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Q & A: sailboats and fans

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Most recent answer: 10/05/2013
Q:
If a sailboat, in calm air, had a fan mounted on the back, blowing air at the sail, would it/could it propel the sailboat forward? If so, why/why not? I could see where the reflection of the air back off the sail might cause motion, but I can’t see how the forward motion of the fan air itself could do the job.
- Mark (age 50)
S. Salem NY
A:
Yup, you can make the sailboat move in this fashion, but there are some ways to set it up that will work better than others.

Pointing a fan mounted on the back pointing it straight at a sail whose surface is perpendicular to the air coming from the fan. This is very ineffective, as the fan pulls the boat backwards by shoving the air forwards, and the sail pulls the boat forwards with almost the same force by stopping the air again. The boat may not stay exactly stationary, but it sure won't make much progress.

The best way to do this is to remove the sail entirely, and point the fan so it blows air backwards. Then the boat feels the full reaction force pushing forwards. This is how those swamp boats work.

Tom

(published on 10/22/2007)

Follow-Up #1: put boat fan in water?

Q:
WELL HELLS BELLS! In that case, I say we throw the fan overboard (well, attach it astern on the boat) and have the blades of the fan thrust the boat forward using water as a propellant instead of air! I aint no engineer, never mind an english major, but isn't driving a propeller through water more efficient than having it push air? (Extra credit, due Monday).
- Brad Howell (age 52)
Kensington
A:

Those are electric fans. It's not such a good idea to put them in water. At any rate, their speed, blade size, etc. are designed for air, not water. Underwater propellers also work well, but they aren't the same design, And they don't have open electrical connections to short out.

Mike W.


(published on 10/05/2013)

Follow-up on this answer.