I have developed a simple yet effective test method to estimate water flow direction under a surfboard, particularly during turns. The results may be useful for fin and bottom contour afficionados or just plain’ole brain candy entertainment.
I can post the test method and results if anyone is interested.
Water flow direction (flow vector mapping) under a surfboard seems undefined and as far as I know, never been tested. I have been intrigued by water flow as it relates to fin and bottom contour effectiveness.
I recently proposed a couple of test methods on Swaylocks. Halcyon expressed some interest in putting the window panels with streamers on one of his boards to observe what’s going on.
As a former mechanical test, product development, R&D engineer, I am a big proponent of modeling. Modeling is particularly useful in hydrodynamics - the same laws apply to minnows or submarines, pond ripples or tsunamis and of course full-scale surfboards and model surfboards. I was intrigued by my mini skimboard idea and after some additional thought it seemed doable, cheap, easy and fun.
The first pic shows the test model board. It is a sheet of clear acrylic with three streamers attached underneath. The three streamers allows water flow observation at each location: near the carving inside rail, the center (stringer) and near the outside rail during simulated turns.
I went to my local intracoastal waterway (very nice place to swim) and began my testing. The test method was very simple: in calm waist high water, grab each end of the board (bending a little rocker in it) and make sweeping turns by twsiting my arms and upper body around in half circles whilse simultaneously observing the streamers. I tested shallow, medium and steep board angle turns at slow, medium and fast speeds. I was very interested in angle of attack on the inside and outside rails where the rail fins on a trifin are placed.
The second pic shows a diagram of what I observed. In a nutshell…
water flows away, in opposite directions, from the surface waterline created along the bottom of the board, as if there is a pronounce vee right along the waterline.
The flow below the waterline flowed at mild angles away from the waterline - from centered waterline out to the carving inside rail, ~10-15 degrees. The inside rail streamer was basically flowing outwards towards the edge of the inside rail.
Above the waterline, the sheet of water (spray) moved in the opposite direction – towards the outside rail and at steeper angles than the underwater flow angle seen at the opposite side.
As the waterline oscillated above and below the middle streamer, the mid-streamer oscillated in direction following the observations in #2 and #3 above.
The same observations were true at the different board angles and board speeds, just to varying degrees. In hard turns, the outside streamer was almost perpendicular to the stringer flowing with the fast moving sheet/spray.
The observations/findings may not be new to some of you but it was kind of surprising to me (I thought water would flow more parallel to the stringer along the inside carving rail).
What I liked most about this test was its ease of implementation. Total budget – less than $5; total time invested – about 2 hours. The cost/benefit ratio is pretty hard to beat. I’ve been reading surfing publications for two decades and have not come across such research or testing. Perhaps someone else has….or maybe I broke some new ground here. Who really knows, the industry is very fragmented.
Verrrrry nice…any idea what kind of velocity you were getting? How ‘bout stapping a video camera on your shoulder and spinning around in a circle? All kinds of ideas poppin’…great job.
OOo oo - I am all for the fin(s) and bottom contours - although some of that might be hard to do as easy - under water video?? And then take it to a fast flowing clear stream… Ah - you could realy play around and see what’s doin’ under there. I wonder why there hasn’t been more underwater filming of people riding. you see it every so often, but with some planning, it seems one could get some good stuff. nice ideas.
I love this place - Peace to all, Taylor E. Olson.
OK, maybe I did say that I though making the bottom of a board transparent so you could see the water flow was a good Idea.
Bert Berger can build one. He already has the technology ready to go. If you want to see what fins do you have to put fin boxes in the board and try different fins in it. Along with that you have to find some way of mounting a camera on top of the board that will take pictures while it’s being surfed. It’s certainly possible. With the digital age here movie cameras can be pretty small. I’ll make a couple of sets of fins for the boards as long as I can make the tabs for the fin boxes easily.
Surely it can be done. It’s just up to someone who has the time time, money and technological expertise.
I’m busy finishing up my first board, (I’ll post photos) making tow-in fins, making rotating box fins and several other fin orders along with upgrading my PC and lots of Plumbing work. It’s way too busy here.
