That takes care of the bottom of the tunnel, but how about the sides. Do you foil them similar to thrusters? Flat on the inside foiled on the outside?
Hello Mark.
Just foil the tunnel the same inside and out using a regular foil section then cut the thing in half and bung it in your board.
Roy
Hello Mark, Or you can do it flat on the inside. You know how it is with good ideas, at least some of the time. Roy
This has been one of the best threads yet on the rocket science of surfboards. Most of what I have read and enjoyed has been “how to” stuff. Good practical information. But this has been a discussion of a different nature. Lot’s of bright people contributing ideas, findings from similar fields (sail boats, airplanes). I wanted to save it so I thought I would print it using the print feature Mike built in, but it looked like it was going to be 160 plus pages. Not very practical. But this stuff is too good to loose, and as it exists now, it is a very long read. Not that I’m lazy, but as written there is a lot of ground to cover before getting to the nuggets, ie, the points where everyone seemed to be in agreement.
So, here is a thought. How about trying to summarize the key points upon which there seemed to be some general consensus? In other words, capture some of the gems and put them to paper. Perhaps by topics such as the source of turbulence and location of turbulence, the effects of rake, how fat and thin fins behave comparatively, aspect ratio, fin size, etc. Here are a couple of examples that with some editing, we might agree on:
- A thin turbulent layer of flow exists along the bottom of a moving surfboard such that when it encounters the foil(s) of a surfboard, additional and noticeable drag is generated. This source of drag can be reduced by minimizing the length of the fin base at the point of attachment or providing a vortex generator in front of the fin.
- Establishing the optimal fin aspect ratio is a matter of compromise between high tolerance of stall (low aspect with more rake) and [WHAT?]
- Optimal rake of a modern high performance fin is generally accepted to be between ?????
The summary could tolerate some level of disagreement by including a discussion of the range of theorys. If the authors of this thread, for what ever reason, are not interested, I’ll try to pull together a little more free ranging “cut and paste” document with contributors names. But, it will be a while before I can get to it.
Let’s capture the good thoughts before they get cold.
I’m not certain about rake and aspect ratio yet.
I read something on the history of surfboard design. It stated, flat out, that using more raked fins, decades ago, was a quantum leap forward in design.
In my own experiments, I’ve noticed…
a standard ‘wide base’ 7.5 inch deep fin has chord lengths of 4-5 inches throughout its entire length. And a rake close to 35 degrees, on average, although it is progressive in angle.
If you make a fin matched in chord lengths, without taper near the tip, it doesn’t work as well, not nearly.
Now, I began to think. Why would INCREASED rake be a quantum leap forward??? And I think to myself, what about the Starfin?? People routinely talk about its drive and drag as being phenomenal (except the kelp-catching thing). And it is MUCH steeper in rake than a standard surfing fin, and it has fairly normal chord lengths. Remember, chord length is measured parallel to the bottom of the surfboard.
Well, it all makes sense, if, the limitation in upright fins with adequate chord lengths is, what is called downwash in airplane terms. The wing on an airplane has downwash. This is air from the bottom (high pressure side), going AROUND the end of the wing, and hitting the low pressure side. It pushes down on the low pressure side, which pushes down on the tip, leading to the name downwash.
If this is a critical limitation of an unraked fin, couldn’t it be the case that the RAKE on surfboard fins is already set to minimize the surfing equivalent of downwash (or tip eddies, of any other phenomena you might think of that leads to drag from side currents running from the high to low pressure sides of the foil) ??
The Starfin is brilliant in that is has endplates to block downwash, and a steeper rake to increase drive (which would otherwise be downwash limited, and NOT stall limited). And that this all happened a long time ago and every fin in surfing has not copied these attributes to maximize performance…
Anyway, I don’t think the story is done yet. I put forth the idea that rake exists to minimize the surfing fin equivalent of downwash, and that if an endplate is used to block downwash, better fins, with steeper rake angles, will be used on surfboards. I do not think it necessary that the endplates be shaped exactly like those on the Starfin, either. Different designs, to minimize kelp catching, may be used.
