Fins are appendage drag , which is always significant and can’t really be eliminated (unless you go finless and loose most of your directional stability). Therefore , I think drag management is far more desirable than attempts at drag elimination or reduction…so , we concur…I think .
Well stated. Some drag is good, more is not necessarily better. (my quote) Your term ‘‘drag management’’ is a perfect description. Wisdom and understanding, rolled into one.
I’d say that’s a big “False” on this one - the board DOES in fact compress like a spring, unless you have a really old board with a rotten stringer or a weak glass job - this is why a freshly glassed board has more “life” and “pop” than an old board. It’s not the foam that “springs” it’s the stringer coupled with the glass… and this isn’t actually “surfboard theory” it’s “aerodynamic law” and explains why a fighter jets flexible wings can actually “slingshot” it our of a turn and generate speed . When you put load on the wing while adding a bit of power, then release the power and load at the same time, you get an increase in speed due to the wings “snapping back” to their original shape. Same thing happens in surfboards and on the dorsal/ventral fins on fish
…right tool for the right job.
herb
https://swaylocks7stage.s3.us-east-2.amazonaws.com/s3fs-public/dog%20pushes.png
I will explain soon. Watch this space.
I see the term(s) kinetic energy used frequently when referring to waves, as a general catch-all phrase**…**
I would like to see a more detailed scientific discussion about how the fin(s) takes advantage of and augments the kinetic energy of a wave.
I read the information at the link lawless provided. My Gestalt is that the Lift:Drag ratio is more about performance at a given velocity rather than velocity itself.
KE equals 1/2 mass x velocity squared. As the fluid passes along, hits, or is displaced by another body the fluid is slowed, this reduction in velocity equates to a reduction in KE of the fluid. The energy lost will be transferred to the other body and heat noise etc.
Always liked this classic series of videos and the accompanying book by MIT Prof Ascher Shapiro. Shape and Flow: The Fluid Dynamics of Drag 1961
http://youtu.be/ftq8jTQ8ANE?list=PLDBC439409A651870
That, if you grasp it (the readers of this thread), will guide you in understanding one of the important ways your fin interacts with the water molecules in an advancing wave.
I own 2 copies of Shapiro's book (paperback & hardbound). It is very readable for a wide audience.
I have not watched the videos. Thank you for the link.
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[quote="$1"]
KE equals 1/2 mass x velocity squared. As the fluid passes along, hits, or is displaced by another body the fluid is slowed, this reduction in velocity equates to a reduction in KE of the fluid. The energy lost will be transferred to the other body and heat noise etc.
[/quote]
The "waveform" has "horizontal velocity" as it moves toward the shore. However, the waveform has no mass.
Other than the falling water in the breaking section of a wave, the (fluid) water movement on the "unbroken face" of the wave is only upward -- vertical velocity. Water movement is downward on the rear side of the wave.
Assuming it is the ideal objective, the surfer and board are travelling on the unbroken face of the wave. Water movement on the unbroken wave face is upward.
The principal forces acting on the surfer/board are gravitational and centripetal. The mass(es) affecting the forces experienced is (are) the combined surfer/surfboard mass.
The principal accelerations experienced by the surfer/surfboard are gravitational and centripetal, resulting from speed and directional changes, respectively.
The surfer/board velocity is in two planes, vertical and horizontal. The surfer/surfboard velocity is a function of both.
The principal force affecting surfer/board "speed" is gravitational and is related to the slope of the wave's face/front.
The maximum velocity of an object is determined by the forces, mass times acceleration, acting on it.
Drag/friction is the resistance to motion -- "the force acting opposite to the relative motion of an object."
Except a surfboard travels in three planes, vertical, cross shore and shorewards; equally it has three axis of rotation - add curved flow from the waveface and the fact that all these variables are constantly changing even if slightly means to my mind that being able to define the direction of lift and drag is nigh on impossible.
I don’t think that focussing strongly on lift and drag as conventionally defined is particularly helpful.
I consider traveling forward and sideways (shoreward and cross-shore) to be moving in the same horizontal plane. Yes, the horizontal motion vectors are perpendicular to one another, in the same horizontal plane. Centripetal force definitely includes vertical and horizontal motion vectors.
Fin lift would be perpendicular to the fin’s forward motion. Fin design can affect centripetal acceleration.
I do not believe the combination of fin lift and drag increase speed. I do not believe drag, by defintion, increases speed.
Drag/friction is the resistance to motion – “the force acting opposite to the relative motion of an object.”
I used to think of horizontal as one direction but pretty much anything can travel shorewards, it is the sideways force generated by fin and hull that allow a surfboard to travel crossshore - its also the bit that makes surfing special, and also what all surfing styles have in common. Besides 3d xyz is technically correct.
Drag by definition will only ever decrease speed. But lift and drag are just components of the normal force, itself the sum of the pressure differences across all surfaces. Personally I think understanding the pressure distribution over hull and fins is key, it is definately more intuitive, and it is far more likely to be what the surfer can feel through their feet and balance, while still being technically correct and obeying the laws of physics.
Basically, we are in agreement Meagain. There are most certainly 3 planes. I overlooked the plane defined by the vertical motion and “sideways” motion.
IMO I believe the sensations caused by centripetal force (G-Force) are what a surfer feels most in a turn or cutback.
Your analysis would lead me to the opposite conclusion haha
If you fly a glider there are also a lot of changing variables such as wind speed and direction, thermals, variation in air pressure and so on. The multiple variables makes the aerodynamics more critical not less.
Yes!
While there can be improvements, I don’t look for anything to be game changing.