Outstanding, man. Modelling is REAL useful in hydrodynamics, 'cos there are so many surprises in it.
Some thoughts/questions -
I wonder what you’d see if you put down a number of short, narrow segments along the board, such that you could have an idea of flow patterns all the way on it. You might have to have quite a few in order to see an overall pattern.
Have you got kind of a feel for what the differering angles above and below the waterline were? I’m kinda thinking relative to one another - there’s something to be learned from that and I’m just wondering what it might be. Pushing flow up against negligable resistance ( other than G ) and down against more considerable resistance from pressure and from water being itself a helluva lot denser than air…
As the flow is…well, not quite symmetrical to the waterline but at least splits there, I kinda wonder if this shouldn’t spark some thinking about how a board turns and what bottom contours- concaves, convexes and vees and channels and the whole mish mosh - are really doing, as the waterlines of the boards I observe turning don’t seem to be paralell with the stringers or centerlines.
I’m wondering if a similar experiment wouldn’t tell us quite a lot about rail shapes and waterflow around them. The nice ( almost wrote ‘cool’ there, which would be singularly inappropriate ) thing about plexiglass is that it can be formed around a hot mandrel or other male mold with the judicuous application of a heat gun, probably into a female mold too. This would let a number of shapes be played with.
If anybody has access to a fast-moving stream, something like this could be set up as almost a steady-state thing, hold it in a given position and wait for things to stabilise.
interesting stuff-- sorry I’m a day late reading this… we had a thread earlier about putting plexi windows in surfboards, and I had envisioned putting one in the front of the board for various reasons – but putting them in the back of the board would be a bit more challenging, especially with cameras involved. Not that it couldn’t be done, mind you…
I’ve spent a fair amount of time staring at underwater photos & video of surfboards in motion, often you can see the water movement pretty clearly, because of entrained bubbles. During turns, water flow off the outside front foil of rail fins appears to be nearly perpendicular to the stringer, but not quite – maybe at a 75 degree angle instead of 90. But, you can’t really see all the flow this way, the streamers are a good idea. Seems to me that if you put a bunch of little streamers on a board and its fins, and surfed it in some nice clear water, over a video camera, you’d get some really good indication of water flow, and not have to mess around with windows & cameras ON the board itself… same concept, just viewing the streamers from a different location. (underwater Tahiti, I volunteer to be cameraman!)
My goal was not to asses how fins interact but how flow affects angle of attack assumptions, why canards work, channel effectivity, etc etc
Greg,
the next step is to place windows on a regular surfboard…what we used to call “goin’ live”. Using surface mounted video to record whats happening. Underwater would be great if proper lighting is available.
Lee,
roughly 6ft/sec medium speed and 12ft/sec at very fast speeds. The arc created by my extended arms was about 6ft.
Doc,
yes I thought of multiple streamers for full mapping but I was pressed for time. The angles were different above and below the waterline. Very roughly, I would say that above water angle was significantly larger may 50% more on average, But at higher speeds is was much more…75+ degrees vs 20-30 degrees. It was not symmetrical.
After about 50 or so tests my arms werent tired but I was getting dizzzzzzzzyyy!
If you guys are really interested do the test…the sheet costs about <$3 at home depot. It was about 2ft long by 1 ft wide. I had streamer material lying around.
Honestly, I was very surprised with what happened below the waterline…It was counter-intuitive for me. I think my next small wave board will have a two straight channel design with flatter rocker imbedded within…starting ahead of the front foot peaking ahead of the back foot and fading into a subtle double concave. The channels will redirect some water out the back, and help release some water ahead of the outside fin for drag reduction (???)…inside the two channels, flatter rocker for a definitive speed spot when planning flat. When the board is on rail it has its natural smooth rocker for effective turning in rail to rail surfing…seems like a good idea on paper.
thanks for your thoughts and let me know if your doing some tests…I really enjoy doing it and reading about it…R&D that is.
Another simple method is to mix oil and graphite powder and smear it on the bottom of your board. Then go surfing and the flow patterns will be etched into the oil. I’m serious here, it works. We did it in sailboards and you can see flow lines, eddies and vorticies. Cool stuff.
