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There’s a thread going on sways now “Tunnel Fins”. One of the
questions on this thread is: Have fins changed/evolved radically over the past 50 or 60
years? Radically being a personal measurement of course.
I’ll hold my opinions to myself for now.
I want to open the subject for objective discussion here, and preface things
with these thoughts –
First: Opening up a treatise on a technical subject like fluid dynamics and just
looking at the pictures doesn’t qualify as real study. Oh, and in order to
understand how fins perform differently you have ride more than a few and study more
than a few foils as well.
Second: Surfboards and the fins used on them are inseparably married in
their evolution. Without the constant experimentation and risk taking of the
modern board shaper surfers could never do the extreme things they are doing
today. Fins have their role and we will of course disagree on some points but
it seems some enlightening exchange from such a discussion can easily result.
Having had an extensive collection of fins on numerous occasions I will agree with you on many levels.
Rider, board, fin and conditions are all key elements.
I have had fins that worked fantastic on one board and terrible on another…fins that just have not worked for me at all on any board…fins that worked well on any of my boards under certain conditions but not others…fins that friends absolutley loved but I hated and vise versa.
I have some current favs but always am up for trying more.
Fins have evolved to varrying degrees…almost always by somebody trying something “off th wall”
I have watched guys take hacksaws to existing fins to see what would happen etc.
And then you have the old fin guys with years of experience making them that know what things have and haven’t worked in the past.
Every factor comes into play when a fin hits the water…and sometimes what “should” work from a tech standpoint, just doesn’t.
There are so many variables in surfing…and there isn’t really any way to take them all into account because the medium they are influenced by is not constant. Water temperature, salt content, current, topography etc. are an ever changing dynamic.
So…what does this have to do with anything? I don’t know…it’s late…I’m tired…and I’m probably rambling right about now. LOL
Hi Rich, interesting issues here. I would like to ask what ‘evolutionary’ path you have followed to develop your own designs… Take your ‘Mental’ fins for example. I am currently trying two on my hulls and find the power and stability they afford uncanny… What can you tell us about the critical moments in the evolution of this foil ?
I am also trying a 9" Larry Allison’s Greenough 4a, a classic fin, is it still evolving? I really like the way it drives in punchy waves, and the flex is the best I have felt. I wonder if he feels that he is still ‘evolving’ that design, or perhaps just reproducing it ?
Are there any evolutionary links between these two fins? Evolution must be a gradual process of improvement, development though trial, error and creative imaginary leaps…
This fin was a bit boring, but then i cut 1/2’ off the back of the fin and refoiled it. It became much more flexy in the tip, and now i love it, especially in this board. Its so much more manouverable now, i think without the big round tip. Just shows how much difference a bit of playing around can do for your fins. Im hangin to save up for a greenough.
Hey BF - Was/is there a picture??? I’m so bummed, lots of thread folk talk about the picture, and I don’t see a thing…
So, for fins, one thing I trip out on is set fins v. adjustable. I’ve use a variety of fins in the center fin position of a tri, and have found moving any given fin fore/aft, even a half inch, make more difference than the subtle differences in outline, rake, foil, etc. Granted, there are fins of radically different shape which make a big difference too, but I’m still intrigued by how little attention is paid to moving fins.
Hey Taylor, there should be a pic, or at least i can see one. It’s a pic of a 2+1 setup. The centre fin used to be longboard style with a roundish type tip, but i traced 1/2’’ or so off the back outline of the fin, cut it off, then refoiled it. It became a pointy tip, more flexy fin. Oh yeah baby!!
