I don’t understand the physics of why a foil moves forward while in a wave. I think I understand lift. Pressure differentials on the top and bottom of the aerofoil create a force that lifts upward. However, to achieve lift, there needs to be motion of the liquid over the aerofoil. I am focusing on surf foiling as downwind/wind foiling has wind to provide forward momentum in addition to a wave. My three thoughts are that it is a combination of (1) water molecules hitting the foil/mast and transferring (kinetic?) energy to the foil/mast and pushing it, (2) gravity pulling the foil down the sloped face of the wave, and (3) that the lift vector is pointing slightly forward instead of directly up. I don’t think it can be fully explained by (1) as the foil needs to be moving faster than the water around it to create lift. Explanation (2) doesn’t make sense to me because it seems like a foil will move forward even when the wave is pretty flat. Explanation (3) doesn’t make sense to me because forward motion relative to the seafloor seems to occur during maneuvers like cutbacks. I am probably misunderstanding some physics here, but I would love to know if anyone has an explanation that can be understood by someone with only a rudimentary knowledge of physics.
When a wave moves through the water, the water goes up then the water goes down. If you are on the front of the wave the water is going up and raising the foil in the water column. Since height can be traded for speed (and vice versa) you turn this up welling into velocity by maintaining height
The foil moves forward because it’s going down hill.
Waves don’t actually move water sideways (until they start to break), they just move it up and down. The “speed” of a wave is the speed that the crest is moving, the water isn’t moving at that speed".
I know it is a really simplistic way to look at it, but I’m always considering that surfing works because we’re “falling downhill”. The front of the wave is sloped and gravity pulls us down the slope. The peak and trough is also shifting forward with some speed (although the water itself isn’t moving forward at that speed), and we need to keep up with that peak without running all the way down into the trough.
Imagine a ball sitting on a tarp. If I pull one edge of the tarp upwards, the ball will squirt away from that edge. The tarp isn’t pushing the ball forwards per se, it is lifting it up and gravity pulls the ball back down and therefore forward.
Thanks! I appreciate the reply.
Thanks! The reason that I considered forward push was that when I surf a high volume standard surfboard in the whitewater, I definitely feel a push. However, from this explanation, it seems that the forward push really only occurs when the wave breaks. When a wave is in open ocean, the wave doesn’t have the forward push.
Correct, when a wave breaks it’s because of the drag induced from the seabed on the bottom of the wave, it’s basically tripping and falling forwards. That generates a substantial push forwards.
You are a watermelon seed. Gravity is the index finger, the wave is the thumb.
Or, a snowboarder goes fwd AND down, while mountain stays still. In surfing the downward motion of snowboard is traded for upward motion of wave.
ok, sort of weird thought here. In open water, could you think of waves almost like pumping the foil for you? So in a wavy environment, the up and down movement of the waves is sort of the inverse of a foiler pumping their foil in flat water?
Brilliant.
I appreciate all the replies. The last point I am struggling with is whether the force of water molecules hitting the foil significantly affects its upward or forward motion when it is in flight. To be clear, I am not talking about upward motion through lift. Rather, I am still thinking of the water pushing me. My understanding of wave mechanics is that the water molecules are moving in deep water, they are moving approximately in circles. The molecules pass the (kinetic?) energy onto the next molecule by hitting the particles around them. Therefore, the wave energy is transferred through the water, but each water molecule remains about in the same place in the ocean. As the wave reaches shallower water, the shape of the circles becomes more oval-like, and the wave eventually breaks. When I am sitting on a floaty surfboard, I feel the non-breaking wave lift me up, but I don’t feel the forward push of a breaking wave. This makes sense with my understanding of the wave. However, if I am on a low-volume (less buoyant) surfboard, I am not lifted as much. Rather, the wave feels to just pass through me. The difference to me seems to be related to buoyancy and/or surface area. On the floaty board, I am less dense that the water, therefore, if the water moves, I will be pushed up by the water. Additionally, with the big board, there is a lot for the water to push against. This seems related to my struggle to understand how/if a wave lifts a foil while in flight. There are water molecules moving roughly upward in a little circle. However, the foil has essentially zero buoyancy and almost no surface area. In other words, I understand how falling down a wave generates forward motion. I additionally think I understand how a wave lifts up a floaty object like a boat or a big surfboard. However, I don’t understand how/if the water in a wave lifts up a foil in flight. Gravity obviously can’t make the foil go up directly. So is it other forces (like water molecules hitting the foil) that make the foil go up? Am I just using the energy of pumping and the initial height I achieved while floating on the surface of the water to move up the wave? Do I need to be thinking about the flow of the water over the wing and lift?
