Hydrofoils, like the kind you’ve referred to above are fascinating. The forces that govern their application are very similar to those governing “surfing”, but the hydrofoil opperates completely submerged. The term planing is usually reserved for partially submerged objects, or objects that operate on, or at least involve the interface between two fluids - here water and air. I’m not suggesting that what Lairid and others are doing (in this case) is not “surfing” (I surely don’t own the term), but what I am suggesting is that its not surfing as I have defined surfing (see original post above.) Whatever the case, I don’t wish to diminish what these fellows are doing, nor what Surffoils and others are doing either. As far as I am concerned, it’s all fun stuff.
All well and good - but my point isn’t that its fun, or whether or not its surfing by anyone’s definition, my point is that they are tapping into the energy of the wave itself. By doing this, they are making it clear that it is the power of the wave that fuels surfing, more than any other force. While their form of surfing is different than a standard surfboard, it is nowhere near as different as skiing, snowboarding, or skateboarding. It is so much closer to what we do, because it is driven by the force of the wave, as are we, when we surf.
Power = Work/Time
Work = Force x Distance
Time = Distance/Velocity
In a breaking wave, the amount of water "falling’ in a big wave is much greater than that in a smaller wave. The mass of falling water in a large breaking wave is much greater than that of smaller wave, and generates more force than the smaller wave. More force more work, more work more power.
This is correct. And what gives the surfer potential energy is the moving water in the wave.
Weight = mass x gravity
All surfers have a static mass and operate under static gravity, and have a static weight. Which is why is it is silly/moot/meaningless to say it is “gravity” which propells the surfer.
Such an absurd statement should give you pause.
Water definately has mass. We have already proven that the water moves in the wave.
You don’t even have to surf to test this… just paddle out and sit on your surfboard, and notice as the swells roll through and underneith you they LIFT you up.
@ Huck
Nor will I. I had hoped a few simple examples and simple equations would help you see it.
While I am no Copernicus or Galileo, it is easy to see why they were censured by the Church/Religion because they stated that the earth revolved around the sun.
I highly recommend watching the TV series/documentary by James Burke, "The Day the Universe Changed.’
Yes. Once again we wrestle with semantics. Prior to this I have used the word waveform. In the interest of typing quickly I shortened to the generalized term often used in casual discussions of physics. Replace the word wave with the word waveform in my previous post. Yes upward moving water has mass… other than bobbing up and down the surfer has no perception of its Force/Power.
FTR when talking about surfer, skiers or tobogganers, I know of nobody who would say gravity was causing them to “fall” down a wave/mountain while riding.
-
drive, push, or** cause to move in a particular direction, typically forward**.
<div>"the boat is propelled by using a very long paddle"</div> <div>synonyms:<a href="https://www.google.com/search?rlz=1C9BKJA_enUS607US607&espv=1&hl=en-US&q=define+move&sa=X&ei=qiVEVNjzMo6ryATc0IKgBQ&ved=0CCAQ_SowAA">move</a>, <a href="https://www.google.com/search?rlz=1C9BKJA_enUS607US607&espv=1&hl=en-US&q=define+power&sa=X&ei=qiVEVNjzMo6ryATc0IKgBQ&ved=0CCEQ_SowAA">power</a>, <a href="https://www.google.com/search?rlz=1C9BKJA_enUS607US607&espv=1&hl=en-US&q=define+push&sa=X&ei=qiVEVNjzMo6ryATc0IKgBQ&ved=0CCIQ_SowAA">push</a>, <a href="https://www.google.com/search?rlz=1C9BKJA_enUS607US607&espv=1&hl=en-US&q=define+drive&sa=X&ei=qiVEVNjzMo6ryATc0IKgBQ&ved=0CCMQ_SowAA">drive</a> More <div> <div> <div> </div> </div> </div> <div> <div> <div> </div> <div> </div> </div> </div> </div> </div> </div> </div> </li>
Yes, you are being confused by semantics.
