how thick is too thick

General Discussion
1

…hello10 days ago I had an accident; still alive but damaged; no more work for the next months. This and possible sequels are what s the problem is.

Anyway, couple of days ago, an architect came here and I was chatting about a boomerang fin for a box that I made days before the crash, I did it as a possible solution for a guy/board that needed/wanted some loose. This fella commented about to make some kind of explanation etc, now I am here trying to draw some schemes etc to fill the time and loose my mind.

My question is: why, when some one decided that the fins thickness should be the way we know?

Yes there s variations, also Bill T always mention his experiences with thicker foils in smaller surf etc.

Is due to too much cavitation problems?

Thanks

2

Well, I thought this was going to be OT and unrelated to surfboards :wink:

Anyway…

You are correct about cavitation. In cross section, a thicker fin will have more curve, front to back. This enhances laminar flow and let’s the fin ‘stick’ better. A thiiner fin with flatter sides will be more prone to turbulent flow and separation, which is undesirable in a fin.

3

Reverb,

A very good foil for a surfboard fin, would have a thickness that is 15% of the base chord.    I’ve had good results with fins with thicknesses down to 12.5%, and even 10% thickness ratios.     The eye opener is to replace a ‘‘normal’’ fin with a thick fin of the same template, and then surf it.      You WILL be convinced.      

4

Here’s an ad from the 70s. The fins are up 1" thick. I made a Greenough Stage 4A and a Brewer from a 3/4" thick piece of G10 last month. Took a lot of work to get the tips foiled down, but I think they work fine. Custom jigs and the right tools would have made the work easier, but I don’t plan on making more.

There’s a thread here somewhere with Paul Gross was saying that the 4A needs to be at least 1/2" thick.

I think the fin thickness is based on the ease of manufacturing for the current fin boxes, and someone decided 1/4" was enough for modern thrusters, maybe cause it was only single foiled, I don’t know.

A lot of the older single fins made for bahne boxes were slightly wider than the box, not like that anymore. Same reason, cost and ease of manufacturing.

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5

Thickness is also directly related to flex.

Perhaps thick fins work better due to much less flex.

 Perhaps thick fins work better, as it is easier to achieve a more accurate foil with a thicker fin, and/or perhaps a thicker fin is more tolerant of an inaccurate/imprecise foil.

Less flex, more accurate foil, happier surfing.

Possibly.

I have been experimenting with MrMik’s 3d printed  high aspect ratio turbucled fins of various thicknesses.  No errors in foiling, perfectly symmetrical, just  Different thicknesses.  The thinner stiffer  fins have been the most engaging and rewarding when pushed hardest, the thicker fins are smoother through turns and more forgiving and rock solid predictable, especially at lower speeds, they have more grip and response. The Thinner fins have presented noticeably less drag paddling into waves and are just all around quicker, more precise, engaging, and less tolerant of user error, Kind of like too much engine, not enough tire thrilling,  where thicker is plenty of tire, but lacking torque.

This impression was formed  in relatively weak waves 3 foot overhead and under on an old school longboard design. A 25Lb 9’7" x 23 x 3 round pin that I have been riding for 15+ years.  The thinner fin performance was unexpected. I was expecting the thinnest fin to be a dog, and it did turn weird, until I moved it 3/4" forward in the fin box and it then quickly began to blow my mind, until It snapped off after ~ 200 rides. The next day I rode a 1mm thicker version in very similar conditions and that Extra torque and top end was gone, replaced with more solid more predicatable turning.

 

So I am on the opposite mindset, at least with his turbucled leading edge super high aspect ratio GW fin design in this specific board…

How thin is too thin?

And I will be exploring this, with the thinnest and among the stiffest of the MrMik’s 3d printed fins yet, very soon.

 

That said, I had one traditional shaped wood fin I broke near the base, which worked OK before it broke being 1/2 inch thick. I rebuilt it ~5/32" thicker, and it worked so much better.  Significantly better. But was It due to less separation of flow at higher angles of attack, or because being thicker it was less flexy, and easier to achieve an accurate foil when hand foiling?

