Thick fins vs. Thin fins

General Discussion
41

Bert,

You are so right.

There is alot of tension involved and it is constant tension. The second the tension is released is when control is lost and we are on our ass. Maintain a constant strain the test pilots used to say.

Any fins job is just that, create tension. And transfer that tension to the board. Without it there would be no direction.

Like I said to Kelly and Merrick when they suggested the tuna’s speed came from the foil: Cut the fins off a tuna and you have sushi. My point was muscular and foiled as it is it’s the fins that allow all that energy to transfer to the water to create forward motion. Even the vortices that it rolls down it’s sides. Hull designers need to know this. Every percentage point counts. Even the ones we can pick up from surface texture of the fin, but that’s another thread or much later in this because surface may play a roll allowing us to make changes of a lesser degree in the foil or template or volume to get the desired results. We need all the percentage points we can muster and now. We only get 100 to play with.

As for vg’s and thicker. Last year one shaper reported to me that he no longer needed MVGs with the new 3.2 Vectors.

I’ll let that stand as data without comment for now. But if anyone wants to comment do it. Don’t worry about being critical. I’m not taking anything personal. Besides this thread has been going very well.

Mark

42

Cord is the “distance from the leading edge to the trailing edge” of the cross section of a foil. The edges that you see on the bottom of the fins in the pictures posted above are “root cords”. Because they are the bottom cross sections of their respective fin foils.

43

This is the kind of thing that really gets me stoked.

Being able to connect with the past directly and to the people who lived it and feel the stoke. This isn’t a book either. More like in college when we had a guest speaker or visiting artist and had Q&A.

You know what I just remembered? We had George Plimpton visit and lecture one evening, then had a wine and cheese thing after. It was fall 1973 when I was 18. He had just published Paper Lion where he trained with and QB’d the Detriot Lions in a live game on tv. I had a zinger for him that was killing me. So I screwed up my courage to talk to the huge pop egomaniac and asked him if he had any plans to try surfing giant Banzai Pipeline? He looked at me for a second like I had two heads. Then he said where’s that? We got into a brief discussion abou big wave surfing and I mentioned some of my heros and the convesation concluded with him saying said no, too dangerous and laughed. Got him. It’s safer to play pro football…in front of millions on TV than risk it all alone in big surf. It all comes down to what we really got. Only two things we really own in this world our word and our balls. I knew his limits. And forever after so did he. But for me, an adrenaline junky stuck in a small upstate NY college looking for kicks, zinging the egos of pop icons passed for fun. That was then, when the few guys I really respected were Gerry Lopez and the Pipe crew. When I finally met Gerry for the first time as few years ago all I could say was nothing.

I feel now like I missed a lot doing the school thing the way I did instead of following my surfing. But no matter where I went and whatever I did I did it as a surfer. Whatever that means.

Okay I’m back Bill. I especially liked the eureka when you figured out what was really going on.

“The truth is that because of the thick foiled fin, and its’ laminar flow charecteristics, the board did not bleed off speed in the cutback turn as I was used to. Instead of leaning back to stay with the slowing board, I now had to lean forward to stay with the board going through turns.”

Any time we can increase speed by slight shifts in weight is a good thing as long as the gear is there to deliver. Made me think. And it’s hard to get my mind around it all at once but that is what I’m after. That moment of total understanding. Like your eureka. You know, the moment after which you can look at anything and just know exactly how it works and combines with other parts of the design. So, that you can then say that works that works and that works, that that and that doesn’t so take that that that and that and bingo. It’s out there somewhere. The most excellent fin.

To tell the truth I’m leaning in a direction where I don’t see any fins. And if I ever get it done it’s going to be so simple everyone’s going to say why didnt I think of that? But it’s not coming together all that simple really.

I can see parts come together in my mind, but I can’t hold them long enough to work the other parts in and hold them too. And that is why I started this thread. I’m searching for the lost chord so to speak. Time to break out the sketch pad and charcoal, I guess.

I posed a question in my reply to Tubedog and now I’m wondering if it’s kind of off a little. Now I’m wondering if slight shifts produce speed what do big shifts do given the same gear? Can we design for both? Or do we lose too much going for one result at the expense of the other.

Surfing currently is about big flashy moves. How do we design for that? Which is better in that light thick or thin?

Or for more mellow riding can we design specifically for that too?

Exploring.

I like your round table idea, Bill. Why, if I was in So. Cal…but I am not. However, if you have any more fin or board eureka flashbacks throw them right here. I’m listening. Mark

44

Ouch is right. Cut a little deep there.

Just want the ideas, don’t want to kill the brain:-)

My first guess is that a thicker fin will rotate less and therefore will put less rotational stress on the box. But let me think about this a little longer.

45

Quote:

thickness is 13.5 % of chord length rear fin is 15 % of chord

I appriciate you taking the time to give me an explanation. Does this mean that the thickness of the fin is 13.5% of the perimeter of the base of the fin?

