I feel that in surfing we plane on the inside surface of the fin , the outside foil adds to that .
Template shape is very important in creating and controlling this lift along with its placement .
Hows that sound ?
Gregory of the interweb ;-)
Could you please post up a few pics of your work, with a little description of the boards?
Hi George, I've really enjoyed the technical aspects of this discussion and maybe I can add something on the cavitation part.
The definition of cavitation is confusing, even in engineering circles, since it is often modelled in many different ways. I've done a lot of study using cavitation as a cutting force for micro-surgical tools. This was generally a 3mm piston moving at 40 kHz at .0005 inch stroke in water. Cavitation was a result of the water collapsing in the void left when the piston retracted. This looks like bubbles under magnification hence the term "cavitation bubbles" . What really happens is that extremely high temperature spikes are created during the collapse of the void (since it's essentially a vacuum) which will vaporize anything in that was in there (3K + degrees). I found that the lower the frequency of stroke, the more intense the temperatures were as the void collapsed more slowly. The higher the frequency, the forward stroke of the piston would interfere with the collapse and the temps were lower. Cavitation on spinning objects is harder to understand due to centripedal motion and such and I hope the linear example helps those interested in this subject.
"Most planes can fly upside down. Most can do this for long periods if pressed. It may not be efficient but the plane flies...
"How does a fin REALLY work on a surfboard?"
Most pilots are still taught this concept and don't truly understand the implications. It's all about the Angle of Attack. Aerobatic planes which are designed to fly upside down have symmetrical airfoils, so that in inverted flight the Angle of Attack determines the lift vector, not the airfoil. Like you said, it's much more ineffecient with assymetrical airfoils, but they can do it, although there is a ton of drag involved.
An intersting concept in aviation is using an Angle of Attack indicator instead of the airspeed indicator (which can vary with altitude and temperature) and purely using the AoA Indicator to determine best flight paths for takeoff and landing - http://www.navyair.com/Angle%20of%20Attack%20Indicator.htm
In surfboard verbage we refere to Angle of Attack as Toe-in on multi-finned boards. On singles you change the AoA by turning the board.
There is such a complex interaction between the board bottom, fins and rails that it's evolved mostly by empirical trial and error.
That's my .02. Obviously, so many different things "work" and surfing is SO subjective it's really hard to say what works "best". It's more what works "best" for the feeling your after. Some guys like to fly stunt planes, some guys like to fly gliders. They both work, but in very different ways with very different techniques.
Actually toe in on a board should be referred to as "angle of incidence". The angle is fixed at manufacture on aircraft as it is on a surfboard. A few aircraft have been made with variable incidence wings, used to control body angle vs AoA at very low airspeeds.
AoA is a variable determined by the angle of the foil relative to the air or water flow.
You mentioned earlier that carbon cloth has greater compressive strength than FG.
If you only had enough carbon cloth to do one side, which side would you laminate with it. Top or Bottom? Why?
Thanks for all of the informative responses in this thread.
Swaylocks Surfboard Design Forum: thoughts & theories ... practical & theoretical
RAIL PROFILE http://bgboard.blogspot.com/2014/03/march-82014-afterr-seeing-recent.html
@greggriffin & @lawless-
Ding Ding Ding!!! Winner!!! (and there's more too)
The majority of "lift" from our surfboard fins come from the "other" side of the fin. Before I explain, this has been verified by two other shaper/board builders, and now there are two theoreticians who are concluding the same. Warning: Fin Foil (as mentioned,) does help, a little...
...the fin acts as a "fence" (yes that's a technical term for it,) to the lateral flow. A fin, (or Vane) can be placed in that slipping flow to "grab" or divert flow momentarily. I am thinking about the outer (side) fins on a cluster of fins, but this is true with singles, on up.
And let me qualify this further, this is just for the "hold" aspect of the fin, NOT streamlining it.
The concept is called "Dynamic Lift" which can be exemplified by sticking your hand out the car window whilst traveling at speed. The AoA (Angle of Attack,) is the thing to consider. With your palm down, going 60mph (95kph,) you will find a "neutral spot" where your hand neither dips down or folds back. If you bring your fingers up (increase the AoA,) your palm gets hit by the rushing air and makes your arm flop back. This is Dynamic Lift.
We tested a set of fins with NO foil, picture the fins cut out to a shape but square edge all the way around. They worked, a little draggy but surprsing that they held so well. We then round the edges off, since we suspected flow separation at the sharp corners, and the fins felt just about like "regular" fins. I "think" the fins may be a little draggy compared to regular, but the trade off was they had great release on off the lips.
This exercise led to the understanding that fin foil was only a small contributor to performance, as Greg pointed out. On multi-fin boards the "cluster" of fins/fences act to contain pressure under the board, and the bunched water acts as a hydraulic dynamic spring, that a surfer can push off of, or leap from into another move. So, in designing a board, have in mind that you are "fencing in" fast water flow, with the fins, then letting it go somewhat elastically. "Could this be why so many shapers have double concave running through the fin area?" Maybe. The way we do boards is, we look at what's going on, build one, then try it. The ones that feel best, survive and propagate. Then something else is tried. Feels right. Continues. NO theories, just trying stuff. If the rider/shaper/designer is honest enough in their assessments, the designs will develop. This is where we are now.