 

 

 

 

 

 

 

6

Maybe Gene (Cooper) will chime in with some photos of the over-thick fins that he currently experiments with on his Carson/Cooper V-bottom model. From what I gather, the fin is hollowed out to reduce weight (and add flex?)

Would be great to have his take on this topic.

7

I also think the stiffness is a big factor.  My youngest who’s become quite a surfer had always ridden the FCS base model fins in his boards up until this past year.  One day we packed up to go to a contest an hour from home and he accidently forgot all his fins on the couch.  When he went to get ready for his first heat he was freaking out when he found he had no fins.  Another family was nice enough to let him borrow a set.  It was the same template but in a carbon fiber foam core composite.  Quite frankly I was shocked at how much quicker, faster and responsive his board was under his feet.  It was as if he had a whole new gear.  The only difference was that these fins were very rigid compared to what he had been using.

1 Like
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We have the fins we have because we have the boxes we have. 

If anyone has made a thick foil fin for a box it’s quite a project and would not fly in profit first production setting. 

Gene’s fin is a work of art and brad is doing some amazing work putting them into production. 

9

Reverb, sorry to hear about the accident, I hope things improve quickly for you.

10

Hello JRandy, thank you for your support. Kind of a white trash possible on drugs and with a crappy old car decided to make a dubious turn…
I have good orders that I would return the money etc, may be lose some clients, not an easy task in these times.

-good discussion, I thought about the boxes and the production of the panels.
However, FU fabricated both so could be done the other way.
May be is like Wsix is saying and some subtleties make the difference.

I am not new in making fins, I have been making them for a great % of my boards, more in the thousands not in the hundreds, but always maintained the normal thickness, sometimes I made thinner ones, but never too thick for smaller thrusters.
Now to the eye could be difficult to get the kids using one inch or more in a 5 5 thruster…

11

How about this: Fins need to be thick and thin at the same time. 

Once you achieve that, then parts of the fin are always performing near peak, while others are not doing much, or beginning to stall.

 

 

12

MrMik, I understand your point, but how much thick?
One inch, 1 1/2 thick.?
For an small board thruster set up could be difficult to paddling (may be with less cant and toe in to aid with that) but I think that the 1/4, 3/8 for smaller boards and fins that most use is kind of a middle point to have an average ride and to have a not so expensive production, also with fibers, to think in the weight associated

13

The question is where to even measure the thickness on a fin that is both thick and thin at the same time.

The only practical way I know is to measure the thickest part of the fin, but that by no means describes it sufficiently. 

The thickness distribution from base to tip is also very important, even on a fin that does not use tubercles. Tubercle effect - Wikipedia

But with tubercles and undulating side walls as created by fin Foil magic, you get the effect that the fin is thicker where the chord length is shorter, and vice versa, with infinite graduations in-between.  

So even though the fin is nowhere thicker than at the thickest part just above the tab, the thickness relative to the ‘local’ chord length is extreme in some parts, because the chord length is so much reduced in the valleys.

I might be wrong about how the hydrodynamics cause the effect, but it works. Low drag (fast), high lift (lots of drive when turning), high angle of attack (tight turning and pivoting), gradual stall (no sudden wipe-out but a recoverable stall).

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14

…I used a similar concept in the boomerang fin mentioned

Thick base, thinner neck and thicker upper part,  all parts tapered out and convex.

But still the question is about how much thickness for the single fins and multi finned set ups.

Seems nobody did any test in the past?

15

I think the McCoy Gullwing fin is also an example of a fin with vastly different thickness-to-chord ratios in the one fin. And, the forward knuckle is a single large tubercle. 

Thickness itself means not much, it’s the thickness to chord ratio that makes a foil thick or thin.

In the past, tests were very difficult to do, because manufacturing a fin accurately was very expensive

Now, with finFoil and 3D printing of fins, one can actually compare fins with different thickness, but which are otherwise identical.

One remaining problem is that the thinner fins become more flexible, but I have developed a ‘FinTune’ system which allows to compensate for that, within limits.

If the fin is very thin, then there is no space to add further stiffening reinforcements. But for ‘medium to thick’ fins I can make two fins which are near identical, including flex, but have different thicknesses. 