I do think it is clear to me the Mental fin, or the long chord length cutaway, achieves a unique drive:drag relation by having the maximal chord length 2-3 inches deep on the fin, where the velocity of the water is close to zero, and that this optimizes the foil action. But you cannot cutaway a normal fin, necessarily, you need to keep in mind that lost area has to be made up somewhat. Measure your chord lengths at each inch of depth - they will give you a good idea of what your design needs to mimic.
And, fins should be stiff as fiberglass/polyester resin, or stiffer.
Or something like that.
Hello Blakestah,
I have just read your theory on downwash and other forms of tip turbulence, and how to reduce them using a 'stopper' on the end of the fin. You suggest that the star fin prevents these unwanted eddies by stoppering the end of the fin with the two extra fins which make up the tip of the star.
Surely the double ends of the star fin have tips also, and these tips are also subject to tip turbulence! So the solution to tip turbulence is a fin with twice as many tips? Hmm . . .perhaps not.
Tip turbulence can only be avoided by eliminating the tip altogether. Please observe that the tunnel fin has no tip, and thus no tip turbulence. The increase in speed as a result of this is clearly visible. The aeronautical version of the tunnel fin is known as the annular wing precisely because it has no tip.
There is a way of significantly reducing tip turbulence for conventional flat plane fins, and that is to use a long torpedo shaped bulb.
I hope that this information clarifies the tip turbulence discussion.
Roy
PS The two pictures here attached show:
1) The Acid Drop, a board with zero tip turbulence and
2) The resolute Salmon, a board with an elementary tip turbulence preventing bulb
Yep, no one’s ever pointed out the contradiction in adding tips to reduce tip turbulence. I don’t know the math but it goes something like this. The amount of turbulence is proportional to the length of the foil before the tip. So adding two smaller winglets/tip extensions can reduce the overall induced tip drag. They can also add some lift to the whole fin too. Cheyne pioneered these fins with Ben Lexcen back in the 80’s.
Bulbs are nice too. The half and whole tunnel thing looks fun. Expect to see more people build/experiment with them.
Cheyne with Starfin:
Edit: what blakstah said in the following post about the spillover due to the pressure differential is, of course, the cause of the tip turbulence.
Quote:
Hello Blakestah,I have just read your theory on downwash and other forms of tip turbulence, and how to reduce them using a 'stopper' on the end of the fin. You suggest that the star fin prevents these unwanted eddies by stoppering the end of the fin with the two extra fins which make up the tip of the star. Surely the double ends of the star fin have tips also, and these tips are also subject to tip turbulence! So the solution to tip turbulence is a fin with twice as many tips? Hmm . . .perhaps not. Tip turbulence can only be avoided by eliminating the tip altogether. Please observe that the tunnel fin has no tip, and thus no tip turbulence. The increase in speed as a result of this is clearly visible. The aeronautical version of the tunnel fin is known as the annular wing precisely because it has no tip. There is a way of significantly reducing tip turbulence for conventional flat plane fins, and that is to use a long torpedo shaped bulb. I hope that this information clarifies the tip turbulence discussion. </blockquote></div>It is not quite turbulence, it is the organized circular currents that run from high pressure to low pressure sure.
Anything you might use to try to block the downwash would need to stick out a few inches from the end of the fin.
I’ve observed many airplane wings with end plates to stop downwash, with both flat plates, and with bulbs.
Of course a loop has no such problem.
The two finlets on the Starfin serve a different roles - certainly they can have downwash themselves, but are not in a position to see much pressure differential.
In any case, I am particularly interested in whether people think the role of the rake is minimizing tip eddies/downwash, or is it something else.
Hello Blakestah,
I write regarding your statement that the tips of the star fin winglets are not in a position to experience much pressure differential. Are you saying that they are not going to experience much pressure differential because they are set up predominantly in the horizontal plane and are thus not subject to horizontal or sideways pressure during a turn?
If so, then it should be noticed that any change in the fore and aft trim of the surfboard will immediately set up a pressure differential in the vertical plane. This is a case of 'out of the frying pan into the fire' as the results are just as bad. A difference in pressure on horizontal fin components will occur even if the board hits a bump in the wave, or when the rider weights and unweights.
