Original post unedited. The word “work” should have been “somebody’s word for it”
BTW the barrage of questions in post 152 (and in at least one previous post) is also know as the Socratic Method. Ask questions to allow the audience to arrive at the answers and grasp a concept on their own.
How much are we talking about here? If I’m trimming at say 20kph just how much drag is the hull generating and how much drag are the fins generating? Lift is about 850N - my weight plus board and suit.
So this does not get buried as a non-sequential post, I am reposting it in chronological sequence.
I believe this post says all that Stoneburner has been trying to explain. The fins only add speed when turning, or being weighted on. In a pure glide, where the water is just passing by with equal pressure on both sides, they would be slowing the board down. That’s where the finless boards have an advantage.
My neighbor is a retired professor of hydrodynamics at UH. He had a company that designed oil rigs for the north sea and ships that serviced those rigs. He’s also the leading designer of modern SWATH ships. We have talked about this in the past and he’s said that it would be hard to design the perfect fin or surfboard because of all the variables. This also applies to propellors on ships, and for other water craft, you can make a great craft for flat water, or for riding certain kinds of waves, but not one that would be best at all conditions.
I don’t think you can argue against the laws of physics, and the science.
I have not done that. Surfboards RARELY operate in pure glide @ zero AoA in still water, where a higher drag coefficient fin would retard speed. And not all fins perform the same either. This thread was intended to stimulate thought, not to stimulate Stonebrains Flat Earth Mentality. Which it unfortunately did. Your fin is a hydrofoil, flying through a dynamic liquid, while your surfboard is planing at the air/water boundry layer where it responds to gravity to propel it on the slope of the wave. A higher lift foil WILL increase drag, but the higher lift also INCREASES speed through a wide range of AoA’s. When the two are combined, that is where ‘‘increasing drag to make your surfboard go faster’’ comes into play. A surfboard fin is almost ALWAYS operating at some degree of AoA. That is why the foil of a fin is so important.
“Surfboards RARELY operate in pure glide @ zero AoA in still water, where a higher drag coefficient fin would retard speed.”
Bill you are correct in the above statement, but I’m not sure why you added “in still water”.
There’s still a lot of people riding straight in. In that case it probably doesn’t matter because that’s mostly beginners.
I do believe that there are times when you are gliding along on a flat part of a very small wave between sections where the fin has very low AoA, and if anything is not aiding the forward speed. I usually get right up on the nose and try to carry as much momentum forward to get to the next part of the wave where it will pick up again and give you another nice long wall to play with. Trying to turn here to get moving faster does not work, a huntington hop might. I like to do what Miki Dora used to do and shuffle my board from rail to rail while standing way up forward. I’m usually riding a longer board like between 7’ and 8’ or longer.
I think that what you’re trying to get at is the proper foil isn’t always on a fin with less drag. The fin’s shape and foil make a tremendous difference. I don’t think that every surfer would benefit from a single combination of the 2. It would be a bit unique to each person, the boards they’re riding and the waves being ridden. Someone used to turning with a tremendous amount of force will require a different foil/outline than someone who is used to finessing their way through turns with less force.
I’m seeing a lot of fins these days with combinations of foils on both sides of the fin. The days of a flat sided fin may be gone.
The ‘‘still water’’ reference was made because that is the extreme circumstance where the ‘‘fin drag’’ would be most easily observed. A thick foiled fin, can be turned with tremendous force as well as with gentle finesse, and be outstanding in both applications. In still water, a moving surfboard has water moving over the fin ONLY as a result of the motion of the board. That is the ‘‘relative flow’’ and is ALWAYS parallel to the mid line of the board, even when riding on a wave.
“A higher lift foil WILL increase drag, but the higher lift also INCREASES speed through a wide range of AoA’s.”
This was what seemed apparent to me looking at all my mass-produced fins. Most have fairly sharp leading edges which do not work well through a wide range of angles of attack. Those thinner fins with sharper leading edges, as soon as you get them past a certain point, likely start creating massive amounts of drag as demostrated in the previous videos of plates and non-streamlined shapes. You could likely just sand your current fins to make the leading edge less sharp and gain some performance for the same reason, better performance through a wider range of AoA.
But, by increasing the thickness of the fin you can get more lift, but also less drag through the concept of streamlining. Made possible by the fact that current fins just have shitty foils, and there are performance gains to be had by increasing their efficiency.
“I’m seeing a lot of fins these days with combinations of foils on both sides of the fin”
Sharkcountry, I think that is for the reasoning above. A proper foil will not have a flat side and a sharp leading edge, so we’re just seeing a slow transition to proper airfoil shapes.
Bill my brother Bernie experimented with thick fins in the '70s after seeing the article Mike Hyson did about the Dolfin design. He made a 1" thick fin with the dolfin outline and foil from foam. He also experimented with ultra thin fins based on the Jeff Ho flex fin design. We are firm believers that a thick fin will work well. It’s all about the foil. While he is very cerebral about the science of surfing, I’m not. Bernie went to college to study physical oceanography, I worked nights and weekends and surfed a lot. Bernie has a ton of fins with all sorts of outlines and foils. I just stick some fins in my board and surf. The only exception would be the Griffin boards I use, they all have custom fins he made and I only use those fins in those boards.
