Fin enthusiast, new research on fish locomotion, drag, thrust, and vorticies. Rob Olliges Excerpted from a excerpt article in Science 28 Nov. 2003: Flags flutter, even in a completely steady breeze. In an analogous way, fish undulate their bodies in order to swim against a steady current. A flag is a passive aerofoil, whereas a fish is an active swimmer. Yet, evidence is accumulating that it is not large oscillating forces that undulate flags and fish, but rather it is their natural oscillations that get amplified to generate visible motion. …………… These fish exploit the vortices generated by water flow around stationary objects, modifying their own swimming behavior most likely to reduce the energy cost of locomotion. ………… A fluttering flag extracts energy from the flow of air, and sheds an undulating vortex ribbon (2) (panel A). A solid obstruction also sheds a characteristic drag wake called a von Kármán vortex street (3) (panel B). Fish, on the other hand, swim actively. Their muscle activity adds energy to the water, and the flow pattern shed behind the fish is called a thrust wake or reverse von Kármán vortex street (4) (panel C). Both von Kármán wakes consist of two rows of counter-rotating vortices with a jet meandering between them. The jet behind an obstruction points forward, against the background flow, whereas the jet behind an actively swimming fish points backward. Yet from the standpoint of the jet, thrust and drag wakes look very similar. Fish and insects exploit this apparent similarity of thrust and drag wakes when they let their body, fins, or wings interact with their own wake or other wakes to enhance performance (4, 5). ………. Trout performing the Kármán gait behind a dowel. The fish weaves through the dowel’s drag wake to keep clockwise vortices to the right and anticlockwise vortices to the left of its tail as if it were generating a thrust wake. ………… The advent of time-resolved flow visualization has enabled the detailed interaction between wakes and active or passive hydrofoils to be explored experimentally. Several years ago, Zhang and colleagues (2) showed that the similarity between fish swimming and flag flapping is not superficial. Flags flutter not because the wind itself flutters, but because the flag’s inertia and elasticity exert forces on the air while the air’s dynamic pressure and drag act on the flag (2). In addition to inertial and elastic forces, a swimming fish exerts muscle forces on the water. Both flag (2) and fish (6) exhibit natural oscillations when they “wave.” Yet, when one places a flag in the wake of a flagpole or a fish in the wake of a stationary object, something new happens. ………… Liao et al. (1) demonstrate that trout behave like a passive hydrofoil when swimming in the wake of other objects. …………… The trout weaves through the wake to let each oncoming vortex pass it on the same side as a thrust wake vortex would. The fish moves considerably from side to side, undulating its body with large amplitudes, a behavior termed the “von Kármán gait” by the investigators (1, 7). ………… In contrast to free-swimming fish, fish adopting the von Kármán gait only activate the most anterior group of their swimming muscles. This suggests that their body wave is generated passively by interaction with the oncoming flow of water. …………. Even individual fish with their multiple fins recycle the wake of their own upstream propellers for increased power and control. An improved understanding of these natural mechanisms should help engineers to design better foil propulsors. The next step will be to seek more examples of phase-locked swimming and flight movements in nature to ascertain how widely vortex recycling is exploited during swimming and flight. 
I mean to ask, how do we use this info? Comment 1 - fin hum is explained by vortex shedding, the pitch being dependent on fin stiffness and speed. Hum results in large energy loss. Comment 2 - a symmetrically foiled fin will loose little energy, as it minimizes vortex generation. Comment 3 - assymetrically foiled fins create lift (normal to the direction of flow) but since they nearly always operate at an angle (toe-in) to the direction of travel, they shed relatively large vortices and cost lots of energy. Conclusion 1 - a symmetrically foiled single fin will be faster since it loses less energy through vortex creation (other things being equal). HOWEVER, life is full of compromises, even for fish. I invite some thought on the implications of vortex generation and energy loss.
Rob, this is great stuff. I am only a beginning shaper but this is the kind of discussion that gets me excited. I think the next leap in surfboard design will be through creating more energy with our boards in the water rather than focusing on minimizing losses of energy(drag). Your excerpt provides some good insight into this. Your article lends credence to Rusty’s disappearing stringer design and the Outer Island flex tails. Passive (stiff) board up front with flex in the rear exploiting the vortexes created. This also adds weight to the MVG talk that has been thriving on Swaylocks. Plus, it makes me wonder if toe-in on fins is over done. Maybe a straight on fin set up with more flexible fins, would be a better way to go. Any thoughts?
fish pump their fins and surfers pump the boards, up and down, side to side. The little vortexes must add up if you tap into enough of them, maybe thats why Dale’s mats are fast. Maybe this is the reason to put on 7 fins. How big are the surfboard and fin vortexes?
