To blow, perchance to sail

General Questions
#1

Help me, dear hive mind, with a question that has plagued me for too long, given its (apparent) simplicity and obvious answer.

If I blow into a sail, will it move my boat forward, even if imperceptibly? Will it produce a net-plus forward movement into the desired direction? Or is there a type of “recoil” that is caused by the air exiting my mouth that will compensate for that net forward movement, pushing me and thus the boat back again? Feel free to apply this to a “pure” environment that assumes no pesky friction or other disturbing factors, as well as to a more realistic environment.

#2

Thread from about 4 years ago discussing substantially this. Everyone there seemed to have a variety of opinions, but upon quickly skimming a few posts, I think there was a consensus that it might work – but rather inefficiently. You’d have done better to just stand facing the rear of the boat and blowing aft. It only took 7 responses for someone to wonder how it would work on a treadmill.

#3

Additional clarification: I’m familiar with the Mythbusters episode that examined this question, but I’m specifically wondering if the outcome changes somehow if lungs (or a lung-like contraption - pressurized air directionally expelled) are used.

#4

Conventional wisdom says that the recoil is equal and opposite to the propulsion and the net is zero. But a closer look shows that this is not the case and you actually do provide a tiny amount of forward propulsion.

Imagine a tank of compressed air, sitting in the middle of the boat, connected to a J-shaped pipe whose long section points toward the front of the boat, with the small end of the J-bend pointing towards the back. Release some air from the tank. What happens? The boat is propelled forward, slightly. Now cut some holes in the sides of the J-shaped pipe so that just some of the air is directed backwards while most of the air escapes to the side. You still get some forward propulsion, but it’s less. Now replace the J-bed with a sail and you still get some forward propulsion. Now replace the tank of compressed air with your lungs. The setup is the same.

#5

Interesting - thanks for the visualization. Which brings me to the question - what is it that interferes with the concept of “recoil=propulsion”? What outside forces would lead to the recoil being a lesser forces than the propulsion?

#6

Think of it this way: The propulsion doesn’t come from the breath stopped by the sail: That balances out. The propulsion comes from the breath that is bounced back by the sail. Which is less than all of the breath, and that’s the reason why you’d be better off just facing aft and blowing.

#7

It brings to mind the cartoon image of the character in a square-rigged sailboat turning on a fan installed in the boat and roaring forward at a high rate of speed!

The movement that results in actual physics is of course the balance of the thrust of the fan and the pressure on the sail. In most cases the thrust of the fan will probably dominate and the boat would actually move backwards, because of the energy lost around the edges of the sail.

But it could be arranged otherwise. What if the sail was closely curved around the exhaust of the fan so that it redirected most of its airflow backwards, back towards the front of the fan?

I’ve just described the reverse-thruster on a jet airliner. It works – albeit inefficiently – because the direction of the bulk of the unimpeded airflow is the primary determinant of thrust. It works in high-bypass engines, too, because it’s the total airflow that’s reversed – jet exhaust and bypass airflow both.

#8

Is this on the assumption that the breath is sufficiently directed so that none of it spills past the edges of the sail? If there’s overspill, I would imagine that would net out to zero (or possibly small reverse) thrust pretty quickly.

#9

Yes, it does depend on there being low or no overspill.

#10

In case anyone doubts this would work, the Mythbusters performed this a few years back, using one of those Everglades air boat fans and an improvised sail to make the swamp boat move in the same direction the fan was blowing.

When that episode came on I immediately thought of the thrust reversers used on commercial jets, which use engine normal jet engine thrust to stop the plane.

#11

Which is good advice for seasickness, as well!

#12

Excellent, thank you - this makes it lots more clear. And at that level of efficiency, I think I’ll save my breath to instead convince my boat mates to paddle me to shore.

Follow-up 1: if the air exiting my lungs or a gas bottle exits and produces recoil with a certain amount of energy, it is probably safe to say that the energy it transmits to the sail must be less as kinetic energy is lost as the particles travel towards the sail. Shouldn’t the recoil force be greater than the forward propulsion? In other words, wouldn’t the forward movement produce by the particles bouncing off the sail also have to compensate for the fact that recoil energy > propulsion energy?

Follow-up 2: Do neck muscles so reflexively compensate the recoil from very heavy out-breaths that we don’t notice the resulting recoil force acting on our heads?

#13

The reason we don’t notice the recoil from our breath is not that we reflexively compensate. It’s that the recoil is tiny. Sneak up behind someone and blow on them as hard as you can: Can you blow them over?

#14

One thing that’s always bugged me about that mythbusters episode is that sailboats very often sail ACROSS the wind (rather than inline with it). Also, the most advanced sailboats tend to have stiff vertical wings rather traditional sails.