Rolliges mentions that the amount of downwash experienced by a wing tip is proportional to the length of the wing preceding the tip. If this is the case then I should point out that the standard star fin tips, added together, are about the same length as the main fin, and so have plenty of scope for generating downwash.
On the subject of whether or not a raked fin has less tip eddies than an upright one because of its rake, or due to some other factor, has it been shown that a raked fin does in fact produce less tip eddies?
Regards, Roy
I think, if you look at the Starfin and project how water will move around it, you will see the finlets are
-
in a position to block “downwash” from the main fin
-
are not in a position to generate much of a pressure differential themselves
The proof is in the pudding, anyway.
The initial points that prompted my posts were
-
Halcyon annd Bert claiming that rake is necessary to get drive (let’s assume all fins are fins and not hoops and have no endplates)
-
I was perusing a history of surfing design document which states that the first fins raked in a modern way were patterned after a tuna tail (ha-ha - I found that funny - like every surfer that has ever designed something for a fin says it was motivated by a tuna tail). The rake led to drive.
Foil theory wouldn’t support rake being necessary for drive, kinda the opposite. Unless it is all about the downwash. In which case you could use much steeper fins if you had an appropriate endplate. I put forth that the Starfin is such a fin. And if you remove the finlets, it wouldn’t work so well. I wonder if anyone has ridden a Starfin with and without the end-plates??
Just an idea I thought I would throw out there…
Hello blakestah,
Precisely what I was just doing was projecting how water moves around a star fin, and it is clearly the case that the finlets are in a position to generate a pressure differential, because of the fact that the fin is subject to movement in the vertical plane. Simply re affirming that they won't generate a pressure differential, without a supporting argument, is insufficient to establish your point. Nor does the 'proof is in the pudding' theory carry much weight.
The handling problems which star fins and other flat plane hydrofoil fins have cannot be made to vanish by ignoring them.
Roy
I was more concerned I couldn’t explain it well enough to make a difference.
The high pressure side of the main foil is moving towards still water. Downwash would generate a current going down the main foil, towards the tip, on the high pressure side.
The finlet, on that side, is angled, and will push water up the foil, and generate high pressure. Note that downwash, of the finlet, would require the current to go down, both on the main foil, and on the finlet. So, I think it should be clear that current can not shunt the high pressure side of the foil around the tip, and that the cant angle of the finlet prevents downwash on the finlet.
The situation is somewhat inverted on the other side. Water needs to flow up the foil, but the finlet re-directs water downward because of its cant angle.
This is what I was hoping you would grasp when I said to consider how the Starfin generates drive with a pressure differential across its main foil. The finlets direct water up or down, mainly, in a way that serves to negate the downwash of the main foil.
I suspect that this set-up allows downwash to be negated with less increase in wetted area than a bulb, and less frontal area, as well.
Blakestah,
Your analysis of the effects of tip finlets assumes that the only pressure exerted on the fin is that which originates from the side. If we take a case where the board is running straight through the water with no pressure from the side, but pressure from above or below, then it can be seen that the winglet tips will definitely be subject to a downwash inducing pressure differential.
Those who are used to regular flat plane fins often tend to ignore the effects of vertical water movement over the fin resulting from changing fore and aft trim angles of the board. These vertical flows generate tremendous power when they meet a horizontal fin component, and in a surfing situation thay can dominate the board's behaviour entirely. It won't do to give more weight in your analysis to the side flows induced by turning than to those vertical flows induced by (for example) fore and aft trim. The tip turbulence and downwash effects for a flat plane tip winglet are thus a lot more complex than your description suggests.
Drive is generated by flat plane tip winglets during turns, but it can be devilishly difficult to control due to some side effects which I have previously mentioned.
Roy
Quote:
Your analysis of the effects of tip finlets assumes that the only pressure exerted on the fin is that which originates from the side.
