Thick fins vs. Thin fins

From: http://www.tomwegenersurfboards.com/fins.htm

The giant difference between my light wood fins and fiberglass fins is my fins create positive buoyancy and move the board up and forward, while fiberglass fins sink, creating negative buoyancy and pull the tail down. In January 1999 I made two Signature Models as close to each other as I could. One had my wood fin while the other had a box with a big fiberglass fin. Several times Col (Team Rider) and I rode both boards in the same conditions, and we agreed that the wood fin had much more drive and speed. I had the same experience when I was developing the Crusader Model. On one of the test boards I had a box and in another I had a balsa fin. The board with the box was much slower. I soon crushed by box by doing bottom turns in big surf with a 12” fin.

I make my fins about ¾ of an inch thick. The thicker fin feels better in small to medium sized surf. I have not experimented with this in big surf. Water sticks to the fin and cavitation is much less likely when there is some curve to suck to. The boards go in and come out of turns a bit better and are livelier. Also, they are not so affected by whitewater. Cavitation is when air pockets form along the sides and back edge of the fin. When a fin is cavitating, the board reacts sporadically and becomes hard to control.

I’m having a hard time putting all this together, Meecrafty. Does the thick fin push more water out of it’s way like retroman suggests, or does it stick to the water with increased surface area, or does it allow for better angle of attack or does the thickness just feel more natural down in the water. Light or heavy, is it still a matter of feel, or as Tom has suggested in your quote, a measurable increase in performance (personal?), but how can we be sure that as a blurb the meaning is unbiased. Halcyon has suggested that thin might be the way to go as the performance scales higher. And I can’t discount the validity of superchargers and vortex generators, after going through some older archives. Now does all this make an argument for an increase in stickie or release or flow or controlled turbulence or cavitation or whatever you guys call it. Can you quantify this or are we still musing?

im no expert but i’ve read enough…foil science is not black and white but generally speaking, a fuller foil, particularly around the leading edge, deals with surfing’s large AOAs much better…as you say it ‘sticks’ better, helps to wrap the flow around the rest of the foil better…flow seperation is the killer…thats why golf balls have dimples, keeps the flow attached (forget about the dimples in fins they create small turbulence). In essence, less flow wake, less flow disturbance, lower drag. Extreme flow seperation is called stall.

Get an old garden hose and cut off the end fitting so that the water flows out of it smoothly then put it on the bottom of your board in front of your fins to simulate flow…on a thruster rail fin the flow is generally at an angle from the stringer towards the rail, thats your AOA while rail surfing and it varies tremendeously. Watch what happens to the water flow on the backside (low P) side of the fin…if it seperates easily you got drag…not the greatest test in the world but its almost free…im on a well here and the waste water goes back into the well during my tests…im a much better tester than i am a debater…i talk softly and carry a big test stick…

PS - you’ll find better info (not just opinions) wrt foils on google

Dave,

KUDOS on your post #163. That is an excellent, simple, and CLEAR summary of the answer to the question. The benefits of thick fins are magnified in larger faster waves, based on my experience.

Believe me I don’t want to spoil your party, but hear me out. I’ll do my best to share my insight, and I promise to never post on your fin threads again.

Speaking of your google, this is an observation I made since I didn’t need diapers, but isn’t the water coming out of the hose different than the wave form energy that moves through the OCEAN that WE move through? At end of the ride I settle right down for a nap in the crowd, or if I stray to far from the pocket my rocket becomes a barge and I’m wishing that my fins were an engine like on the back of Flippers tail. Which one of you is brave enough to hand Laird a fin and say to him that you tested it with a circumcised hose? Sounds like a trick you’d pull on a gremmie.

Have you priced a set of Halcyon’s fins(?), but I’m willing to pay, cause to an artist the difference in brushes, example - a fine sable or disposable, is quantum. (Spiritual earth - Aloha Waves Nalu Pu Loa - Denjiro Sato - for the indide of a wave) (Second Thoughts - Timmy Turner for the outside look)

Welcome to the world of sleepless nights, if you dare? Or, it’s a twang thang, baby. Up and down motion can create a wave which amplifies and oscillates. If you accept this premise then does thickness dampen or amplify? The laws of nature were made to be broken but it’s a matter of degree. How do you get a horse to move or get that board glassed by a lazy worker? Crack that whip. With a flick of the wrist just a little up and down motion goes from thick to thin and what a speed increase! you’re breaking the sound barrier. OK, you can power a horse but power a boat? The answer is back to google scale. Sperm, cause the minimal effects of gravity, can whip their tails and head for that egg , but at our scale all you have is sushi.

Common features - only difference is in the DNA. Do you participate or does your wife do all the work? She may be the genius in the family if she works with mans first quantum leap. FABRIC. Clothe the mass. Rug on the loom, movable tent and sail the seas. Even cover foam surfboards. I know the answer to this, only cause my wife has asthma, so I shake the rugs. How can you amplify that crack the whip thing I was first talking about to gain power instead of only speed? (Read that book about Chaos that I gave as a suggestion before). Or maybe you’ve shook the sheets with your lady and if she’s smaller than you she put more speed in the shake(?) and didn’t it rip out of your hand with surprising force, when your shake frequency was more leisurely than hers? So you said wait a minute here, you tightened your grip and then it happened again. So you tried more force and it still happened? Woah, down on your knees and bow to the magic sheet that overpowered man. One more try? Just shake your sheet faster than your partner and she’ll lose her grip.