A few years ago I started working with the computer department at Swansea University on developing a fin design program. During the course of development, Dave Carswell wrote his doctorial thesis on the hydrodynamics of surfboard fins. For anyone that is interested I’ve asked Dr. Carswell to web host his thesis. You can find it here:
Back in '69 when I first moved to Santa Cruz I met John Mel. He built me my first board and I struggled to ride it. It was a narrow single fin board about 7’3" or so as I remember and I never did get it going very well. I fooled around with riding long boards but didn’t take to them well as I’m a white room ~ green room kind-o-guy. I decided to move to an intermediate size and ended up with an 8’3" that I made the fin for out high density foam/poly resin and fiberglass. It was built around '73. Well, with this said as background. I should metion that I was cutting and grinding a few fins for John at the time. I found myself compelled by some basic questions about fin dynamics by looking at the most intellegent of the sea creatures and how the fins they used to best manuever themselves are appointed. These fins, of course, being those of the cetations, and most particullarly bottlenosed and spinner dolphins as they are such versatile swimmers. What followed w was a departure from the full based template to one with three arcs combining on the trailing edge. With surfboard fins the rake of the fin is not something that can be changed once it’s on the board. These dolphin are able to rotate their pectoral to various angles and stand them and various vertical attitudes. If their pectorals were full based they couldn’t do this. Thus I had to settle on a compromise with it comes to rake. Over the following years I have noted that all the great swimmers have some measure of cutaway on their pectoral fins.
My first attempt at such a fin was crude but the second one turned out to be a perfect match for the 8’3" board,. a glass-9n of course, which, grey ghost, I’m happy to say is in my quiver and in fine shape to surf to this day. The template has evolved over the years. It’s rake and outline has been modified probably 4 or 5 times as I have surfed it on many boards in may different locations with many different foils until it has reached it’s present state. The foil has evolved as well and now sports a fully rounded leading edge, a gentle concave refoil tucked in behind the vertical cord for center fins on thrusters, single finned boards of various designs and bonzer type boards. I have found that because the fin has a, what is called by Coque, speed sensitivity, that is the fin is rather unengaged and somewhat washy at slower speeds but as speed increases it becomes more and more engaged. So it turns out that the fin is one that doesn’t not provide low speed drive very effectively. Such is the nature of a fin with a good measure of the base cord taken away. This template “Mental” with an agressive helical foil works beautifully for lead fins on modern quads and twinzers as well as on longboards with a 2+1 set up as rail fins where they can be complimented with one as trailer. A three stringer 9’0"x23" that John Mel built for me was my daily driver for about 10 years with such a set-up on it. I spent much time in the shack, coming off the top and cheating up in trim on it. I have since given it to a friend who is absolutely stunned wth it’s magical performance.
I have more plans for set-ups and experimentation with this template along with others in the works.
Your fins are evolving, gradually developing through trial and error, intuitive leaps and measured adjustments…A very personal trajectory for you as it must have been for GG. back then…
Here is one of your Mentals and Larry’s 4a together… I feel they are somewhat related…
Did you write, or have you read, that the base cord should be about 3.5", and that part of the idea for the cut away at the base was to let the @ 1.5" of turbulence along the bottom of the board flow by better, and get the hold/flow/lift/etc. out of the fin deeper in the “blue” water?
Well - So much for attaching a drawing… I was trying to make a visual of what I think you’re saying about the sides of the fin having concave -
If you lay a straight edge from the thickest part of the vertical cord to the trailing edge, would there be space between the straight edge and the fin?
And, last but not least, what is “aggressive helical foil?”
Your words: “If you lay a straight edge from the thickest part of the vertical cord
to the trailing edge, would there be space between the straight edge
and the fin?” Yep you got it right.
An agressive helical foil is one where the fin (always a fin place toward the side of the board.) Is cut diagonally from and cylinder segment (that what a helix is) that has a relatively small radius thus makng somewhat tight arc in the undercamber and making the fin twist out of its root. The fin will have less cant at the base than it will at the tip. This sort of foil doesn’t work for long based fins but for fins of more vertical aspect or those with somewhat shallow horizontal cords this type of foil will amplify the fins engagement. At present I only use this type of foils for boards intended to ride waves up to about shoulder high or slightly above. On bigger waves fins foiled like this will become grabby and cause the board to want to track off in one direction or another without any impetous from the rider.
