This is a serious question involving fluid dynamics.
When I paddle my boat, each oar creates a ring-vortex which moves away through the water, and in response the boat moves the other way. The moving ring-vortex carries significant mass, so the oars act like a “reaction engine,” while the ejected mass of moving acts as the “exhaust.”
But wait a minute. In a fluid environment the density is constant and no fluid builds up anywhere. Therefore whenever the oars emit a ring vortex, for every parcel of fluid which moves away, there must exist another parcel of fluid moving back again. As the ring-vortex pattern moves away from the boat, there is no net flow. So… how do rowboats work? But in addition, how are fish able to drive themselves forwards, and how can ship propellors create thrust. All of these involve the creation of fluid flows which are constrained to be circular. The same is true of flapping bird wings, jet engines, helicopters, etc. I KNOW that bird wings fling out vortex patterns and generate thrust in the opposite direction. And I know that the travelling vortex from a Wham-O™ air blaster gun can knock over distant cardboard targets. But with the net flow being zero, how can these deposit any momentum into the fluid environment?
(The above is actually a key question in the longrunning physics debate over how airplane wings really work.)
I’m not sure what the fundamental question is. Your oar pushes against the lake. This moves you forward, and since the lake as a whole can’t move, it pushes against the bank, moving the earth (very very very slightly) backwards. Momentum is conserved, and (except for temporary disturbances in the lake), the only relative movement of mass is you versus the earth (including the lake). Think about a solid, non-moving lake; you’d have the same story except for the temporary disturbances.
What’s contradictory about that?
I have no training in fluid dynamics but I think that if you do, you’re overanalyzing this. You are trying to analyze it like jet propulsion, but the propulsion here does not work by moving water in one direction and the boat in the other. As Quercus says, you’re pushing against the water, just as when you walk, you push your feet against the ground. There is no reaction mass. The ring vortex is probably pretty much like a wave rather a movement of water mass.
I would think that the vortices moving backwar represent a loss in the process. What you would like to have happen would be to stick your oar in the water ahead of your present position, have it stay stationary while you pull the boat up to it and then repeat the action.
I would think that the vortices moving backward represent a loss in the process. What you would like to have happen would be to stick your oar in the water ahead of your present position, have it stay stationary while you pull the boat up to it and then repeat the action.
I would think that the vortices moving backward represent a loss in the process. What you would like to have happen would be to stick your oar in the water ahead of your present position, have it stay stationary while you pull the boat up to it and then repeat the action.
CookingWithGas and Quercus have the correct idea. It isn’t the ring vortex that’s pushing the boat, that’s just a by-product of the rowing motion. The key is how energy is transferred. In rowing, you anchor your blade in the water. In racing shells, you would then push off the stretchers with your legs and drive the oar blade through the water with a lot of force. All of that force is then trying to move that blade through the water very rapidly. Except it can’t, since the water has a lot of resistance in it. So since things like to move in the path of least resistance the boat moves forward.
Think of how it would be if you put the boat on wheels on land. You would anchor the oar against the ground and shove. Since the friction on the oar would prevent it from moving very far, the boat would be pushed forward. The same thing happens in the water essentially.
Water is relatively incompressible compared to, say, air, but it isn’t incompressible. So it’s a mistake to say that density is constant.
Secondly, even if water was incompressible, friction would still exist. For example, the floor that I walk on doesn’t flow anywhere, yet I can still achieve a reaction from the floor.
Thirdly, this part:
is nonsense.
Even supposing that those things were constrained to be circular, exactly the same physics describes things like rocket, pulse jet and ram jet engines which clearly aren’t constrained to be circular.
Replace the water with a large flat block of clay (anchored to the earth); your oars dig into the clay and you are able to exert force against it, moving you forward (conveniently ignoring the friction of the boat itself against the clay). There is no vortex created in the clay, but the clay and the Earth move in the opposite direction by an imperceptibly small amount.
Huh? Your oar pushes against the water in contact with it. The lake as a whole DOESN’T move, instead only a very limited volume of local water is actually affected by the oar. If the lake was a solid block of putty then things would be very different. In that case the lake as a whole WOULD move, and there would be no launching of underwater ring-vortices.
OK, if the oar is too confusing, then let’s look at a Wham-O air blaster gun instead. It launches a ring-vortex across the room, and if that vortex hits a target, the target is knocked backwards. Also, the Wham-o gun seems to experience a fair “kick” as if it’s shooting out a bullet.
My trouble comes about when we inspect this “bullet.” For every parcel of air moving forwards, there is another parcel nearby which moves backwards. If the vortex carries a hunk of momentum along as the vortex-pattern moves, exactly where is this momentum stored?
It’s all in the vortex. Fill your Wham-O Blaster with smoke and fire it. You’ll see a smoke ring that leaves very little smoke behind. You can also do the same thing with a small paper or cardboard box with about a 1/2" diameter hole in it. The vortex is self-contained. It’s no different than any other object moving through the air. There’s no net movement of the air outside of the vortex, which pushes the air out of its way as it moves forward, and the air pressure pushes it back in place after it’s gone past, the same as happens when a thrown baseball passes through the air.
Certainly, but as they say, is this a difference that makes a difference? In other words, if density changes in the water are critical in explaining this stuff, then even a small density change is important. But if density changes play an insigificant role, then they are merely a distraction, and we can simplify things by pretending that the density is constant. In the fluid dynamics texts I’ve seen, they assume that density changes are irrelevant except in trans-sonic flight (big density changes in a shock wave.)
