## Mythbusters: Self Propelled Sailboat

Last night, the Mythbusters tested the myth of propelling a sailboat with an on-board fan. At first, the myth seemed busted when the scale model sat still thanks to Newton’s Third Law. But, as is usual Mythbuster style, the team decided to ramp up the test to show that it didn’t matter how big the fan was, the boat would remain still – but it didn’t. Using a model plane engine followed by a model plane ducted fan jet, the Mythbusters were able to get their scaled contraptions moving (albeit inefficiently). This test was finally confirmed using a swamp boat with the fan turned backwards. On a windless day, blowing the 60+mph wind into their sail resulted in forward motion of about 3mph.

Naturally, this was something to talk about at the Nerd Facility I work at with all our whiteboards and geeks. Discussions abounded about why the myth worked from recycled force vectors and sail design, air compression and expansion collisions to air escaping the sail imparting an additional force (like a rocket nozzle). But in the end, we kept negating all of our explanations by falling back on Newton’s Third Law with a twist of Conservation of Momentum (of the thrusting gas). So what’s the math behind how it worked?

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##### My guess - Bernoulli's Principle by gnifyus

The large fan on the swamp boat may have been able to create enough of a pressure difference in the air on both sides of the sail to set up an inefficient demonstration of Bernoulli’s Principle
The lift effect goes in the right direction anyway.

##### RE: My guess - Bernoulli's Principle by VnutZ

That had actually crossed my mind too. But I dismissed it because the airflow is across the wrong surface. I would think for Bernoulli’s Principle to work, the airflow would have to be perpendicular to the sail (while it’s shaped concavely) in order to create the low pressure zone across the front, curved surface. With the fan blowing directly into the sail, it would be like the wind blowing upwards holding a wing up as opposed to a pressure differential.

##### RE: My guess - Bernoulli's Principle by gnifyus

Does it make a difference how the pressure differential is set up? In an airplane wing it’s set up by the length difference of air traveled due to the shape of the wing. The airflow direction only matters here because it’s necessary to provide the forces used to create that difference on a shaped wing.
What if it were possible to create a pressure difference in some other way on both sides of the wing? Would there still be lift? Your example of blowing a fan under the wing might actually be somewhat valid. (Just not valid enough to fly a plane.)
On our sailboat the difference on both sides of the sail might be set up by air being slightly more compressed on one side by a powerful fan.

Occam’s precession force idea might be proved by rotating a giant flywheel instead of a fan on the back of the boat (light enough not to sink it, of course.) I would think the boat would need a keel for this to work.

##### RE: My guess - Bernoulli's Principle by Occams

I didn’t see that Mythbusters program but I presume that the sail was like in the cartoons: set square to make the boat run down wind. But a sailboat can also sail upwind (within about 45 deg) using the same effect as an aeroplane wing – together with a keel to give lateral resistance.

So if the fan were made to blow over the topside of a large headsail, like a 150% genoa, this would generate a lot of lift or forces for forward motion. THis is Bernoulli again. It also requires a keel: which a swamp boat does not have.

I think the conservation of energy law would probably kick in and this lift would be less than the drag caused by the fan. Without the sail, the boat would go backwards – fast. If the fan blows to the rear it will go fastest – normal swamp boat mode.

The precession idea is probably a crock since it needs a force at right angles to the axis of the fan. It might work in a side wind or current.

##### RE: My guess - Bernoulli's Principle by Occams

A better idea for fan and sails working together would be to mount the fan on the tip of the bows, perhaps even out on the jib boom, and blowing aft. This would make the boat go forward anyway, but it should go faster if the boat carries fore and aft sails close hauled to give lift as in beating to windward. Being a sailboat this vessel has a keel.

##### RE: My guess - Bernoulli's Principle by Anonymous

I didn’t see that Mythbusters program …

You can see the relevant video pieces of the program on their website using the second link in the article.

##### RE: My guess - Bernoulli's Principle by Occams

It does not need much wind or current to make a boat move like that. There might even be a precession force from all the rotating mass in that fan.