It is only in the last 3-4 years that I've designed boards more around this concept and have met noticibly good results. YET, I STILL USE regular fins... ...ARRRGGHH. Hypocrite! I can't sell boards that look like that, I can explain as much as I want: no sale. And I have to sell boards, because that is my livelyhood: pressure.
Lastly, I wanted to mention that the regular thruster fin set up you see, is NOT "camber matched." That means the center fin should be TWICE as thick as the flat sided front fins. That bugged me for a long time, so I tried it, with only marginal results. But then after I realized that Dynamic Lift is the main player, it all made sense.
The squared edges of this Dynamic Lift fin should probably be built with the leading edge shockwave in mind, AND with maintaining "flow attachment" at high AoA's. So sharp is not a good idear. If you are into trying this, try 1/8" Radius all the way around the outline of a fin that's 1/4" thick. Both sides? You tell me, you want symmetric or asymmetric attachment properties?
Furthermore, most all surfboard fin templates are not optimum, but that's a whole other "exercise in futility" that I won't go into right now. ha
Oh, an afterthought, I meant to follow up on this.
"There is such a complex interaction between the board bottom, fins and rails that it's evolved mostly by empirical trial and error."
Yes! On point. There is an augmenting effect with this "bunching up" of energized water. The effect seems to be caused by the base of the fin interacting with the board. This is truly counter intuitive to everything I was taught, like "baseless" fins and "elliptical football" outlines. The augmenting effect feels like massive amounts of drive as you pump or rollercoaster a board for speed. The longer the base, the more this effect, which is probably why I've gravitated toward the fish keel fin to promote this one aspect. This drive effect is countered by less freedom in yaw however. I just wanted to mention this when considering "fins as flow fences." My fish fins have 8" plus long bases, up to 9". This is sorta why channels/gutters work, but they don't work as well given the wasted area presented.
I have a lot of pics on my and my company's Facebook pages. I kinda don't want to put too much on here, as it can be seen as me "selling my wares," which I am not into in General Discussions.
That reminds me I'll be in contact with you for some more of your custom valves, thank you so much for providing this product! Oh, and I have a planer that was in a fire, which I have questions about... ...anyways,
"I've done a lot of study using cavitation as a cutting force for micro-surgical tools. This was generally a 3mm piston moving at 40 kHz at .0005 inch stroke in water. Cavitation was a result of the water collapsing in the void left when the piston retracted."
Oh man, that is such cool stuff! And yes that tracks well with classic Cavitation scenario. And a great use as an industrial tool, amazing!
I always refer to "two ways to boil a fluid" one is to heat it, the other to lower the surrounding pressure. I tell guys, you can boil water at ROOM TEMP, just lower the room pressure. Same with the human body, or any fluid. With pistons I totally see how low pressure or vacuum can be created. It is a little harder to see with racing boat propellers for example, but it has to do with Dynamic Lift (as mentioned,) but focus on the "other side" of the prop blade, that has just dug out a groove in the water. One side of the Prop Blade grabs water and provides thousands of pounds of thrust to push the boat (drag boat,) and once that void is "dug out" of the water, the back side of the Prop Blade is in an incredible VACUUM, and the water (say 60 degrees F,) flashes to an instant boil. The bubbles are microscopic, and as quickly as they form, they collapse. The bubbles collapse "asymmetrically," directing the collapsing energy toward the surface of the prop. The little "pinging" collapsing energy tears molecules of metal off the Prop Blade. The metal doesn't go peacefully so some heat is given off (kinetic energy transfer.)
I love your description of how to "throttle" a cavitation for milling purposes, WOW. Thanks!
14 to 16" tail blocks sound about right, but I'm sure there is no "rule" to it. I had Joe Bauguess shaping out of my shop for two seasons, he shaped the first modern Mini Simmons, and he had set templates. He said he basically used what Bob Simmons did and sectioned the board shorter. That might be a good reference, either looking at Joe's templates or at Simmon's boards. Good luck with it, and realize Joe has the rights to the name "Mini Simmons" if you plan to make any money off the use of the name
Hmmm, I said that carbon graphite is an Engineer's favorite, but as a surfboard builder, I don't like complete encasement of a board in graphite, WAY TOO RIGID. That said, to answer your question, I'd go against what I'm seeing coming out of China and Thailand and use carbon on the deck. Assuming you surf without bindings, you can only apply weight to the deck, causing the board to deflect downward. The fibers in the bottom of the board would pull tight (tension,) and the fibers in the deck would cram together (compression.) Carbon graphite would counter this compressive load. Again, it might not be so desireable. Plus there are other load scenarios whic might take precedence like the board snapping in a gnarly wave.
With racing SUPs and some sailboards and kiteboards I would use graphite encasement.
Hope this helps.
precedence like the board snapping in a gnarly wave.
precedence like the board snapping in a gnarly wave.
So for this scenario, what takes precedence? Carbon on top?
i played a pretty good prank on one of my buddies during a surf trip by switching his side fins (so the flat side faced the rail)
he is a really good surfer and was still ripping - i was bummed that my prank did not work. but then he mentioned that he was having a hard time that session then he saw the fins were backwards and thought that they had been backwards for the past few days and thought he had stumbled onto something great since he was loving his board for the majority of the trip (except for the last session which he blamed on the waves). Turned out to be a good prank - the whole crew had a good laugh at that one!