I only make single fins at this stage, and I am not aware of anyone else capable of 3D printing large fins like that. There are a few working on 3D printed fins, but most seem to print them small and often with a flat side, because that is sooo much easier. 

wrcsixeight and myself are currently doing just that: Testing fins that are largely identical except for a couple of variables. 

16

I’m with Bill T. on this one. 12-15% thickness to chord ratio seems to be ideal given the Reynolds Numbers we’re dealing with from a theoretical fluid dynamics perspective.

Of course, the smaller and more numerous the fins, the less you will notice the effect of poor foil and less than ideal thickness to chord ratio. A large single fin however will provide you the most telling evidence.

Remember that if you’re dealing with single foiled side fins that they are “half” foils so their ideal thickness to chord ratio should be 6-7% if you want to play by the theoretical rules. 

 

17

Good on you Mr Mik, that’s real research breaking variables into compartments and testing each version. I thank you in advance for the results.

18

I’ve been wondering about this for a long time as well.

I wonder how much of it comes from the advent of molded fins? There are thickness limitations to plastic injection molding and I wonder how much that plays into where most mass produced fin system fins are today in regards to thickness.

Thick fins make a lot of sense in the surfboard context in the way they need to pass through a wide range of angles of attack. It seems that thicker fins with a more rounded leading edge would have a lot of benefits in our context.

I’ve seen lots of old single fins that are really thick and even lots of glass ons from thrusters in the late 80’s were noticeably thicker. I think Bill’s comment hits the nail on the head:

“The eye opener is to replace a ‘‘normal’’ fin with a thick fin of the same template, and then surf it. You WILL be convinced.”

I just need to try it and see.

 

19

  The injection molded Brewer Fin, that was produced by Fins Unlimited, had a base Chord of 6.25 inches, and a base thickness of 15/16ths of an inch.   Exactly 15% of the base chord length.      The ‘‘market’’ did not respond well, to the thicker fins,       The concept is counter intuitive, for most sheeple involved in surfing, as a recreation.     But, it is ‘‘settled science’’, in my opinion.      You only need to experience the change in a board that you think you know, to grasp the merit of a thicker fin.     

20

Thick vs thin debates happen often, on the Bob is the oil guy automotive forums, regarding oil viscosity.  The thin guys state that 0w-20 still allows engines to last longer than the vehicles, the thick guys say that one overheating event will have the 0w-20 not have the film strength to keep metal from touching metal, and that the only reason thin oils are used are for slight fuel economy advantages and the same engine in other countries without C.A.F.E. standards will spec thicker oils.  These debates go on for many pages,  they are quite amusing and also informative and both the thick thin guys really dig in their heels.  Eventually the insults begin and the thread is locked or deleted, but it is a just matter of time before a new one starts up. 

 

With Mr Mik’s GW fin, which is extremly high aspect ratio, and with the turbucles on the leading edge and with the scallops trailing them, in my high mileage 9’7" Lboard I have been riding 85% of the time when i surf for the last 15.5 years, the thinner GW fins have been the better performers.  It is noticeable just paddling, and paddling into waves, I find myself going to take the extra stroke, but not needing to.  Just paddling from A to B, it is noticeable that thin has less drag.

 

Turning as hard as I can turn this board, the thinner GW fin projects more like a mulitfin board with sharp rails off the tail, than a single fin with soft rails.  The thicker GW fin is more forgiving of error, more controllable, but I am also entering and exiting turns at a slower speed, so it should feel more controllable.

 

I have never tried identical regular/traditional shaped fins of different thicknesses.  The one fin I broke and rebuilt thicker, also had its outline changed a bit, and it did ride significantly better when thicker and the slightly altered template.

 

I have NOT ridden MrMiks fins in anything bt a longboard.

 

But in My longboard the thinner GW fins have been the best overall performers by far, except for the thinnest fin which is far too flexible for my 215Lb ass. That said i could not notice much difference in paddling speed between the 8.25mm thick fin compared to the 9.9mm, but I could easily tell the difference between teh 10 and 11mm paddling, and up and riding.