We’re all related. NOW look at Greenough’s spoon - FLAT and STIFF and THIN with that (trailing) straight rug edge. He said it was to, “Handle the horsepower.” Hey wait a minute there’s more… Look at his paddle fin - straight trailing edge? Get the idea? Look at Simon’s thruster… Or Simmon’s balsa. Or “Why the concave deck on your woodie, Bert.” Or why did your honey comb longboard break with that gunshot sound? Or look at a tuna tail. Why did I ask and why did I open that door? It’s starting to gell, I can feel the tide. Breathe… I think he’s gonna pop!

I’m spent. And here’s the kicker they prefer Greenough’s mat…

Why, Why, Why? What if your tail rail unfolded a membrane that filled like a half-pipe hose? Would the water increase the relative or apparent weight of your tail and additional curved surface hold you in? Then, zip up, tuck that heavy hose back in for your speed run? Where’s the tank to test this? Tag, you’re it.

Which one of you is brave enough to hand Laird a fin and say to him that you tested it with a circumcised hose?

ha! see thats the thing…i really dont care what Laird is doing cuz I’ll never surf like him…low AOAs at Jaws…so i go back to speak softly and carry a big stick…in my case a new set of thick buoyant fins with good foil…on this coast thats the next logical step…work in progress…google rules…one of the greatest tech innovations in history…wanna learn to make a good Cuban Chicken Fricassee…just a few key strokes away…

…now where the hell is that hose fitting?

I have to echo the Google love. I just installed the google toolbar in firefox and discovered the “search highlighted text” feature. Its amazing!

Which one of you is brave enough to hand Laird a fin and say to him that you tested it with a circumcised hose?

Aggressive Sales/Marketing 101

Sales Rep: “here you go mr.laird…the fins we suggested you use”

Laird: “hm…these are kind of fat and funny lookin’”

Sales Rep: “Well yes that’s what makes them work so good…theyre laboratory tested y’know”

Laird: “really, how does your test work?”

Sales Rep: “well I’d love to provide that information but our test methods are state of the art and are very proprietary…nothing personal, its just business”

Laird: “wow…sounds impressive…I bet they’ll work on this board over here…”

Quote:

……I’ve been hacking on the fraction of the rider’s weight supported by buoyancy (water displacement) and the fraction supported by planing forces…

As a first approximation (rectangular wetted surface, no rocker), the buoyancy force becomes equal to the dynamic lift (planing force) at the speed, V, where:

and where:

V = speed at which buoyancy lift equals dynamic lift

L = wetted length of the hull

g = gravitational acceleration

CL0 = lift coefficient slope for the wetted area of the hull

AR = aspect ratio of the wetted area of the hull

** Ooops. Just noticed that I forgot the exponent of 2 on the lower-right aspect ratio that appears in this equation. Namely the “4+AR” should be replaced with “4+AR*AR” (as appears in the equation below). Sorry about that! **

At higher speeds, the dynamically generated lift dominates over buoyancy; at slower speeds, the buoyancy dominates over the dynamic lift.

The first square-root is essentially a Froude Number based on the lift coefficient. The second square-root corrects for the finite aspect ratio of the hull. The lift coefficient slope, CL0, for the wetted hull that should be used in the equation above is defined via the lift equation:

where:

L = lift force generated by dynamic lift/planing (sorry about using the same symbol for two different things)

rho = density of the fluid (sea water)

alpha = angle-of-attack of the hull relative to the local sea surface

Area = wetted area of the hull

Hope this helps,

mtb


somehow it worries me that I enjoy reading posts like this…

Are we heading towards Savitsky here?

“Trim tabs”

The Dynamics of Marine Craft,

by Edward M. Lewandowski, pg. 380

(see link provided)

http://print.google.com/print?id=UbjvXKReIAUC&pg=380&lpg=380&prev=http://print.google.com/print?q=Savitsky&ie=UTF-8&lr=&sa=N&start=20&sig=TSJra7-qvP-VlLpslmOmLk7oVag

P.S. I don’t know if this is an example of Google’s Optical Character Recognition technology in action, but I sure like it.

Mark,

Savitsky is better known for developing basic equations related to drag (power requirements) of planing hulls, surfboards are more complicated, but we gotta start somewhere…

That’s the understatement right there… surfboards are more complicated. And we all know they are.

That’s what I find fascinating in reading everyones comments, because underneath it all we are all right in some way.

I used a lot of experience and the relative scientific info available to me at the time, to find that when I seriously combined both I was onto a good thing. Space only knows how much better it may have felt manufactured properly, a la Swaylocks epoxy revolution.

The individual interpretation is exactly that, individual.

Not until someones pours their excess into development, will we see a truly serious proven scientific recognised and accepted design standard. I’m sure the government doesn’t really care, after all, it’s only a surfboard.

