Steep drop into an on rail bottom turn, the stiff fin(s) sustain and redirect the captured peak G force of the drop.
On the same drop, a more flexible fin(s) creates addtional thrust force above the peak G drop force by the mechanical loading/unloading of the now compressed fin(s).
Single fins have the fastest straight line planing speed.
Multi-fins, usually attached to less then ideal planing shapes, create additonal force by capturing cross sectional flow and converting into additional thrust.
Griffin with his flat bottoms, thin rails, less rocker, Rigid fins.
Arakawa with his concave bottoms, boxier rails, and increased rocker, likes tuned flex.
So many variables, so many ‘truths’, impossible to capture with pure physics alone.
Which is why 200+ mph street legal supercars, after extensive and extremely costly testing cycles, come out with nearly identical performance, yet all look different.
Because different solutions can reach the same outcome.
If only that myth were true…(sigh)…what a wonderful world it would be. You are not feeling ‘‘additional thrust force’’, but instead, you are feeling the recovery of most (but not all) of the energy you put into initiating the turn and flexing/compressing the fin. Some of that energy was lost as heat. Less efficient, and slower reaction to rider input, is the result.
to be clear on fin flex. Not gooey, washed out plastic crap…tuned glass with appropriate ‘twang’ on load/release.
Slater, the quinessential surfboard design feedback loop rider, during a discussion on fins stated he has so much time invested in flex tuning fin sets to his precise requirements, he carries them in his personal bag while travelling - willing to lose the boards, but not the fins. John Carper also talked at length about fine tuning fin flex, minutely sanding the fins between waves, until finding the exact ‘fling’ he’s looking for.
Comes down to the personal arcs the surfer scribes, and the board under their feet. Some do it better with stiffer, others with a bit flexier.
…I still maintain that there are 2 worlds; so Kayu, what do you say about board in a straight down the line ride is applicable only to one of those worlds (type of Surfing and type of board). I mean, you can pass sections with a small HP potato chip (hot doggy board) constantly riding in the pocket and searching for the curl changing the AOAs.
Physics define the limits. No algorithms required.
Cetripetal force allows you to turn your board and is the result of centripetal acceleration. The limit is determined by mass, velocity and turn radius. Centripetal/centrifugal acceleration is what creates “G Force.”
Unless you can alter your mass with your mind, try as you might, you can not defy gravitational and centripetal/centrifugal force.
Fins affect tracking (resistance to sideways motion, side-slipping). The better the tracking, the tighter the turn and the greater the centripetal acceleration and G-Force for any given velocity.
Force = Mass x Acceleration
&
Centrifugal/Centripetal Acceleration is
where a = acceleration, v = velocity and r = radius
Earth’s gravitational acceleration is 32 ft/second per second. Times 3 = 96. So if my calculations are correct, a turn/cutback with a fixed radius of 10 feet at 21 mph will produce centrifugal/centripetal acceleration of 96 feet/second per second. Apparent rider and/or board weight will be 3 times greater than stationary (gravitational) weight. Centripetal Force will be 3 times greater than gravity – 3G.
Changing turn radius and/or velocity changes centripetal acceleration and “G-Force.” Changing mass also changes G-Force but has no effect on acceleration.
When humans can alter a given mass instantaneously, with or without their minds, I will rescind the statements in my previous post.
If we could create a state of masslessness for any object, the universe as we know it would change. The current laws of physics would be significantly altered. Light speeds and faster than light speeds would be possible.
I will stand in humble awe of any human (especially a Sways member) who accomplishes this…
I already filled in some values that could make sense.
With the Reynolds number, go to literature to find good low-Reynolds foil profiles.
Unless if you hold a phd in aerodynamics or hydrodynamics, this is the best way to choose a foil profile.
Please note that I don’t say anything about the effects of flex, surface area and thickness profile (~flex) on surfing. Since these parameters should be determined by trial and error, but don’t invent your own foil profile!
The 13-15% mentioned above is indeed a good figure for low-Reynolds profiles.
MH30 and MH32 foils look real sweet to my amateur eyes. For side fins, of course.
Thicker center fins can handle more aggresive AOA’s without sliding or spinning out. me thinks you’d have to be going real stinking fast in a straight line for the thickness to cause any noticeable bad drag.
OF couse, we’re totally off topic at this point, but I don’t get the impression Bill T cares too much.
Guys, really interesting topic here & I’m the firest to admit I know nothing like what you guys do, but my question is does the surfers weight & level of skill need to be factored into the speed issue? I’m always amased, watching the pros, how they generate so much speed to get around sections that us 'normals" wouldn’t have chance of making. Does skill take away or add to the fin/boards performance.
where a = acceleration, v = velocity and r = radius
