Can the quobba fin concept work 4 wsurf?

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8 years ago

General Gps & Speed Waves Foiling Gear Reviews Lost & Found WA NSW QLD SA VIC TAS

fangman

1906 posts

fangman

WA, 1906 posts

8 May 2018 3:57pm

I just had a read of the papers cited:
The first : El-Atm, Billy and Kelson, Neil A. and Gudimetla, Prasad V. (2008) A finite element analysis of the hydrodynamic performance of 3- and 4-Fin surfboard configurations. In: 9th Global Congress on Manufacturing & Management, 12-14 November 2008, Gold Coast, Australia.

Credit to the authors for noting that the influence of the flow at board/fin junction is still guesswork. They simplified things by removing that variable, but in doing so have added a degree of uncertainty to their findings.
"The default upstream turbulence intensity of 5% was used in the absence of experimental data to determine this value. In order to eliminate the turbulence influence on the flow around the fins from the underside of the surfboard, boundary walls for the fluid domain were set at free slip conditions."

The paper references Nick Lavery and Dave Carswell's original works and I am not sure it adds much to our understanding of fin performance. It does compare the performance of three and four fin configurations of a NACA 0006 foil, and in itself is an interesting read, but does not appear to substantiate any of Quobba design attributes.

eprints.qut.edu.au/16824/1/c16824.pdf

Of lesser importance is the other paper - it's a general piece on hydrofoils and its relevance to the Quobba fin is tenuous at best.

decrepit

12872 posts

decrepit

WA, 12872 posts

8 May 2018 5:37pm

Ian K said..
>>>Only ever heard of thrusters being credited with extra grip in turns up until now.

That's why I started experimenting with extra side fins back in late 60s early 70s. But I didn't twig to the need for toe in and they were a pain to turn..
A single fin can come out of the water, if the back is too wide. but can be used to give good acceleration when you push on it coming out of a turn, (especially if it's raked back and has flex). Twin fins turn from the back, you can't really push on the fin. The thruster was the answer to this, had the grip right on the rail where it can't come out of the water, and a center fin to push against.
This is where the extra power comes from, the riders muscles, pushing against the fin in the turns.

When my step son was learning to surf I made an extra maneuverable skate board, it could turn 360deg on a footpath. By pushing against the turns, without scooting, you can keep it moving even on a slight incline. I said to him, the better you can keep that moving, the more power you'll have surfing.

Simon100

QLD

490 posts

Simon100

QLD, 490 posts

11 May 2018 11:19pm

donut4u said..

Simon100 said..
I didn't really look into it but the explanation in the add sounds like they are trying to sell a perpetual motion machine. There's no free energy . No matter what magic you used to create a low pressure in front the energy is going to come from somewhere.

Is this serious? long term Seabreeze stalker just jumping on for a comment 'cause of a 'what the' moment (am too busy usually doing that sciency (inc. physics) type stuff)?

These are surfboard fins for surfing waves. What what would you call the energy from the wave? I'd call it free. What you're suggesting is the board sits on the surface of the water and propels itself forward by the fins ??

Aircraft fly because their engines force it forward through the air (i.e. the input of energy and definitely not free). It is kept in the air by lift. Lift is generated as air flows over the wings and an area of low pressure is created above the wing due to the flat foil shape compared to the pressure of the air flowing under the wing. This pressure differential causes 'lift' as (like water) air is always trying to equilibrate pressure so air (+wing) is 'sucked' towards the area of low pressure above the wing. This is aeronautics 101. If the engines aren't working, there is no forward movement and no lift generated.

It seems pretty obvious to me that the wave provides the forward motion like the engine does for the aircraft. As has been known with thruster side fins on shortboards for decades, a small amount of lift is created by turning the side fins in slightly (3 degrees or so toe from memory). These side flat foils which are shaped like a plane wing trade off a little bit of extra drag (by the fin being on an angle instead of straight) for a small amount of lift in an outward/slightly forward direction which as a net total provides a small amount of additional forward speed for the board thanks to the lift. This is well understood in the surfing and physics world and where they collide - i.e. not contentious or questionable. This is why just about every single three-fin shortboard for the past 4 decades has side fins arranged this way in the (hence named) 'thruster' arrangement.

I saw some fluid dynamics flow charts around one of these qobba fins on another page of this site and it seems to me that they have just managed to find a fin shape that generates more lift in a generally forward direction (through a larger lower pressure area created at the front of the fin due to the shape) than what is provided with the standard flat foil shaped side fins of a thruster set up that nearly every surfer uses every time they surf a wave.