If that day ever comes, it still won’t change the fact that there are subtle pressures we all put on our boards, through our toes, feet, legs, torso, arms, head, forgot the fingertips. And every one of us is different.

We all have different interprtations of the magic board or the magic fin. I reckon you should use whatever science you like, as long as it makes magic happen.

More fun that way.

Simonac,

But are surfboards as hulls really more complicated?

My brother says so, but he is addressing the

idea of modeling the flow of water up the face of a wave and under a board carving top to bottom on it. That, I have to agree, is mystifying. I can visualize it but can’t model it. Have I said that before? I’m either having a deja vu, or a petite mal seisure:-)

On the other hand I know that there are NASA guys who surf and sailboard. I believe I may have mentioned it before that the top NASA vortex generator design engineer (F-22 Raptor VGs) is a sailboarder. I’m sure these guys think about these things. It’s just a matter of time.

I believe, you, Simonac, are deeply intent on making some kind of breakthrough contribution. YOU are not missing anything and you are right there when we need to hear what you are thinking. And you are always just a little bit ahead and a bit around the curve, almost out of view, which makes me believe you are getting some good ideas going. Believe me these ideas won’t just stand in the middle of the road waiting for us to run into them. They are hiding in the bushes waiting patiently till we come along so they can trip us up or cause us to catch a rail.

But, and here’s the only downside I can see, whatever it is it will come come from insight and not from empiraicism. The good part is, once you have the eureka, you can always go back and figure out the math. That is the way Einstein did it. He saw the fly, while on the trolley, and had the brain flash that became his paper on relativity. From there he had to learn the math to do the proof. Lucky for him, for us all, he could.

So, I guess I’m just saying keep plugging along and lets see if something doesnt spark an idea that will help us all in ways we can’t even imagine just now.

I think Wildy is right, too. I think he gave us a clue. We are all right in some interconnected way, circling whatever it is that could just be the big clue.

Daddio has my attention, too. He gets things stirred up and in that mix who knows what he’s going to say next? But if the uncertainty principal has anything to do with advances, it is precisely that it does. M

A few ideas that have been milling through my mind without a great deal of coherency I’ll admit… I’ll apologise now if many of my ideas are based on small craft naval architecture but that’s where my experience and training lie. I know some people have a dread of associating numbers with the very subjective nature of a surfboard, but we do it with everything else in life, automobiles for instance, something I’m guessing most Swaylockians are far less passionate about.

What we really want to measure is pressure and pressure distribution; this is really tough so why not take some rather old fashioned Naval Architecture as a guide.

Why do we measure boards the way we do? It seems a little strange to measure the length of a board just from nose to tail, for a nose rider where the whole length of the board is used this may be valid, but for a flip nose short board maybe only three quarters is actually in contact with the water much of the time. Might it not make sense to measure the length of a board from the tail to a point where the width is (say) 50% of max width? Or maybe to a point where the nose rocker equals tail rocker? Or maybe half way between these two points? What I’m suggesting is effectively a water line length, for any kind of water craft this tends to be far more important then overall length.

Plan area: we all think we know what each other mean by “drawn out rails” or “hip”, but how much insight would it give us to measure the plan area relative to length x width, Plan Coefficient = Plan Area / (Length x Width). A high number would reflect something like an egg or longboard, a low number a gun of retro pin tail. We’d also need to consider centre of area.

Plan shape versus rocker versus thickness. Naval architects measure something call prismatic coefficient, this is (Length x Breadth x Depth) / Submerged Volume, this is really important to anything that moves through the water, and could tell us a huge amount about any board. Again where is the centre of this volume?

Rocker? The thing about rocker is that it is intrinsically related to the shape of the wave and the attitude (technically and figuratively) of the rider.

Then we have bottom shape, rail geometry, fins, tail shape, etc etc.

I could, and probably will go on for ever along these lines but I have to get some work done, in the mean time ponder this: It doesn’t matter what we measure of our surfboards if there is no objective way of measuring surfboard performance. We all surf differently, to each other and even on a daily basis, and pretty much every wave is different. Before we look too hard into the boards themselves we need define how we measure board performance, and that is really really hard. Simon

“Before we look too hard into the boards themselves we need define how we measure board performance, and that is really really hard”

WAY WAY subjective but let me try to simplify a good starting point…

Basically you have two types of surfers: 1. high-performance 2. stylists/cruisers/trimmers

I can only authoritatively speak wrt #1 (actually i prefer smooth styled hi-perf), and the key to performance in average everyday conditions is SPEED…to me speed governs just about everything i do on a wave…too little and i have minimal performance and fun…and honestly, i dont think i can have too much, at least not in average conditions. So if i were to measure any one thing it would be the speed/drag relationship…now we’re right back to thick vs thin…

A favorite axiom of mine ( from fighter pilots) applies to surfing also, “SPEED IS LIFE”. The more you have the more you can do.

This is still and important thread.

bump

bumping so i do not lose track of this thread, have not figured out how to ‘toggle’ or track threads without placing a reply w/i the thread so they appear on my home page

Warren