The “waveform” is only an idea/concept to help explain how energy is transferred. What is really happening, in reality, is the water is lifting the surfer and the surfer is using lift and drag (with pitch, roll and yaw of the surfboard) to harness (and release) the energy of the moving water.
You are stuck on this gravity thing. Gravity just gives us weight. It is static, it is constant. While essential, it doesn’t add anything dynamic to the system. It is the moving water and surfer who add the dynamic elements to the system.
Planing force is the resultant of lift and drag and is the reaction to m.g and propulsion. You have it pointing in the wrong direction.
Without gravity (aka gravitational “force”), an object (with mass) without a “force” acting on it (source of propulsion) will not move down an inclined plane/surface on this planet – absent irrelevant semantics.
Newton’s First Law of Motion:
An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced **force.**
Absent gravity (aka gravitational force), a surfer would not drop down and travel along the face of a wave…
The forces in the diagrams are correct, given the explanations. To not believe they are is fine, but at no point have I written or indicated in any diagram that the “Planing force is the resultant of lift and drag and is the reaction to m.g and propulsion.” (See my reply/post above regarding lift and drag, and the original post.)
Gravity is utilized as a counter force to that component of the planing force which is parallel to the gravitational force. The magnitude of the component of the force of planing which is parallel to gravititational force is controlled by the surfer by through the presentation of the bottom or wetted surfaces -i.e. the way he chooses to orient those surfaces.
Taking this point a bit further, the surfer is able to control (to a large degree, but ultimately limited by wave conditions) the forces of planing via the presentation of the bottom wetted-surfaces; how much of the bottom is to participate, which section of the bottom is to be involved, etc. This is why the overall plane-shape or template matters, as do bottom contours, etc… Changes in bottom presentation are usually done via changes in position on the surfboard or via postural changes (changes in the center of gravity of the surfer.) How effectively (and efficiently) he is able to change bottom presentation is of course also a function of the tools he has at hand -i.e. the design of the surfboard itself, and the surfer’s skill in using those tools.
I don’t think anyone has suggested that surfers are weightless.
Isn’t the planing force (hydrodynamic lift) defined as perpendicular to the flow?
Which means it isn’t always parallel to gravitational force?
A surfer makes a surfboard go , without a surfer a surfboard will just float on the surface , if a swell comes the surfboard will just float over the swell , if white water comes the surfboard will be pushed to the shore , but that is not surfing , ( it may be for some people ) . We know the surfboard will NOT go without the surfer . If a surfer stands on a surfboard on a waveless , windless stretch of flat water the surfboard will not go because we do not surf on waveless windless stretches of flat water . But when a wave comes along how does the SURFER make the surfboard go ? What does the SURFER do to make the sufboard carry him or her on the dance all over the wave . When we use the word go we mean to surf/ride a surfboard under the feet of the surfer , that is the subject , right . As I stated befor if you are a surfer you know what makes a surfboard go and how to make a surfboard go , dont over complicate things .
Your answer is in the section which you’ve included in your post, the ‘component’ of the force of planing. (A given forces can be decomposed or resolved into any number of arbitrary components. I believe you did as much in one of your ‘block on a plane’ diagrams.)
There is only one case in which the force of planing is perpendicular to the flow, and that is when total flow is perpendicular to, in this case the plane of the plank. The pressure that results is often referred to as the ‘stagnation’ pressure. In fact, Savistsky included the ‘stagnation’ line in his diagram.
Glad to see NOAH’s drawings (D.S.) making an appearance on this forum…
1st off I haven’t studied the hydrodynamics of surfing much, As i tend to just throw a kite up and ensure an excess of force is present at all times…
However am more than decent with vector diagrams, energy conservation and have studied savitsky in detail to the point of working out the calculations in an effort to make things go fast in the water.
Kcasey thanks for this post it made me think…and to whomever posted the wave motion gif…this F’n blew my mind…knowing this I will adjust my “wave riding” method in the future in an effort to actually try “surfing”…
Kcasey your rotated savitsky diagram has a major flaw you are neglecting the VISCOUS Drag in the savitsky diagram and as a result confusing the crap out of meagain. As meagain pointed out, the vector diagram you presented is obviously missing something otherwise it would accelerate forward towards the beach unrestrained by any opposing force.