Greg, I think you have said in the past (although I may remember it out of context) that single concave through the fins is somehow better than double. (Do I have that right?) If so, Meecrafty’s results seem to bear that out with the observed well-defined break in flow at the middle. Your thoughts?
I started doing single concaves in the early 70’s. Picked up the idea from Brewer and it just happened to fit what I wanted to do on a wave, which was to hit the top hard. Singles have issues about creating acceleration and the concaves improved this dramatically. It kind of cracks me up to see so many short single fins being made today with vee. Single’s DON"T have problems with rail to rail so why a vee?
In the case of single concave vs. double the single is more efficent at producing speed and lift. The double creates turbulence under the tail of the board and in my opinion that would be the last place you’d want turbulence. I also feel that the double and triple concave were done simply because most of the leading designers in the early 90’s were afraid to let go of vee which was so successful with twins and early threes in the 80’s.
I started doing single concaves in the early 70’s. Picked up the idea from Brewer and it just happened to fit what I wanted to do on a wave, which was to hit the top hard. Singles have issues about creating acceleration and the concaves improved this dramatically. It kind of cracks me up to see so many short single fins being made today with vee. Single’s DON"T have problems with rail to rail so why a vee?
In the case of single concave vs. double the single is more efficent at producing speed and lift. The double creates turbulence under the tail of the board and in my opinion that would be the last place you’d want turbulence. I also feel that the double and triple concave were done simply because most of the leading designers in the early 90’s were afraid to let go of vee which was so successful with twins and early threes in the 80’s.
Where do you begin and end your single concaves and what is the depth and what type of rocker do you use?
I start the concave at about the midpoint in the board running to it’s apex between the front fins and then tapering to flat at the tail. They usually run about 1/4 inch deep.
I started doing single concaves in the early 70's. Picked up the idea from Brewer and it just happened to fit what I wanted to do on a wave, which was to hit the top hard. Singles have issues about creating acceleration and the concaves improved this dramatically. It kind of cracks me up to see so many short single fins being made today with vee. Single's DON"T have problems with rail to rail so why a vee?
I think one of the ideas was that tail vee creates an accentuated rocker line on the rail. So, when you lay it on the rail, the rail near the tail helps with the turn. And, since harder turns on singles consist of banking the rear rail with the fin sweeping through the turn, the rear rail line matters.
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In the case of single concave vs. double the single is more efficent at producing speed and lift. The double creates turbulence under the tail of the board and in my opinion that would be the last place you’d want turbulence.
I think statements like these need more precision. Concaves create lift only when the water flows sideways across them (and is re-directed downwards). Mostly they reduce lift by putting planing forces over more curved surfaces. In a straight line, flat beats concaves for paddle-in surfing speeds.
The aspect of concaves I appreciate most is being able to weight the front end more heavily in turns (b/c the concave resists sliding sideways). This probably brings you through the turn faster.
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I also feel that the double and triple concave were done simply because most of the leading designers in the early 90's were afraid to let go of vee which was so successful with twins and early threes in the 80's.
The single-to-double is the de facto standard on competition shortboards today. The double in the tail complements rail fin water re-direction, although it is shallow, whereas the deeper front foot concave gives the rail a little more bit in turns.
I don’t usually get into particular design discussions on this site because so much is personal preference and opinion but I was asked my opinion so…
If you want a different rail line just change the lines in the tail in relation to the bottom curve while incorperating concave. In single fins this is easy and the results are significantly enhanced acceleration which is needed in that design. Vee is generally used to enhance rail to rail which singles don’t have issue with. Twins do.
As for concaves on three fins, the designs today all incorperate concave with subtle differences. I think discussing one over another is many times splitting hairs. They all basically increase drive off the bottom turn which enhances speed. As long as modern surfing is based on bottom turn acceleration any advance in this aspect will advance perfomance. In performance shortboard surfing, trim is dead. As a result the concept of water flow as it pertains to anything other than accelleration has dropped far down on any list of importance.
There are certainly pro surfers today riding single, double and triple concaves sucessfully. I haven’t taken any recent polls on which is the most popular but again they are all to enhance drive and accelleration. As to which is best, personal preference is the winner.