Ok. I am continually hearing about and reading comments here referring to fins creating lift... either too much or not enough, and have heard varied and sometimes unusual explanations of what lift is. So, am wondering what is it exactly when referring to sailboard symmetrical find?

Not sure of the exact definition. But my understanding is, it's a force at right angles to the flow of a liquid over a surface. In the case of a fin, this force counteracts the sideways thrust on the board. Too much lift usually means, the rider can't control the rotational effect of the fin pushing sideways underneath them, lifting the windward rail. Not enough lift means the board goes sideways to a certain extent, and if the angle of attack gets too great will lead to spin out.

So if the fin has the right amount of lift, it will help the rider keep the windward rail up, allowing the board to produce aerodynamic lift. Reducing wetted area.

A symmetrical foil needs a positive angle of attack to create lift, the greater the angle the more lift, until you reach the stall angle then you loose all lift.

joewindsurfer.blogspot.com/2008/04/jim-drakes-windsurf-physics.html

howtowindsurf101.com/physics-of-windsurfing/

upward force generated by movement = my best guess (excluding hot air , helium and drugs here)
because gravity sucks :-)

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decrepit said..
Not sure of the exact definition. But my understanding is, it's a force at right angles to the flow of a liquid over a surface. In the case of a fin, this force counteracts the sideways thrust on the board. Too much lift usually means, the rider can't control the rotational effect of the fin pushing sideways underneath them, lifting the windward rail. Not enough lift means the board goes sideways to a certain extent, and if the angle of attack gets too great will lead to spin out.

So if the fin has the right amount of lift, it will help the rider keep the windward rail up, allowing the board to produce aerodynamic lift. Reducing wetted area.

A symmetrical foil needs a positive angle of attack to create lift, the greater the angle the more lift, until you reach the stall angle then you loose all lift.

That... is the best explication of what I think and have thought 'lift' is. Have heard some very different ideas over the years, so was interested in what were others' perceptions.

Also a long fin can flex a lot , so I would think creating upwards lift also ???

Foils give you tremendous lift.

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Imax1 said..
Also a long fin can flex a lot , so I would think creating upwards lift also ???

Yep, agreed as long as it's fairly upright, a flexy raked fin will give downwards lift, that's why weedies should be stiff.

"downwards lift" - is that an oxymoron ??

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joe windsurf said..
"downwards lift" - is that an oxymoron ??

You are clearly confused.

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joe windsurf said..
"downwards lift" - is that an oxymoron ??

The lift we're interested in here, isn't necessarily upwards. it's the force at right angles to the foil. If the foil's angle of attack is downwards, that's the direction of the lift.

But there's some upward component that comes into play, too. It's easiest to see in the videos of formula fins which can bend almost 90 degrees. The lift created by the foil in the bent sections points upwards. With shorter fins, that is obviously less pronounced. But on my short and fat Black Project RE Delta fins, I noticed that I plane earlier and better if I push the leeward edge of the board into the water. That gives the lift vector an upward component, and it feels like the board is lifted up some.

Assuming a non flexing fin, if there is no lateral force (board and fin running straight with no foot or mast foot pressure pushing sideways, there should be NO lift. However, that rarely happens, so as forces are applied to the board, it is pushed slightly sideways, causing the water to pass by the fin faster on one side than the other , resulting in lift like an airplane wing. A small fin just might spin out, but a large fin may want to rise to the surface. In the past, this issue was most prevalent when racing longboards, wanting the maximum upwind ability with the largest fin possible, without too much lift causing the board to "turtle". The boards were narrow compared to many of today's boards, so it is much easier to "turtle" a narrow board compared to a wide board. I had a course slalom board for racing where I used three different fins depending on the conditions (wind speed and sail size). Formula board can carry 70cm fins because of their width, but they can "turtle" or take off with enough wind, so they too may choose smaller fins for windy conditions.

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joe windsurf said..
"downwards lift" - is that an oxymoron ??

Downwards lift happens in the Southern Hemisphere

I think Leo's got it

Looks like takeoff from the little wave behind him.
This guy's got lift!
www.facebook.com/Kooklife/videos/2053147801617885/

Mikey, This video doesn't exactly explain why, but it does explain how to do it.

www.bing.com/videos/search?q=flying+the+fin+windsurfing&&view=detail&mid=FF879B2E610D291FCBADFF879B2E610D291FCBAD&FORM=VRDGAR

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decrepit said..

joe windsurf said..
"downwards lift" - is that an oxymoron ??

The lift we're interested in here, isn't necessarily upwards. it's the force at right angles to the foil. If the foil's angle of attack is downwards, that's the direction of the lift.

The foil's angle of attack has to be in the direction of the flex otherwise it will unflex. ( maybe that's reflex?)

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Ian K said..
The foil's angle of attack has to be in the direction of the flex otherwise it will unflex. ( maybe that's reflex?)