OK. That’s seeing the water as a viscous liquid, and assuming that propellors could not operate under “frictionless water” no matter how much water they sucked in their front and blew out their back. Does an airplane propellor drive the aircraft forward because it’s blasting air backwards? You’re essentially saying no. And your viscosity-oriented explanation doesn’t explain why ring-vortices knock things over as if they were a moving hunk of mass.
Wrong. If we neglect the transient density changes (or if we operate the devices under a near-incompressible fluid) then pulse jets and ram jets are constrained to have circular flows. “Circular flow” just means “fluid doesn’t build up and up and up continuously anywhere.” Now rockets are different. Rockets I understand. They fling gas parcels out, and those parcels came from the rocket, so no circular flow is required. My problem is with helicopter blades and ship propellors which don’t eject any NET parcels of mass, instead the obvious stream of fluid mass that they eject was drawn from the environment.
Here’s an equivalent question: if we’re under water, and if we launch a jet of water against a solid plate, will the plate be forced backwards? Will the jet behave as if it’s carrying momentum? (This is essentially the reverse problem compared to oars and propellors, since the plate is forced backwards in colliding with the jet, while a propellor is apparently forced forwards as it generates an underwater jet.)
The whole lake does not need to move. The oars push on local water, the local water pushes on the non-local. The transfer of force through the local water to the non-local creates eddies. Arent the eddies an inefficiency in transfer of force?
Running in the sand, the sand at the interface between foot and beach is moving with the foot, and a force is transferred through this moving sand.
Even if you blew against the cardboard, the air pressure difference between the moving air and the stationary is not significant. It is the mass in the moving air that is applying the force, just like the mass of the air in the moving vortex.
If you want to design the best oar you would have to go into the flow of water over the blade, vortices and all that stuff. If you just want to find out what makes the boat go forward I think all that is needed is to find the drag force on an immersed body. The book states that for a thin, flat plate the drag is virtually all form drag.
My book Elementary Fluid Mechanics gives the formula for form drag force on a flat plate as:
F = CdAd*v[sup]2[/sup]/2; Cd is the drag coefficient, A is the area of the plate, d is the density of the water and v is the velocity of the flow past the plate.
I assumed the boat at rest to begin with with the oar moving through the water at 6 ft/sec (4 mph). The blade was 1.5 ft x .5 ft so A = .75 ft[sup]2[/sup], d = 62.4/32.17 slug/ft[sup]3[/sup]. The book gives Cd for a flat plate with our aspect ratio as 1.2 for Reynolds Numbers > 10[sup]3[/sup]. Our Reynolds Number is about 6 * 10[sup]6[/sup].
Using these factors the initial force per oar is about 30 lb. That is the force required to move the oar blade through the water at that speed. If the oar is pushing on the water with that force, then the boat is accelerated from rest with that force on each oar.
For a boat and cargo weight of 200 lb. the initial acceleration would be 60/200 = .3 g.
The above is getting closer to the trouble I’m having.
Running in sand is still too much like walking across a solid: the grains lock together and allow your feet to interact with other grains over an immense distance, and long distance shear forces are easily supported because the locked grains act like a solid. Liquids are very different (where we can’t have transverse sound waves or anything associated with the kind of long-distance propagation which communicates shear forces to all parts of a solid.)
Better analogy: try walking across a layer of marbles on a polished floor with flat-bottomed shoes. When you try to walk, you only go forwards if you launch numerous marbles backwards. Walking on the marbles has the physics of a reaction engine. As near as I can determine from my reading, the same is true of propeller-driven ships, planes, and helicopters, and “paddle” driven birds and rowboats. They only go forwards because they’re throwing mass backwards. (But are they?)
Bingo! “The mass of the air in the moving vortex.” Yes, if we can treat the moving vortex as a forward-moving hunk of mass, then everything works fine. But can we do this? In other words: does the fluid in a moving vortex carry net momentum along with it as it moves? I think the answer is no. After all, for every parcel of fluid in the vortex which moves forward, there is another parcel nearby which is moving backward, so the net momentum carried by a vortex is zero. The middle of the vortex moves forward, but the outer region moves backward.
Think like this: if I throw a big blob of water through the air, then obviously I’m driven backwards as I throw it, and when it hits you, it knocks you down. Momentum is obviously transferred. But now if we both go underwater, then when I try to throw a big blob of water at you, instead I create a ring-vortex. And isn’t the net momentum carried by this vortex pattern required to be zero?
Yes, we can look at the water in one part of the vortex and see that it’s moving forward, so it has momentum in the direction of travel. But then if we look at water in another part of the vortex, it is moving backwards. Add it all up and the momentum associated with the travelling ring-vortex is zero.
Here’s a very crude GIF animation I made:
Red fluid moving through fluid environment
And another to show the internal flow:
These two animations obviously are simplified. But maybe they are seriously flawed as well, since as far as I can tell, the net momentum carried by these travelling vortex patterns is zero. Let the moving vortex in these animations penetrate a closed surface, and as it does so, for every parcel of mass that flows into the surface, another parcel simultaneously flows out.
But experimentally we find that vortices seem to carry momentum (they can knock over objects, and a vortex-launcher experiences a “kick.”)
Where’s my error? Maybe I’m supposed to ignore the back-flow in the travelling vortex, and only concentrate on the forward flow. But I need a good reason for this, otherwise it’s an arbitrary kludge-job.