l tested an idea that l had fifteen years ago, l used the board that l had been riding for six months because l new exactly how it went and had surfed it in a large variety of conditions. Now you guys are going to think that l’m pretty wacked out (as if you dont already)but what l did was get a peice of fin panel and cut a shape out of it like a beaver’s tail leaving a 3" projecting portion at the front so that l could attatch it to my board. l foiled the bottom of it with a slight vee and the top was foiled down to the bottom edge, it was 6" long and 4" wide at the apex, l drilled into the tail of my board and attatched the beaver (thats what l affectionatly called it) with roving and cloth. My train of thought was to see if l could get extra drive from the beavers flex and because the water could pass through where it met at the tail the board would still turn alright. Ahh l love the smell of experimentation in the morning. FIRST SURF - apart from looking like a greenpeace member trying to rescue a sea mammal l was amazed at the results,the surf was perfect for the test 2 to 3 feet onshore mush, my main aim was to grovel better through using the extra spring in the tail, the first wave that l got was a complete mush burger, l could fart with more power, but soon as l angled and pushed down on the tail to it took off and when l released and pumped again l could feel the beav kick in,l shot accross the crumbling wall as if l was on a sucky shorey a section approached for a bottom turn backhand reo combo and it bottom turned with drive and when l hit the lip l swear l pivoted on the beav to come back in, l proceeded to have a much better surf than what l would have had in those conditions. The next day the swell came up and it was 6 foot Burliegh, once l got past the wise cracks l was out in the water and the first wave l got was a bomb, it was a fast peeler so l had to be quick, l angled accross and pumped, the beav kicked in and each time l pumped it got faster, l easily made the fast section and faded to bottom turn, l layed it over and pushed with all my might to see how much drive the beav could handle, it was projecting with a forward force that l was amazed with and then it happened, l felt like l had done a gear box l lost control and crashed, the beav was no longer with me (snapped out due to not having the strength)the back of my board was never the same and l had to lay it to rest (but not before cutting it in half and taking out the stringer, she went to good)l have’nt done anything else with the beav since but l learnt from that experience that yes people will laugh at you for being out of the norm but who cares. KR http://groups.msn.com/MyKRSurf/krcomweb.msnw
I Love beavers.
I think KR is onto something, as a large-ish flex blade is gonna be real responsive to pumping in waves where the rider’s power input is a meaningful fraction of the energy available - "2 to 3 feet onshore mush…l could fart with more power’. Think ‘swim fin’ or ‘sculling’ and it makes a lot of sense. It’s kinda interesting, as fluid mechanics/dynamics are understood not much more than empirically… doc…
Water works against regular boards differently than with one of Dales mats. Something else is going on. DAMN THEYRE FAST!!!
the reason for 85% of the surfers use tri fin configuration is ´cause the stars shapers, the magazines, the championships, you know, all the media are not risky…
What would be a good beginner mat?I have to try it.They look so fast,and fun. Probably make some of our big choppy days a blast.
Well, thrusters were an evolution of the single and twin fin boards of way back when. I suppose in the beginning they were pretty “risky”, but I can’t imagine a quiver without some thrusters in it nowadays.
Please contact Dale. He will do you right and the price is absolutely worth it. With a good set of swim fins and waves, you’ll be all over it in no time. http://neumaticsurfcraft.homestead.com/NeumaticSurfcraftwebsite.html
“The funny thing about mats is that they’re the easiest thing to surf on a basic, beginner level, but they’re the hardest thing to surf on an advanced level…” “…I’ve been riding mats day-in and day-out for over 40 years, and I’m still learning things. I’ve never been bored riding a mat, ever. They’re just too challenging and too much fun.” -George Greenough Jeff, Don’t waste your time with poor quality “beginner” mats. Get a Neumatic. You won’t regret it.
Thanks for info guys.After Santa strips me of my bank account,I’ll contact Dale.
Are the physics of hydrodynamics different between fish and, say, whales? Whales and dolphins pump their tales vertically, like a surfer, rather than side to side like fish.
Good question - for that matter, different between some fish and others. as you get fish with fairly flat planar bodies ( think 'fresh water sunfish’as a definitive example) and tail blades ( if I may coin a term ) close to their bodies which would demonstrate ( I think ) more of a von Karman-type gait as shown in the original post. Their whole body is involved in creating thrust. I think… have to look at one swimming next spring when the ponds heat up hereabouts. On the other hand, whales and fish such as tuna which have tails out on the end of a relatively narrow structure probably have something more like the thrust wake shown above…I suspect the vertical or horizontal tail doesn’t matter a whole lot. These are very efficient swimmers, capable of high speed. Then, it gets even better…well, better if you like thinking about these little mysteries. A surfboard with a beavertail, a swim fin in use by somebody on a mat, bodyboard or kneeboard or a whale’s tail - they are all operating at the interface between something pretty dense ( water) and air, which is realtively not dense at all. That in turn gets quite interesting indeed. that wasn’t a whole lot of help, was it…sorry about that… doc…
Your comment about how a whale’s tail extends from a narrow section intrigues me. How much work has been done with concavity of rail sections? Looking at the outline from above, think of the shape a whale has, (fatter at the nose, narrowing towards the tail with a wide tail), and try to copy that on to a surfboard.
Maybe it has more to do with weight, l’ve said it before that weight is momentum and maybe for whales this is very much so, the bigger fish in the sea seem to swim with less motion and the lite little buggers who zip and zap about but dont really get anywhere (hmmm sounds like something l’ve seen alot in the water lately) look how long it takes a big ship to stop when coming into port, take away momentum and you take away drive. KR http://groups.msn.com/MyKRSurf/krcomweb.msnw
Uhmm… one thing for sure, there has been a lot of research done on whales and how their bodies are optomised for making serious knots through the water. For instance, the US Navy - check out any modern submarine. However, the thing is that those are displacement hulls. Meant to operate completely surrounded by water and often at considerable depth. However, surf-craft are not displacement hulls, they are all planing hulls of one sort or another. I don’t know that whale shapes have a whole lot to give us for surf-craft design beyond how well their fins work for producing thrust on the surface. What critter in nature does? Well, think about a flying fish. A critter that’s gliding on large pectoral fins ( I think, may be another type of fin) nd propelled by the very bottom of it’s tail in the water. They are getting a LOT of speed from very little of their fin actually in the water making thrust. Worth thinking about?? doc… flying fish are also nice fried… http://www.fishbase.org/Summary/SpeciesSummary.cfm?genusname=Cheilopogon&speciesname=melanurus