Good point Ian, that's the trouble with half modeling something in your head. So with an upright flexi fin the angle of attack can increase the flexing pressure and create that vertical lift. But with a highly raked flexi fin the side ways pressure and water flow are pushing the fin in opposite directions, so there won't be very much downward lift if any

Well this is an interesting one - I always thought of 'lift' as upwards force from the fin:

Smaller fin = less lift = later planing, more board in the water - better control at higher speed.
Larger fin + more lift = earlier planing - less board in the water - less control (turtling) at high speed

Maybe it was all just in my mind.....?

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sinker said.. Maybe it was all just in my mind.....?

Not at all, the effects you mention are very real, but not because the fin is producing upwards lift. Because the fin sideways lift is below you and the sail sideways lift are above you and in opposite directions there's a big rotational force on the board, a combination of board width and sailor weight counteracts this force. more fin lift will raise the windward rail producing aerodynamic lift as well.

Maybe a stupid question.
As a fin is tapered from thick to thin from top to bottom, is there any upward force generated by the water pressure trying to push the fin out of the water.
Say if the board was towed by a boat without any sideways forces from the sail, or any lift from the side ways forces, would the board rise up to any degree on the fin ?

Many of the older fins from 10-20 years ago have a thicker foil producing much more lift than current fins.

I tried a 20 year old 36cm Leading Edge B4 fin the other day in my 65 cm wide slalom board.

It produced a lot of lift - but I did not feel I was going very fast.

I then put a 32cm G10 Volt in and it felt a lot more slippery,producing a lot less lift.

The narrower the foil in the fin,I'd say the better it will be in really powered up choppy conditions.

My carbon 38cm Volt has a very thin foil producing very little lift and it sort of feels like the board is going a bit sideways.

Speed produces lift - so when the wind picks up this fin feels much better and has exceptional control.

Thicker foiled fins make trimming the board a lot more difficult as the wind picks up.

In 10 -12 knot conditions I would try and use a fin with the thickest foil.

Interestingly, early surfboards just had a FLAT piece of ply as a fin. These were later 'foiled' to try and stop them cavitating and sliding out at speed... and creating heaps of drag. Just over 30 years ago I put my Windsurfer rig on an old mal of mine so I could have a 'shortboard'! It worked ok with the flat, ply fin, though the fin did tend to let go. The foil, as I can determine, is primarily there to keep a smooth flow of water across and behind the fin creating less drag.
I believe what truely makes the board 'fly' is the lift from the air rushing under the board. Wind speed cms above the water surface is negligible, so a board doing app 30kts has about 30kts of wind coming under the board, making it want to fly up... but your weight (and that of the rig etc) is holding it down. If you can balance these then it's a magic carpet ride.

From what I know of aircraft foils...
-thick foils hold flow better at slow speeds.
-thin foils work better at high speeds.

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decrepit said..

But with a highly raked flexi fin

I can imagine that an upright fin can flex but the angle of attack can remain pretty much the same from top to bottom. That means for a given expected top speed and lateral lift requirements you can choose a fin of an area such that it'll be operating at an angle where lift to drag is maximum. 4.5 degrees maybe?


A flexing raked fin probably has a different angle of attack all the way up and down. Too much angle of attack at the root and less than ideal at the tip. That's what I put it down to. Flexy raked fins just don't work.

I think Mal Wright told me once that the AOA of a speed fin at 40-45 knots was around 1 degree, but that may have been for an assy foil.

'Lift' when referring to foils, is in relation to the foil surface, a force created by the foil moving though a medium such as air or water. It most definitely is not always 'up'.

But, most fins will create and 'upwards' lift to small degree, even when running perfectly vertical due to their tapered shape. It is generally a very small force though compared with the main direction of 'lift'.

Tnx all. The general consensus is exactly as what I have believed. Some interesting and concise explanations.

What about when you're pushing hard upwind, the more force/load you put on the fin the more lift it generates?

Then you bear off, accelerate and unload the fin, does it then not produce as much lift even though you're sailing much faster, probably doesn't need as much to keep you tracking?

What about canting keels so popular on ocean racing boats now? radical angles compared to the direction travelled.

Interesting topic, this is the nuts and bolts of what we do.

In many ways sailboards have had the most efficient and advanced sails and underwater foils for the last 20 years, surely no accident.

Yep, upwind, you have high wind pressure on the sail, so the fin needs to generate more lift to counteract that force, but it has lower water speed so needs to run at a higher angle of attack or be bigger.
Down wind you have lower wind pressure, so fin needs to generate less lift, but it has higher water speed so it runs with less angle of attack or can be smaller

It's all about context. Lift is an upward force, but where is up? If the discussion is purely a 2D discussion it is all pretty straight forward. In a 3D context it is more complicated, some of the time lift is being referred in a 2D context, and others times it is a 3D context, There is no universal correct definition, you must consider the context of how the term is used.