Whilst not being able to comment on the design theories and practice of current fin makers I can however offer you my thoughts in regards to the following non traditional (Wildy’s style) #1 template versus the traditional #2 template.
Template #1 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.
Among Pilots, and I am one, we call that a ‘‘Hershey Bar Wing.’’ Classic example is the wing on a Cherokee 140/150.
If you are curious, go buy a Hershey Bar, and check it out.
Added bonus, it tastes good.
Good sketch analogy…most standard fins are like that, but to its detriment the fin thickness tapers to the tip leaving those long tip cord lines as thinner and thinner foils, which to me is not so good for tip turbulence.
Wildy,
For the fins in your avatar, does chord thickness gradually taper as you approach the tip?
We should not forget other effects than the efficiency.
What is really interesting about curved fins, is that they twist under load.
The twist reduces the angle of attack of the tip compared to the base, which postpones stalling at the fin tip, making the fin more forgiving with regard to spin-outs.
Airplane wings are also twisted towards the wing tips to ensure the plane can still be controlled when stalling appears at the wing base.
AKA, Root Stall.
Yes, it’s the same percentage to the tip.
That’s a great visual.
Standard fins seem visually like the chord gets smaller towards the tip, but it’s interesting how little it decreases from base to tip if you actually measure it.
Hans, would you consider the delay in stall provided by a traditional shaped fin with a poor thickness to cord ratio but flexibility at the tip to be greater than that of a Wildy type template with good chord to thickness ratio at tip with no flexibility?
That’s hard to say, it all depends on what you mean with “poor thickness to chord ratio”.
The twist becomes more important for thinner profiles as they tend to stall abruptly due to “leading edge stall”, where thicker profiles stall more softly by default (“trailing edge stall”) and therefore can be made more stiff and less raked.
All those effects work together, it’s very hard to make a general rule which effect will dominate in general.
A nice example of a different balance between all those effects is what is commonly referred to as the Thrailkill Twin. The Thrailkill twin softens the stall characteristic using the “biplane effect” where one fin stalls later than the other. This allows to create thinner and stiffer upright fins while still being forgiving.
Thanks Hans. I would consider anything much under about 12% chord to thickness ratio to be a compromise considering we’re operating at pretty low Reynolds Numbers (less than 150,000 - 200,000).
I’ve always preferred thicker, stiffer fins with fuller foils (12-18%) and the softer stalling characteristics you’ve described. Fins which rely on flex to delay delay stalling are too “inconsistent” for my taste, especially in bigger waves.
For windsurfing I prefer the opposite.
At high speeds the thicker foils drag considerably even at low angles of attack due to the larger wake they generate (trailing edge stall).
So when going into the waves, a fin that twist when under heavy load is desired.
But the loads we speak off here are considerably higher, these fins are still full G10 or carbon laminated cores.
I still need to try the thrailkill twin on a windsurf board, I have high expectations that it can get much more upright fins with tighter turning radius.
A couple of questions , on a Bonzer board they use long runners , starting at zero at the front , growing to about 3 in tall at the rear , should the runners get thicker towards the taller end , keel fins are wide at the base but generally not very tall , would a keel fin benefit from having the rear vertical trailing edge be straight .
I’m no expert but I would think that bonzer runners you probably have to treat like a water flow redirection feature rather than a fin with foil. So a uniform thickness with a blunted leading edge and a “sharpish” trailing edge is about all you could do.
To try and apply a proper foil with a consistent thickness to chord ratio to such a design would not be an easy task. The fin would be very thick at the base and then rapidly diminish.
I would be interested to see Hans apply his finfoil software to a bonzer runner and view the results.
You are exactly right. A similar opinion was held by Bob Simmons, about his twin ‘‘half moon keels’’ on his surfboards.
Abrupt let go and spin out was what I experienced when I was using those white plastic FCS AM fins with almost no foil in the tip. The flex would manifest as a delay in turn, but when pushed hard the fins would just let go.
I moved to stiffer fins with more foil in the tip.
Seems like less tip or more thickness at tip is the way forward with fin design.
I really like these fins. The thickness to cord ratio makes sense in the base due to the narrow cord but the tip has thickness and an even longer cord. I guess the twisting of the tip is more, which reduces the turbulence there.
Regarding the spitfire wing, according to the link posted by stoneburner
If one is seeking less tip vortex, then having the wing/fin thinner at the top, producing progressively less lift, will lessen that tip vortex, and a ‘hershey bar’ wing would have nearly the same lift at root as at the tip, and perhaps a huge draggy tip vortex at high angles of attack.
I’ve been doing a bit of holiday reading on submarine design.
It has a nice layman’s description of pressure drag and skin friction as components of the total drag on a moving submerged body. Pressure drag rises with “thicker” form and skin friction falls. These curves have a crossover point and the resultant curve you get when you add them together leaves you with a distinct minimum point.
The bottom line seems to be that the most efficient (least total drag) length to beam ratio for submarines is around 7.7 which translates to about 13% as a chord to thickness ratio for the equivalent of a fin foiled on both sides. Anything thinner than this will actually have more total drag.