In regards to Starfins and tip induced turbulence Blakstah is correct. The Starfin is double foiled, without tips and going strait it would produce little tip turbulence. There are other pressures that act on a fin but tip turbulence is generated with different pressures on both side of the fin. This occurs when the board is turning (almost all of surfing).
Did some other quick research and seems like winglets (Starfin wings) have another effect. One that many people who ride them describe. I’m not sure if this has been explained at Swaylocks yet:
“NASA researcher Richard Whitcomb invented “Winglets.” The winglet, like so many great inventions is obvious once someone else has thought of it. It represents one of the few effective means of fighting induced drag. Unlike the other methods mentioned above, the winglet does not strive to reduce induced drag so much as it uses it to create an offsetting thrust. In the diagram to the right we can see that the air flowing over the winglet, due to the presence of the tip vortex, strikes the winglet at an angle of attack.Like any airflow, over any wing, it produces lift. In this case the lift ( or a component of it) is forward. Thus, the winglet produces thrust. The stronger the vortex the more thrust it produces.”
Rolliges,
I was actually talking about what happens when the board is pointed up or down, or weighted and unweighted, and thus about the vertical flow over the fin which results. It seems that neither of you have yet gotten my drift . . This vertical flow component is always present, even when you think you are just going straight ahead. So it needs to be considered.
Furthermore, after having ridden many straight wingleted fins, I can report that they have severe handling problems which are not really addressed by analysis of straight line aircraft flight in still air. I can also tell you exactly why these problems occur.
Roy
PS You have highlighted a portion of Richard Whitcombe’s assertion which refers to ‘methods mentioned above’. Without seeing what he is referring to it is not possible to evaluate what he is saying fully, but it has definitely already been explained that winglets produce thrust. My two points are that the winglet tips cannot escape tip turbulence themselves, and that the thrust produces severe unwanted side effects in the case of a flate plane winglet setup like the star fin. These side effects are inescapable, and cannot be ignored in a surfing situation.
You are a skilled and determined debater. I resign. (Insert sense of humor here).
Where can I get a half-round fin? Do they work on short boards?
-Rob
Rob, I can make you one (see my site under speed surfing/get tunnel fin). BugPower seems to be talking laconic into making one if I am reading the coffee grounds correctly. Doesn’t Halcyon make fins? Or you could make one. Doing it on a shortboard is going to be difficult, but then I think shortboarding of any sort is difficult since I took up the big sticks. Roy
Hey Roy & Rob, et al. I have ridden a Starfin and didn’t find that it had any “severe” handling problems. What kind did you experience? They take a little rider mentality adjustment, but that’s all I’ve noted. If set up right (relative to rocker) the “vertical lift” problem is not a problem at all.
I also don’t understand why the theoretical argument that the tips must also create tip turbulence. The point of the winglet is to vector the tip vortex up and away from the main foil, converting it to a forward thrust component. It’s a small component (evidenced by, eg fuel savings of approx 4% in airplanes w/ new winglets added) but it’s there. If you wanted to, you could put tips on the tips, giving you another (much smaller) forward thrust component. But the forward thrust added would be almost nil, and the construction probably not worth it. (this also implies that the turbulence created by the tip of the finlet is probably not that big of a problem, doesn’t it?)
as I’ve said before, I think the tunnel fin is interesting. I also think finlets are “under-explored” notwithstanding the Star fin. The point being, all these have their pros & cons, no one is perfect, and it’s not necessary to prove that all other systems are defective in order to show that a particular one has merits.
By the way, Roy, the “other methods” mentioned in Rob’s post are, e.g. Hoerner tips, elliptical plan-shaped foils, bulb tips, and other methods of reducing INDUCED drag (which sometimes increase parasitic drag). Winglets are more a “thinking outside the box” solution in which the lemons (drag) are converted to lemonade (thrust). At least in aviation they are a proven commodity.
