No conclusions or physics theories in this post.
I just wanted to share this image (https://twitter.com/finfoil/status/629989678586953728).
More pics / theories to come.
Remember see this sort of thing when i was the factory grom.one of my first things i did was wet rub boards it was all fixed fin ,was playing hosing boards down one day and started
playing with angles of water across the board and up the fins…have seen it curl off the tip like that…i also believe that the shape at the base should have the fillet, much smoother flow of water…funny cause when the removable fin systems came in one of the selling points was how there is no fillet at the base so its faster better… something i do not agree with… the last few boards i have made for myself had fixed fins…i like 30 percent foils on the inside twinfins so much smoother…boards flow so much better if leading edges are not so sharp…
I do not agree on the fillet, hydordynamically a fillet only introduces extra drag. In speed and formula windsurfing fins are really optimised for maximum performance and fillets are never used or mimicked.
But I totally agree with your point about the 70/30 side fins. A single foiled fin is invented purely for manufacturing reasons. The recent inside foil hype is all about this.
So few grasp this reality. Though few in number, don’t dispair, you’re in good company.
wow what a beast…
Thanks 
I’m planning to write a series of blog posts to try to bust some myths about fin design using these fluid simulations. But nothing more, since these simulations are often used incorrectly or used to falsely prove that some design is “optimal”.
But there fun to play with, and I hope I can share some valuable results in the future.
Did a quick simulation of a simple tunnel fin.
This is a half ring with an AoA of 10 degrees.
No tip vortex found indeed. But there is rotation (a vortex).
As Roy pointed out on twitter, these images visualize what he means in his blog.
I didn’t test or validate any theories, I’m just visualizing the flow for now.
Edit: I just read Roy’s full article. It’s an interesting read! But, it’s difficult stuff to simulate.
Hans, with your CFD software can you get lift/drag numbers?
That would be really interesting to model different fins and run them through the same models and get an apples to apples comparison.
“One way to eliminate fin-tip vortices is to eliminate fin tips.”
There’s a lot of truth to that. In aircraft, look at the newer wing tips, they are able to increase efficiency by reducing wingtip chord. Surfboard fins are the total opposite of that.
Whether the lack of tips at all results in less total drag remains to be seen. It’ll remove tip vortices but the added lift also has a drag component. That’s where CFD is really nice, it’s gives you rough numbers on whether you’re looking at net gains or losses with an overall design concept. Great way to prototype things and do rapid iterations to get to a point where you’re a few steps ahead of the game by the time you build a working prototype.
When water is disturbed it spins in to a vortex.
The less vortices produced as a object passes through a fluid
the less drag it is is producing.
Centripital force is more powerful than centrifical force.
Centrifical force produces an exposion in fluid.
Witness the vortex alongside a caneo paddle – the paddle pushes against
the water. As the angle of attack runs toward the paddle edge the vortices
occur off the trailing edge as wash.
Now If a fin goes through the water creating minimal votices it is more efficient.
It creates less drag revealed by less vortices occuring in its wake. It is then
creating centripital force: The good stuff!
Stay Stoked, Rich
P.S. But, the question is: How do you do it? ====== (-;
Yes, I can get any number in the flow e.g. flow velocity, pressure, forces, turbelence intensity, …
But there is no point in sharing the plain numbers. I’m planning to make some interesting cases like:
- comparing good and badly foiled fins
- comparing aspect ratios
- effect of fin flex on stall angles
- ...
Any suggestion is welcome.
But my time is limited, so for now it’s only fancy colors 
Yo Hans,
If you have a fin in hand can you generate a flow picture from it?
Stay Stoked, Rich
Does this simulation only show water flowing over the fin directly straight? Can it incorporate water flowing up the wave and fin at various angles to such? Turns? at various parts of a wave? weight distribution? What you need out of fins is variables, not straightlining toward the shore. Just curious
Hi Hans…
It would also be very interesting to see a graphic (or numerical/quantitative) analysis of leading edge tuburcles. I would be particularly interested in the effect of tuburcles (or random tuburcle-like ‘slots’) placed on one side and then the other (but not both) of an asymmetrically foiled (70/30ish) single fin designed for an asymmetrical board with a one-way wave in mind… I.E. Raglan Pt in New Zealand, Rincon in California, etc.
Specifically, I am wondering if the benefits of a single or asymmetrically foiled single fin might be further enhanced by increasing potential Angle of Attack through the use of one sided tuburcles???
I have seen videos clearly demonstrating laminar flow over fan blades through an increased AOA via the use of tuburcle-like slots.
Am open to ideas or suggestions on this possibility from anybody else as well.
Yes, I can.
I can also generate these pictures starting from a .foil file (from finFoil).
The pictures I posted is water flowing with an AoA of 10 degrees and a velocity of 6m/s (= 21.6km/h = 13.42mph).
But any rotation relative to the flow is possible. The bottom surface also acts as a wall (surfboard surface), but anything is possible here.
However, the difficulty here is to translate a specific surfing manoeuver into a flow regime. I wouldn’t even try to do that.
It’s more interesting to compare different fin characteristics and than go and test it in the water to see what it really does.
Hi John,
An analysis of leading edge tuburcles would be very interesting. But would require a very fine mesh and a very high computational cost.
I admit that it is a very interesting case, but it is a very specialized topic. So I’ll start with the simpeler cases first before tackling these kind of cases.
Edit: I just investigated these tuburcles a bit more, an other thing Roy has been using for years. They’re very interesting, it’s definately on my list to research. It might become a feature in finFoil to add these.
Thanks for your interest!
https://twitter.com/finfoil/status/614310059103748096
Hans, could you show us this (or similar) elliptical one please?
Are you thinking long term finFoil +real time flow analysis? Pull on a control point and watch it go?
Good stuff, thanks for showing us. -J