I’ve been wondering this for a long time and I would appreciate some good facts to clear my mind. Maibe my windsurf background is not helping me in this one
How thick can (SHOULD) I foil a twin fin (not keels but a MR’s style with a bit longer base) ? Is thicknes something good? G10-fins are thinner (and faster?).
Question 2. Advantage/ disadvantage of a " Semi-speed foil" in a a longboard fin. Faster but turns worse? Why have 90% of longboard fins similar foil? (max thickness very early)
G-10 is stiffer than regular fiberglass panels of the same thickness. There’s an ongoing debate about the merits of flex vs stiffness in a fin. Personally, I’m a biflexual; I like flex fins for longboards I’m using in slow conditions and I like stiff fins for shortboards and fishies where I want less lag.
There was a time when you could buy longboard fins with a 3/4" or 1" max thickness at the base. Someone here probably has pics of those. But by the time we got to the Bahne boxes with the 3/8" channel that’s when 3/8" became the standard for the fins using those boxes and 1/4" became the max thickness for the FCS and Future boxes.
This is a pic of a modified MR template I did for the big guy (I tweaked the trailing edge of the template to reduce the recurve). The max thickness at the base is 1/2" thick, so this amounts to 1/4" tab fitting the 1/4" slot in the Gearbox fin system I used with the other 1/4" hanging over the outboard side of the finbox. You could do the same with a centerfin: 3/8" tab fits into the slot with another 1/4" or more hanging over the sides. I’ve only done that a couple times because I normally just buy a regular flex fin.
For those who have suffered through my ravings on this in the past, I apologise, but I’m going to express my not completely uninformed opinion on this subject once more.
For a given fin cross sectional area, an approximate 13% ratio of fin thickness to chord length will result in the least drag. Pressure drag and skin friction are the two components of the total drag on a moving submerged body. Pressure drag rises with thicker form and skin friction falls. These two curves have a crossover point, and the resultant curve you get when you add them together leaves you with a distinct minimum point. This crossover point occurs at around a 13% ratio of fin thickness to chord length.
So ideally, a centre fin, foiled on both sides, should be 13mm thick, if it has a 100mm base length. Side fins, which are foiled on only one side, should be half of this, because they are a half foil. 6.5mm for a 100mm base length. Conveniently, I’ve also found these thickness ratios seem to give just enough scope to easily get a nice foil when shaping a fin.
Maximum fin thickness is generally placed about 1/3 of the way along the chord length for the best response/drag compromise. Placing this maximum point further forward than this will result in more drag, but a faster turning response. If you have a very upright fin you can afford to have the maximum thickness point positioned further back than a swept back fin, because the more upright fin gives a faster response via its plan shape, so you don’t need to push the maximum fin thickness position forward to achieve the same. This has been my experience anyway.
Single fins will reveal all of this much more clearly than multi-fin boards, because the margin for error is much less when you only have one large control surface, rather than many smaller ones.
Disclaimer: I am not a professional hydrodynamicist.
But in commercial practice they never are, because of the increased cost to make a thicker centre fin for example in a thruster set. Because less than ideal foil and thickness is not as noticeable in smaller fins like this, the fin makers don’t bother. They just make them all the same thickness.
If you check a set of thruster fins you will see that the side fins have a decent foil on them because their thickness is about correct. But the centre fins foil is very flat, because there isn’t sufficient thickness to foil it properly.
I started foiling this one. Using 13% /2 for thicknes and different foil than usual for my twins. Really looking forward to compare it with the thinner ones
I am also not a professional airfoilologist, but have learned a lot since I started designing fins in CAD to be cut out with a CNC router. In my opinion its a lot less intuitive, you can’t sand until it feels good, but it can be more data driven. I start with an fin outline. Then, theoretically based on conditions I plan to surf in but realistically picked on suggestions by other swaylockaholics, I pick an airfoil with appropriate lift, stall, angle of attack properties. My chosen outline and airfoil dictate how thick my fins are and where the thickest part of the fin lies. This approach also allows you to use different airfoils along the height or sweep of the fin to almost tune the fin to how you want it to perform. Finfoil is a great resource even for hand made fins. I previously used finfoil’s printed contour map to hand shape a few fins.