What size tunnel fin do you think might be appropriate for a 6’2" shortboard-that-would-have-been-a-thruster? My thinking is that such a setup might handle more like a twin fin, by the way…
ok i know i said i was gonna have a break…
but i think roy is heading down the garden path…and theres a few willing to follow…
ive been following this thread the whole time absorbing all the arguements and the various logical conclusions…
firstly …the definition of induced drag and some of the circumstances it comes about…
if you induce something , youve caused it to happen, which means a change of circumstances,
from an areonautical point of view, induced drag would be created if you put more weight in a plane ,then you would need a higher angle of attack to get the extra weight off the ground and keep it flying, the higher attack angle would cause more drag…
in a surfboard induced drag occurs when you load hard into a turn…
you can minimise induced drag by setting your fin up to handle higher AOA…
the end plates help , when you turn your automatically creating a bigger pressure difference between either side of the fin, now coz there is a bigger pressure difference ,the water on the high pressure side is more vigorous in attempting to get to the low pressure side thus setting the trailing vortex in motion, the bigger pressure difference means you will get a flow along the wing/fin towards the tip,the end plate interupts that flow minimising the size of the trailing vortex and tip turbulence…
if your looking at a single fin then a starfin is a definate improvement on the single . it allows that bit more forward projection than the equivilent template without the wings,
still not as much as a raked fin in forward projection…but then with the raked fin you lose pivot and that snappy response you get from a more vertical template…
so the starfin is a valid improvement to the stock single…
ok roy , i can see you arguement for the half pipe having less drag all your arguements were valid and very realistic …for going straight at high speed with low angles of attack…yea i can see how functional they would be as long as you gave the pipe a neutral foil or double sided foil , as soon as you introduce an assymetrical foil , an increase in drag but also an effect to lift the board or pull it down depending on which way you foiled it…
but i can absolutly say for a fact the half pipe wouldnt allow a board to drop straight down the face of a wave turn 180 degrees come straight back up then 180 back down and repeat that series of turns 4 or 5 times in a row…
you may get something close to small wave surfablilty with a half pipe were your front fins should be and a conventional tail fin behind , or another smaller half pipe at the rear behind the main pipe…
the traditional thruster has no equal over a wide range of applications …
im not talking small wave straight line speed …im talking belting the lip top to bottom time after time increasing in speed with every pump…
drag is not a bad thing…drag adds an element of control…the more control you have , the more ability you have to place your board in the power zone of the wave and harness its energy…
its all about setting your board up with controled drag…but being careful to put the drag in the right places so as to not hinder the overall high performance of your board…
bottom line …
the half pipe is not a high performance option for small waves…
roy theres a few pics of me at raglan last year…
and i dont even rate in comparison to the guy i went to nz with …
he does sick things on a longboard …i just gota find his pics and get em in the computer…
i got a friend in whakatane who rips on a longboard …or you could get kelly ryan to ride one of your boards…or go down to raglan and look up mickey t …
if you can show me some high performance action in those same nz waves …then i may start taking you seriously…
but until then the half pipe is staying in the same corner as garden gnomes and magic mushies…
im not saying the half pipe is not an option , but i think your giving the design to much credit over to broad a range of conditions…
regards
BERT
I believe Roy has some valid points as well, for narrow variations of AOA. Which is exactly why he has refined his design to long narrow tailed boards. Both design elements favor shallow sweeping turns and minimze drag. But, if you attempted to apply the half pipe foil to the bottom of a 14-1/2" wide sqaush tail, I believe there would be conflicting design elements. I do believe that curved fins such as Greg Webber’s CRV’s are relatively wide open for exploration regarding optimization for different wave conditions and rider styles for wide tailed boards. And, I’ve been playing with the idea of a curved side fin with an end plate winglet for the last year. Unfortunately, I haven’t had the time to make any proto-types to date. However, I have made this suggestion to a few custom fin designers.
One of the big improvements that thrusters have made over single fins is their ability to deal with greater AOAs. Such as the ability to bottom turn mid face. Much of the reason they can handle these greater angles is due to the side fin toe in and greater girth parameters relative to the centerline typical of thruster side fins. Which gets back to my previous assertion that as you foil the inside face of side fins, for a given fin girth you give up girth on the outer faces. When you reduce girth and move the apex of the foil aft of the leading edge you give up ability to handle greater AOA without stalling. Which means that you give up efficiency when making tight abrupt direction changes.