I haven’t been very picky about fins since the thrusters came out, but one fin I really liked in the mid '80s was the bong fin Downing was using on his boards, I thought they worked well. They were glassed on, so I think they had thicker foils than modern box fins, but I gave that board to a friend years ago, so I can’t say for sure. They had a bulb on the tip similar to a ships bulb and that helped to reduce cavitation. I always thought that board was great.
Robin has been making some interesting fins for several years. He was one of the principals in the design of the Probox. I have a set of his quads with the oblique fronts and hammer rears in G10. Love those fins, beautifully foiled. He has newer designs these days.
What is a higher lift foil? What does it look like? I work in both marine and aerospace and some of these concepts could be worth a large fortune.
Seriously, fluid dynamcis is fluid dynamics. The science has all been done. Its just a matter of breaking down the vectors if thats what you want to do, but I don’t see anyone doing that with any rigour. But surely if someone is claiming a game changing development based on scientific principles they should expect to be challenged on it.
I’d suggest that a more scientifically and technically correct phrase should be fin efficiency rather than high lift fin.
In general, when people (myself included) refer to a high lift foil, I believe they mean there is a higher camber to either one or both foil (fin) surfaces. This usually means a thicker foil, of necessity, is the result. Increased lift also (in general) is accompanied by an increase in drag. Foil efficiency will determine how much drag. As you know, some foils are more efficient than others. I think both terms have a place in the discussion. I’m not so much concerned with scientific rigor, and nomenclature, as I am in conveying the concept or experience under discussion. I’m willing to accept a slight increase in fin drag, to reduce or eliminate fin stall, when making high AoA maneuvers. This allows more speed to be carried throughout the ride. I am not in disagreement with your suggestion, just trying to bring some clarity to the topic. Thank you.
These are reasonable statements that clarify.
Premise:
A fin design that increases drag could reduce speed loss overall.
So in other words you are happy to trade a bit of straight line speed for the abilty to carry more speed with a bit more control through turns?
Personally I think sematics and nomenclature are vital. In aĺl walks of life the abilty to communicate and discuss ideas and it is having a common language that enables us to do this. When complex and counterintuitive scientific, technical and engineering concepts are being debated I believe this common understanding is even more important to avoid confussion and misunderstanding.
More food for thought. I think multiple fins change the overall AoA affect. I know that the 5 fin setup Griffin uses does quite a bit to add lift in the tail and the board seems to flow through turns with better speed and hold. I also noticed that the offset fin arrangement George Gall’s assyms use help make the board paddle with less drag, and the board also has a good feel when turning. I think Herbs superchargers and the little fins on the C5 boards and twinzers affect the flow of water on the main fins positively.
So maybe the downsides of a fin’s foil can be enhanced by adding more fins in the right places. I noticed that you had a gun with 2 fins inline. I’m sure the reason was the same.
At the Groundswell Society’s presentation during the Sacred Craft Expo in Ventura a few years ago, Glenn Hening presented a discussion of surfboard fins and speed effects. He had a board there with long, high-aspect ratio quad fins on it that he claimed proved to be faster than any other combination they had tried (on good waves).
That is not what I said, at all. As to terms I may use, if you do not understand or grasp my meaning, I’ll be happy to define my terms. Fair enough?
I think that’s probably true. In the case of the Gun with the Pre-fin, the pre-fin performs the same function as a leading edge flap or slot, on an airplane wing. It allows a very high AoA without stalling. The main fin on that Gun is the ‘‘normal’’ 3/8th inch thick. That’s why I put a pre-fin on it, to overcome the deficiencies of the thin foiled main fin.
Mr Thraikill I may have my head around what you have been explaining in this thread, because i surf a lot and know that the wave is pushing my board and fin together. While fins or a fin are good for turning to gain speed the wave is also pushing against your fins thrus propelling you foward faster than just graviety and the up and down with fins draging against the water rushing up the face, as explained by science.
I have been having a similar dicussion with an older (than me) Fin guru, and it goes like this, Him “My Fins do not obey the laws of Physics”. Me “blank stare”. What he really means i figure out later is that the laws of hydro dynamics dont really apply because surfboards dont go fast enough. This is very hard to deal with as a scientist who has a vauge understanding of the principles. When he backs it up with “I have been doing this for over 20 years and my fins work so belive me when i tell you”. This sort of banta has cause a few arguments, and lessons about how someone can learn a phrase in the indusrty that is very diffrent to what is taught at universitys around the world. We have excepted that we are both wrong and have moved on. i.e what a Chemical bond is.
But i do think we got a bit off topic here, which was originally about flex and drag. My old fin mate is all about flex, swares by it, if i show him a perfectly foiled fin with no flex he says it is no good. I would really love to test these theories out and have the resources to conduct some sort of scientific testing.
I have been thinking about making 3 - 10" Greenoughs, one with No Flex maybe make this one 15mm thick. One Normal 10mm Box and one superflex, all same shape just foiled thinner thur the tip for the flexy ones. While i Dont ride single fins much i can find some pretty decent long board riders for the testing.
I may very well do the same thing with 7.25 Greenoughs and a set of side bites on smaller boards. which fin do you think will work better?
Any chance you could give us some hints about that old secret foil of yours? Because one of the things about Science is that we can not accept or test Secrets and be still be creditable.