All of that being said my fins all come out on the thicker side, 1/2"-3/4", and I think they all surf well. I made a quad set for FCS boxes that were 1/2" at their thickest, but I haven’t glassed or surfed them yet.
And a dolphin fin should decrease thickness faster. I see a lot of dolphin fins that carry a lot of thickness to the tip vs others more foiled.
So the fin in that link has a very straigh leading edge. How does it compare with a fin with similar rake and are but with curvy leading edge, for exemple this one. ( https://www.trueames.com/collections/noserider)
Keeping the 13% ratio of thickness to chord length is ideal. Fin templates which don’t reduce much in chord length towards the tip will end up being thicker at the tip. But because of this they will also end up producing more drag because of the additional tip vortices that a thicker tip produces. For example template #1 below allows for a gradual reduction in thickness as you move toward the tip and the ability to keep a constant chord to thickness ratio as you go. If you wanted to keep a constant thickness to chord ratio with template #2 you would need a nearly constant thickness almost the whole way to the very tip of the fin.There’s a much longer discussion on this subject here. Fin Efficiency - General Discussion - SWAYLOCKS
I think part of the equation includes the amount of energy and speeds involved in various conditions. How much drag do you want? You will want to run slower and apply more rider input in slower conditions whereas in fast conditions that becomes a big problem. If you’re using a fin cluster as your motor then you’re using them differently than if you’re using a board’s rail line as your motor and the fin is there primarily to provide hold, stability and control.
I don’t longboard, but I see the function and use of a D-fin on a noserider to be distinctly different than a keel on a windsurfer or a thruster cluster on an HPSB.
Then we all agree the 13% ratio all the way is the best. A bit of critical thinking. We say that straighter leading edge and an outline like the Wayne Rich Harmonic or the fcs H2, 3… Fins is more efficient. Why is not everybody using this outlines? A curved leading edge is less efficient but should be better for something else. If not peoøle wouldt choose that outlines. Better turning or… you name it. Is my point correct?
These more efficient outlines with reduced tip chord length are probably not being used widely for a few reasons.
Anyone spending money tooling up to make multifins will be taking a risk on a fin outline that surfers are not used to seeing. The high performance shortboard generic fin outline has been with us for a long time and most surfers just assume that it is the optimal outline and would be risk averse to spending money on anything else. Although FCS are edging towards fins with reduced chord length at the tip with designs like the H2 and H3. In fact I believe the H2 was designed in conjuction with a naval architect, which would explain why its outline is what it is. You will see more variation in hand foiled longboard single fin outlines because there are no tooling costs involved.
Also, as gdaddy has alluded to in this thread, these less efficient outlines might actually be what surfers want, despite the fact they have more drag. The additional drag created by long chords lengths at the fin tip might create a feeling of control, or hold, or drive. Terms which are difficult to qualitatively define, and don’t have a lot of scientific meaning, but are important to what a surfer wants to feel when turning.
Hi, thanks for bringing this very interesting topic up. The 13 % thickness… i have been playing a bit with finfoil.io and i have to say these thick foils look different.
One thing… when are fins considered to be a half foil? Only when they are flat on one side? A lot of fins now incorporate a inside foil, from what i understood these should be thicker too, right?
In a twinzer or split keel set , would it make sense to have a thick fin in the front and "leaner "fin in the back ?
Side fins which are flat on the inside are half foils, so 6.5% thickness to chord length is what suits them.
If your side fin has convex foil on the inside then 13% is ideal drag wise. Concave inside foils on side fins however is another story. 6.5% would probably be best for them I would think.
Please remeber that all of this is “ideal”, and for example the drag inherent in a thruster type fin set up itself is magnitudes greater than any drag reduction you might achieve by impmenting the 13% ratio on all the fins. Twins and quads will benefit more, and a single fin will benefit the most.
Any advice from me on particular fin set ups, beyond these basics I’ve outlined would be purely speculative, and uninformed. There are others here with much more experience on these matters from a practical point of view than me, who may be able to help.