Not magic. The science in fluid dynamics around a forward moving flat foil is well understood. They just appear to have a generated a better result from a different shape. Not surprising considering how little fin shape has been changed by FCS and Futures who sell 99% of surfboard fins. I don't have a spare 230 or I'd give them a crack at least on my surfboard. 2.5mill is quite of bit of $$ but there's 10s of millions of surfers on the planet, most with more than one board, so they only need a bit of the market to get their money back. More likely, the Aussie company sells to some cashed up buyer (cough..FCS..cough) and they retire on the sale.

I for one love new tech and the sciency stuff behind it. If it works, good luck to them. If it doesn't, the market will let them know.

Sorry ive been really busy doing sciency physics things but finally got a chance to look at their site .
i guess if you were going slower than the water in the same or similar direction of travel you could get a low pressure at the front of the fin thats about the only way the fin is going to create a low pressure in front of the fin isnt it ? I have no idea what your on about with the plane but almost any shape wing will create lift/ fly on the right angle it doesnt need to be an airfoil shape. 3 fins will always be slower than 1 but like other said there is heaps of reasons they angle them Unless im missing somthing on there page they have revised there claims to : 1, the faster you go the faster you go >>> well yes that is definitely a true statment
2, the water behind the fins changes direction >>> 100% will happen if it didnt you wouldnt bother putting them on the board and what a bold claim it is definitly worth the indipendant assesment
3, some fish swim fast becuase there shape lets them >>> yes very observant

So no claims really , maybe they surf really nice and people do like sciency references with graphs and technical looking things so thats just normal marketing . I know nothing about surfing the closest i have done is when i sheet out on big swell and prentend im surfing but im guessing the physicy science stuff is similar to the rest of earth . Also some times im wrong i thought that down wind faster than the wind thing was a scam turns out it completely works and the logic trick that got me is that the blades are going cross wind.

decrepit

12872 posts

decrepit

WA, 12872 posts

11 May 2018 9:45pm

Simon100 said..
Also some times im wrong i thought that down wind faster than the wind thing was a scam turns out it completely works and the logic trick that got me is that the blades are going cross wind.

I thought that was because the wheels are driving the blades, not the blades driving the wheels. That's why it starts off so slowly. Initially, there's almost an equilibrium, the drag downwind is only a fraction greater than force on the blades trying to push it upwind! But drag wins, and blades start to rotate, creating a breeze that's pushing against the wind. So in this case, the faster it goes, the faster it goes is true. I'm not sure what limits top speed, has it some relation to wind strength?????

Ian K

4169 posts

Ian K

WA, 4169 posts

12 May 2018 9:13am

Simon100 said..

. Also some times im wrong i thought that down wind faster than the wind thing was a scam turns out it completely works and the logic trick that got me is that the blades are going cross wind.

Embarrassed quite a few of us.

Not so much cross wind but off the wind. Some part of the propellor blade will be going downwind at 135 degrees, in an exact parallel to speed sailors! A speed sailor going off the wind at 135degrees, at more than 1.414 times the windspeed has a VMG downwind faster than the wind. A propellor is just a clever device that eliminates the need to gybe.

Could an AC foiling cat tow a water skier down a course on a long rope and put in quick enough gybes such that the water skier could ski a straight line downwind ? If you called the water skier "Skipper" would that count as sailing directly down wind faster than the wind?

Simon100

QLD

490 posts

Simon100

QLD, 490 posts

12 May 2018 1:28pm

Ian K said..

Yeah i saw one explanation that was a boat gybing within a box which is the same as that , i still dont fully understand it but kind a feeling of whats happening haha . Amazing how something so simple can be so hard to fully visualise.

jn1

2753 posts

jn1

SA, 2753 posts

12 May 2018 7:14pm

Ian K said..
A speed sailor going off the wind at 135degrees, at more than 1.414 times the windspeed has a VMG downwind faster than the wind.

Where does the sqrt(2) come from ?

Ian K

4169 posts

Ian K

WA, 4169 posts

12 May 2018 6:46pm

jn1 said..

Ian K said..
A speed sailor going off the wind at 135degrees, at more than 1.414 times the windspeed has a VMG downwind faster than the wind.

Where does the sqrt(2) come from ?

45 degree triangle, one of the short sides is aligned with the wind.. You're sailing the hypotenuse, You and the wind have to get to the 3rd side, the downwind side, at the same time. You've got this much more distance to travel.

sqrt (2) = sqrt( 1^2 + 1^2)

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