Regarding hard numbers relating to turbulence along the fin base cord, consider that the greatest amount of turbulence on the bottom of the surfboard is at the fin base if the fin has a hydrodynamically acceptable foil occurs where the fin meet meets the board. The turbulence can be relieved significatly but cutting away the base trailing edge. No fish has a full based pectoral. I wonder why. It’s all about release.
Your words: “Did you write, or have you read, that the base cord should be about
3.5”, and that part of the idea for the cut away at the base was to let
the @ 1.5" of turbulence along the bottom of the board flow by better,
and get the hold/flow/lift/etc. out of the fin deeper in the “blue”
water?"
No I didn’t write this. Though as the horizontal cord increase and moves away for the base the fin becomes harder to release and has more holding power.
I think that the biggest steps in fin evolution (and even surfboard evolution) have been the changes in fin placement, and the distribution of the fin area to different parts of the board.
I think that the changes of fin placement has driven the evolution of the surfboard. From the log, to the single pin, to the wide tail twin keel, to the swallow twin, to the squash thruster it is the fin that has influenced and spawned the other changes in design elements (template, rocker, foil).
hey Rich the link Tom posted worked for me, it’s a 4.4mb PDF file so maybe it was just taking a long time to download. I can email you the file, just pm me your email address if you want it. Pretty technical stuff lots of math but interesting
Thanx keith. It downloaded after after a short time.
Patience always pays dividends.
Been doing some experimentation with single fin hulls up here in Santa Cruz. To date nothing out performs a Greenough Stage IV with the foil/flex tuned to the surfer’s power level. (-;
You say nothing out performs the well tuned 4A on single fin hulls, and I have felt that magic feeling…
But you also say that the cutaway Mental produces less turbulence at the base of the fin and so better release, and that it may engage better at higher speeds. So, am I right to asume that that both fins would work well in different conditions? Or maybe in different parts of the wave?
The Mental may be better suited to bigger, faster, more powerful and more hollow waves, and a higher line? The 4A may be better in smaller, gentler point breaks, or for carving flex fin turns and fast trim.
There is a great variety of hull templates being made at present, and they are being ridden far from the original point breaks. Do you think that your Mental fins might be a positive evolution for hulls in these locations, reefs or beach breaks? And are you developing new hull templates in conjuction with these fins to explore other breaks?
The base cord of Stage IV is fairly short so it doesn’t create as much turbulence at the board-fin juncture as on with a longer base cord. Frankly, though I haven’t measured both. The base cord of an 8.0" Mental and a 9.0" is nearly the same. You are right to assume that the two fins will suit a hull in completely different conditions.
Having both a Stage IV and a Mental can be an excellent compliment on a hull. The Stage IV will make the board, as you have said, loose and free in it’s momentary directional change. When the waves get big, if moving the Stage Iv back in the box doesn’t give sufficient directional stability then it’s time to go to the Mental. Placing it well will only come through experimentation and your experience in larger surf. Marking the fin box at the spot of leading edge with a black dot can be very enlightening and facilitate this process. For the record, I made a 7.25" Mental just like yours for Mark Andreini, the shaper of your boards. He shaped a big wave board just for the fin, which I believe was approaching 9’0" in length. He placed the fin well back in the box. He says the board rides great. I believe it was on his recomendation that I made your fins.
Because of the template of the Mental the flex pattern is not like that of the Stage IV. Flexible fins, for the most part – there are exceptions, (Cheyne Horan’s Star Fin for one) do better in smaller surf. The foil of the board has to complimented by the fin or fins under it. Otherwise a grand board can be a real dog.It’s impossible to speak to all the variety in hull shapes out there. Frankly most of them aren’t very refined. I’ve watched this hull craz in it’s present surge and have to say that a guy has to shape at least 50 of them before he begins to produce something of quality. Hell my first fins were junk.
Oh, Juan, thanx for posting the photos on the thread. Pictures say so much. How goes the wild Helical Twinzer set-up?
