from the fan is the wind. The reason why the the boat did not move forward initially (apart from the leakage round the sail problem) is because of efficiency, not because of Newton’s law. A fan is far more efficient than a sail. I you had used a solid sail you would have had better results, but still not efficient. You loose a lot of energy reversing the air flow. Your conclusion should have been confirmed but silly. The boat would move a lot faster if you forgot about the sail and reversed the fan. And you were definitely NOT defying Newton’s Laws. Even Einstein did not do that!

    • Phil says:

      Actually you’re wrong. The physics they used is correct. You state
      “The air is not “on” the boat, so it decouples the fan and the sail. The sail dos not know where the moving air comes from so it reacts as if the air from the fan is the wind.”

      This is incorrect. When the fan pushes the air forward, the air pushes the fan backwards. The fan is attached to the boat. Therefore the boat experiences a backwards force due to the force the fan experiences from the air. The sail is attached to the boat, and therefore experiences the same backward force and acceleration.

      The air that collides with the sail pushes the sail forward. The effect of the two forces will be roughly equal assuming that all of the air the fan pushes collides with the sail.

      The reason the boat moved forward in the end is just as Grant said. Some of the air colliding with the sail reflected backwards off of the sail, so the sail pushed back on the air and the air pushed forward on the sail.

      The decoupling statement you made is negated by the fact that the fan, sail, and boat are all rigidly attached. Efficiency is poor in any case, but it is not the primary answer.

      You’re correct in saying they didn’t defy Newton’s Laws. However, Einstein clearly proved that Newton was incomplete in his description of gravity, as Newton predicted no gravitational effect on light. Some would argue that an instance where Newton’s Law of Gravity predicts incorrect results would be “defying” his law. Einstein correctly predicted that light would defy Newton’s Law of Gravity.

      • Phil says:

        To be clear, once you account for the backwards force the boat experiences due to the air on the fan cancelling with the forward force the sail experiences from the air, it becomes a conservation of momentum problem.

        Mboat(0 m/s) + Mair(Vair) = Mboat(Vboat) + Mair(Vair)

        This equation would have two variables, so another relative velocity equation is needed to solve.
        (Vbf – Vairf) = -(Vbf -Vairf)

  10. Dave says:

    with regards to “blow your own sail” what kind of efficency would you get with a “hamster cage type fan” ie furnace type fan that draws air in from the sides??

  11. Manuel Ribeiro says:

    I’ve always thought mythbusters to be a show both written and hosted by people that loved engineering…fine, quality engineering.
    So why on earth, out of millions of badly engineered and poorly built cars on the US alone, did you guys chose a miata to be blown-up?! That episode gave me a heartache…Unbelievable. First time I switched channel while watching Mythbusters.

  12. JNH says:

    How the heck is the silencer myth plausible? The movie silencers generally have a pew pew whisper sound; which no other movie characters can hear. In reality, 126 decibels is louder than a jackhammer. Even using the best design and the weakest ammo available (.22), 95 decibels is about the lowest you can get, and that is still about as loud power tools/table saws.

  13. Eric Bailey says:

    I really do like this show but every now and then they surprise me and not in a good way… Re. the blow your own sail myth – I couldn’t believe it when they turned as one and said “confirmed”. They always take things to an extreme to get the result they are looking for… but in this case they ignored the science/physics. People who agree with their conclusion do not understand how a sail, or a sailboat actually works!

    • Tim says:

      Please read Phil’s explanation above and refute it.

      • Eric Bailey says:

        Nope, both Mark and Phil have some good points that I agree with, although I agree with Mark a little more. The really important point in my statement involves how a boat sails. Of course you can bounce air from a fan off of a sail and the boat will move – but it is NOT sailing. Part of the problem with this myth is the actual meaning different people put onto the word(s)in the myth. Semantic hell for sure… I sail, so I want the boat to be actually sailing for the myth to be confirmed. In fact, online is a clip of Grant explaining that all through the process of taping the episode a producer who sails kept saying “That is not sailing”. I will put it another way – and excuse me for restating the obvious – assuming you have a large enough fan and a power source for said fan you could point it “backwards” so just the fan is pushing the boat forward and with enough power the boat will move forward. Note that this is NOT sailing. Turning the fan 180 degrees to bounce air off of the sail and move the boat forward is NOT sailing either. Just a lot less efficient than pointing the fan the opposite way you want to go… I mean the fan they used was off of an air boat! So BUSTED and gone to silly lengths to get it to work should have been the conclusion…

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