IMHO your foil is unaffected by the circular movement of the water in the wave until you are in flight. To take off you need forward movement - water moving over the surface of the foil to create lift. Without a wing or kite, paddle etc etc your source of propulsion is the board interfacing with the water surface - e.g gravity as the board slides down the wave usually with help from paddle power. Once in flight the foil can take advantage of the circular movement of the water in the wave similar to how a glider can ride a ridge lift in flight - at least that is my understanding. In addition you also have gravity as you can dive the foil down the face of the wave creating more speed.
…smarter people may know more but I believe that is the gist.
What is going on in flying mode vs archimedean mode are two different things.
Floating. You sitting on your board are a mass-spring-damper system, where the stiffness of the spring is proptional to your waterplane area. A big floaty board will have a much larger waterplane than just the cross sectional area of your torso where it meets the water surface when yoy are sitting on a sinker. Mass remains much the same in both cases, damping is usually a fair bit higher for the sinker case, and the spring is much weaker, so the response for the same input (wave passing by) is less. Look up TPI (tons per inch) for ship design if you want a more detailed explanation of the spring stiffness waterplane thing.
Flying. Lift is proportional to speed and angle of attack. Both speed and angle of attack are the result of the vector sum of your velocity plus the wave particle velocities. So, when you are in the part of the wave where the particle velocities are vertically upwards, the vector sum means speed is unchanged but angle of attack increases and vice versa for the part where the particle velocities are verticall downwards. At the part where the particle velocities are mocing horizontally against your direction of motion, speed increases but angle of attack is unchanged so lift increases, and vice versa for the opposite phase part of the wave. There are many many great visualizations out there showing 2D vector summation, i suggest having a look. With a good visualization you can see that the influence of the wave particle velocities is reduced as you either go faster or oeperate the foil at a higher angle of attack.
Interesting spring analogy, i like that. I would think it’s not mainly the waterplane area though (so less dynamic effects), but mostly the magnitude of the (static) vertical archimedean displacement force just being higher for a floaty board than a sinker, thus reducing your inertia perceived by the wave/ the force of the wave needed to move you to a higher state of potential energy when lifting you upwards?
So, take the waterplane area, and then sink it by a fixed distance, say 1cm (or inch), and you have a change in volume that is only dependent on waterplane area - more waterplane, more volume, more force for that same 1cm immersion. Your mass is unchanged. Inertia is largely unchaged but I wont get into inertia further because then we have to talk about the full mass matrix and that includes added mass and if we go there heads will almost certainly explode.
Thanks! The flying discussion makes sense. The discussion of floating will take some time for me to understand.
I think of it this way …
Following a couple of the comments above:
@Slappy - “The foil moves forward because it’s going down hill.”
@FoilFondler - “a snowboarder goes fwd AND down, while mountain stays still. In surfing the downward motion of snowboard is traded for upward motion of wave.”
Consider flat water, but gravity is turned off and now the water is angled like the side of a mountain, up at 45 degrees. Consider for now that the water is magically held in place like that.
Now you’re on your foil board, and you can foil “down” that hill of water, pulled down by gravity as a snowboarder down the side of a mountain of snow.
Now instead of you foiling down the hill of water, send the water constantly all moving upwards underneath you, all still at that 45 degree angle. So you stay at the same height, and are pushed forward, foiling downhill as the water rises below you.
Now instead of this fictional 45 degree infinite flat mountainside, consider it is a real WAVE that is causing the water below you to be moving upwards, against gravity. Maybe right where your foil is, the angle is 45*, maybe some other angle, but the water is going up at that point. The wave is also moving you forward as it travels and you need to keep in balance with the speed vs. wave steepness to stay on the wave. Since the wave is curved you can pick the right angle you need to match the speed.
In all that, nothing special about 45*, works for surfboard, bodysurfer, boogie board, kayak, SUP, etc. as well as a foil.
I think you’ve just described a “standing wave”!
Here is a good graphic of what the water molecules are doing on a wave.
The objective is to ride the thermal updraft like a bird would on a cliff face with oncoming wind.
Pressure + shape of the foil = displacement . Look at a glider in the sky for exemple, it’s doesnt stay up hanging withouth moving forward.