The diagram presented only shows PRESSURE drag which acts normal to the board and assumes water is a frictionless surface. I recognize the photo from “Hydrodynamic Design of Planing hulls” Savitsky was only addressing pressure distribution and put in a simplified diagram that did not include the viscous drag until later in his paper
Savitsky
“The total hydrodynamic drag of a planing surface is composed of pressure drag developed by pressure acting normal to the inclined bottom and viscous drag acting tangential to the bottom in both the pressure area and the spray area. …”
None of the vectors from planing resolve to result in a force that would align to create a resultant increase in board speed (v)
You guys are WAY overcomplicating things trying to resolve the planing forces and actually screwing it up in the process. (no offense intended it’s complicated shit)
An energy analysis simplifies this much more easily so people can understand it intuitively
Here’s what propels a surfboard.
You have potential energy as you get on the wave because it has height, you use this height to drop in pick up speed converting potential energy into kinetic energy in motion, you then do a sharp bottom turn, (you all know this is critical to your succes). Let’s pretend you went STRAIGHT down the wave face and did a true 90 degree turn in the flats . you are now going horizontal relative to the incoming wave. The wave will eventually catch up to you and pick you up…Note if your angle was truly 90 degrees this would NOT change your speed, The wave picks you up and GIVES you this energy. …free of charge the wave will pick up damn near ANYTHING…Once the wave catches you and picks you up…you have more potential energy, and you’ve stored some of your previous energy in the form of velocity. You can now drop back down and create more speed. Provided you didn’t lose more energy to drag than you gained from the drop in you will be able to go faster. At this point aerials and other cool shit is possible. Cause you have more energy than is needed to climb that wave and then some.
A surfboard is an energy storage device. Rip down the wave, get as much speed as possible, change direction but keep as much kinetic energy as possible while the wave catches up, do it all over again.
to address some other posts, Anything that can reduce net drag for the ENTIRE sequence is a good thing. If you’re fat fins create more drag in a straight line than others but don’t stall out on the bottom turn and so you exit the same turn faster or cut back faster and so the wave catches up to you faster than, you have a better design… but it all comes down to how you ride it.
Thanks for the expanding on Savistsky. Aside from leaving out details such as viscosity consisterations, which in my opinion would not have added to the main point “What Makes Surfboards Go”, I stand by what I’ve written. (And I fully expect the same from everyone else posting on this site, or at least I assume as much.) I happen to believe your treatment is niave and fails to account for transverse motion, or trimming that is necessary to stay in a barrel, or just trimming in general. (If you’re inclined, please also see “The Decelerating Wave-Form” and “Rails Plane”, or not, the choice is totally up to you.)
Good topic for thought, as I said earlier. Nothing more fundamental to surfboard design, than an understanding of what actually makes the surfboard go, you would think.
A lot of good points in this thread, I don’t think any one answer given is a hundred percent correct, or a hundred percent wrong. All the factors mentioned are real factors. The power and push of the wave, the movement of the water up the face and then down and shoreward in the lip, the lateral movement of the wave’s curl (the place where most all surfing takes place), the downward pull of gravity, the kinetics of the surfer manipulating the board, the effect of the fin as a sail, the rail as a plane, drag, pressure, momentum, all forces that come into play, and probably many others not mentioned.
Because there are so many factors at work in a typical ridden wave, surfboard design becomes a mixture of nuances and compromises - what creates lift one moment creates drag the next, what increases speed one moment decreases control the next, etc.
Its all part of the mystique of surfing and surfboard design!
The question is; What makes a surfboard go. Paddling makes a surfboard go. In the case of a no paddle take off drop. Well gravity and then inertia and maybe add some momentum into that equation. Still think this is a dumb question. But it sure has got everyone going, funny.
No, not gravity. The fin is performing as both a wing/sail, generating directional ‘‘lift’’, and as a keel providing lateral resistance to the water ‘‘flow’’ up the face of the wave face. Helpful?