46

47

The shortest distance between two points is a straight line.

Cord is the measurement of a curved line between two points.

Historically airplane designers used a rope or cord to measure the width of a wing’s curve and give it a number.

The term is very accurate. Lay the cord on the wing, cut it to length and measure the cord.

Cord measurements can be made anywhere on a fin or wing horizontally and vertically. You can graph them and create and an x y axis. You can measure the thickness where the graphed cord lines intersect and transfer them to other materials that are graphed to similar or scaled dimensions and duplicate them or scale them. Just for starters. Everyone should do this at least once.

48

Mark-

That is a point I’ve kept in mind, what I compared were identicle unusual templates, one thin with a uniform thin foil, and one with a combination thin/thick foil, so it is not a pure thick vs thin test, perhaps the transition zone creates some drag. Other than that, though, all of the variables are the same: both have the same straight hatchet lines, both are short center thruster fins. They both are about (forgive my rounding I misplaced my calipers) 4 1/2" height, 2 1/2" length base, 3" length trapezoidal tip, 1/4" thick @ base, 3/8" thick @ tip. Transition zone at about 2/3 distance to tip. Modified fin is the same, but foiled 1/4" thick the entire fin. As a note, the manufacturer says the thicker tip allows for the reduction of the fin area, So perhaps we should then compare thicker smaller area fins with thin standard area fins?

Quote:
Might that yield us one or two of Mark’s % points?

49

Thanks again (both of you)! I know that it can be disruptive to pull a thread down to an elementary level, but it really cleared things up for me.

50

Did someone say no fins? I’ve played around with that too. “Cut the fins off a tuna and you have sushi.” Great quote- I have pretty much realized that conclusion too. :slight_smile: Mark- the reason for the angular templates on the fins in my previous post was an attempt to keep the foils accurate- I put a template on both sides of the material I was working with, tapered and foiled. Back to the subject of thick foils on airplanes and thin foils on jets- the way I see it, I don’t plan on going 200 mph any time soon, much less mach 1, so what does it matter? Oh I remember, we have scale things up and down… has anybody? I haven’t- I just figure that we are going a whole lot slower than anything around- even Mr. Tuna would smoke us in a race. Dolphins are probably much closer to us speed wise, so why not examine their foils? I find that they seem to be pretty thick. Until I figure out how to go 30 mph on a 3’-6’ wave I am sticking with thick foils. I’d say more but you guys have it covered. Enjoy the pics. -Carl

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51

What is the thickness of a tuna fin?

How about a shark pectoral fin?

How about a dolphin pectoral fin?

How about an Orca pectoral fin?

How is it that nature saw fit to make its best swimmers all have fins much thinner that those quoted in this thread…all of them turn to relieve AOA…all of them concave or flat on high pressure side, convex on low pressure side.

And on another note, smoking RR epoxy startled me today :slight_smile:

52

DanB, Not at all disruptive. Depends on the moderator or poster. For me every good discussion requires I keep the dictionary near. Internet is great for that as is Google. Sometimes going back to basics really clarifies my thoughts in other areas. Remember the simplest answer or solution is usually the best. There is simplicity in definitions and their etymology. They are tools and they help. Use them all. That’s what they are there for. Mark

53

Blakestah,

I can’t tell you how I know this, but suffice it to say that all those fins foils that you mention are remarkably the same. You are just going to have to take my word for it.

Unless the people who paid for the research release it.

Good point, too. Maybe info on fin size relative to weight and size of these amimals would be relevant also. What is the proportion of fin size to body weight? How does that affect speed? Mark

54

Tube,

Anyway if it’s drag that the major hatchet fin company was after, straight lines are one way to get it. But, I can drag a brick from a string and get even more drag. Or tie a bunch of doors to a rope and drag it from a sailboat if I lose the rudder. Remember that if you ever get in that jam. But is a trapezoid shape the best way?

While I was running down the canal tow path, for my lunch break, some guys rode past me on mt bikes. I thought, yeah, right, wheels. Those guys weren’t even sweating and they were gone like the wind. I got passed but good. Then I remembered reading that some MIT guy is making artificial atoms and adding a specific amt of electrons (artificial atoms have no nucleus) and charging them a certain way so that they can get the atoms (electrons) to all spin in the same direction. Imagine what of effect this kind of coating on a surfboard or fin might have? Like little wheels driving us, eliminating the drag from flow, propelling us. Super efficient surfaces. Of course $50 billion bucks or whatever…but nano technolgy may in ten years not even be a term as they expect the technology to be so prevalent in our economy we won’t even think about it. Point is round round round. Not square.

The universe likes things round. There are no straight lines in the universe. The water exposed to the flat top (bottom hanging down in the water) of the tip of that hatchet is screaming trying to flow to the back around and over it. It has to be forcing a separation somewhere along that break line kind of scenario. I have heard guys have reported hearing a hum from vectors 3.2. That is what it would sound like. Especially if it’s a fat flat line. But I have not isolated the hum yet. That is just one possible source to consider. But it’s most likely not coming from the base like the waer rushing under FCS fins. So it has to be coming from the fins themselves. I remember reading someone saying it was a cool humming sound.

Laughing my ass off. “But it was cool humming sound. Man and if you dudes don’t have it, maaannn, you’re a bunch of fags.”

I like your test.

What is the actual difference in volume between the fins? If you can do a displacement of water test and measure the volume of water displaced by each you know Archimedes test. That would tell us something too. Provided the templates are the same.

Back to thick vs thin. Volume is something to consider. Mark

55

Boy, I am just paralyzed by indecision as to which displacement tank I should use…

just kidding. I don’t have any, and it’d be a tiny amount to try to measure unless the tank was closely fitted and very finely calibrated, so that’s why I furnished the dimensions so if you were interested you could make a rough estimate. What’s the volume in cc of 1/8" x 3" x 2", roughly?

56

12.29cc

57
Quote:
Boy, I am just paralyzed by indecision as to which displacement tank I should use...

just kidding. I don’t have any, and it’d be a tiny amount to try to measure unless the tank was closely fitted and very finely calibrated, so that’s why I furnished the dimensions so if you were interested you could make a rough estimate. What’s the volume in cc of 1/8" x 3" x 2", roughly?

12.3

Using very liberal estimates you could get 1/4th of a kilo’s worth of lift (in a standard gravity field) from three fairly large fairly fat fins.

58

By the way, I’d be happy to put thicker thruster fins to the test of good consistent fast waves, provided someone could point me to where I can obtain some thicker, bouyant ones (foam or light wood) with FCS fittings. I’ve never got around to ordering 'em custom.

(and I don’t want to get into a discussion of the merits of fin systems. FCS has done just fine by me, I’ve tried others too but I’ve got lots of good boards with FCS and it allows me to eliminate a lot of variables, as well as modify fins myself).

59

That 12.3 cc number appears bit high, probably because the added 1/8" thickness probably isn’t uniform- I’d suspect it is less towards the trailing edge so the volume difference is probably more like a third that. Now, I also suspect it is not so much a pure volume difference as much as how and where that volume is placed, too. What that hatchet fin does is sort of like the picture Tom posted with the bulb on the end of the boat’s keel. Placing more volume and surface area near the tip. I’m guessing that allows for better all around performance at more AOAs, as opposed to a thin even foil which may or may not be better at straight-line speed runs. Bert says maybe not. I’d like to try it. I know how fast I can go on those waves. Anyone see the DVD Second Thoughts on the bigger Indo faster freight trains where they get passed up during even their top speed runs? Anyone think thicker foils would give them a better chance?

60

Hey Sways,

Blakestah says, “How is it that nature saw fit to make its best swimmers all have fins much thinner that those quoted in this thread…all of them turn to relieve AOA…all of them concave or flat on high pressure side, convex on low pressure side.”

Only one glitch here IMHO. Never flat. When one inspects very closely nature’s creations always have a radius, a taper and usually both.

Questions:

What is the thickness of a tuna fin?

How about a shark pectoral fin?

How about a dolphin pectoral fin?

How about an Orca pectoral fin?

I don’t have info read on the others species the series of questions but we have dolphins and orca in capitivity so that shouldn’t be too hard to find out. As for the sharks there are lot’s of them, some quite fast, some quite slow. The faster ones have more high profile fins and all them are relatively thin. The slower swimmer’s fins have more full outlines. As for the tuna a 20lb albacore’s pectoral fin has a root cord of 33 mm., a vertical cord of 235 mm., and a thickness of 1 mm. There’s a serious thickness to cord ratio for you. I ate the sushi but I still have the fins that’s where the numbers come from. The outline of one of these pectoral fins is one that has a very slight cutaway at the base and tapers very gradually toward the tip which is pointed. Rake is not measurable because the fish can change it at will. Camber on either side of the fin can be changed and undercambering on one size will be reflected by overcambering on the other, thus changing the effect lift of the fins and allowing the fish to ascend or descend abruptly It seems to me that with the very narrow porfile of the fin that camber and attitude adjustment are the principle ways in which direction and speed is controlled. Though I don’t think a tuna has much need for brakes very abrupt direction change is how it is affected. Once a speed is attained it is optimized by laying these fins into a perfectly formed depressions in the side of the body thus making them dissappear completely from laminar flow. Obviously this combination works extremely well. By comparison a surfboard is a pretty crude approach to maximizing performance.

I have borrowed leading arc from this fin to use in my “spinner” template. It is the one I use as a center fin in several of my higher performance boards. I’m sure it’ll be one I’ll test as a trailer in my quad experiments and has some possibilities as single on some specially configured boards. I continue to new ideas from this fin.

When it comes to asymmetrical foils I contend that putting a flat surface on one side of the fin and making it fat so it will create more lift is a pretty gross approach to high performance but what do I know?

Good Waves, Rich