foil shape and "lift"

[adam aunins]

Can someone help me with this?

I've read about different foil shapes for the center board and rudder. Now I can understand how they would have less drag with a better shape, but some will talk about how they will create more/better "lift". I know about wing shapes and how they create lift, I just don't see how a symmetrical shape can create lift and if a CB did create lift it would only be good when healed over on one side and would be counter productive when healed over on the other side. Am I misunderstanding when they talk about lift? If not can someone show me which direction that lift is "lifting?". If anyone kows or has a guess please chime in I would like to hear.

[jpclowes]

I'm no engineer (I tell people I got into my career because I don't have to count higher than 60 ) But I think it has to do with the resistance from the wind pressing on the sails wanting to push the boat sideways. The higher pressure on the leeward side of the Centerboard, works with the shape of the board to provide lift on the windward side. I think it is similar to the way a plane can fly upside down, even though the wing isn't shaped on the side that is now providing lift. By pointing the nose slightly up, (relative to the ground) the wings can provide lift.

[Adrift]

To oversimplify: Lift is not "upward" (away from the center of the earth). Lift is perpendicular to the plane of the foil. In this case the lift force is approximately horizontal (neglecting heel) and roughly to windward. We are not trying to remove the boat from the water, we are trying to keep it from blowing sideways.

Unlike the asymmetric airfoils used on airliners and GA aircraft (which tend to stay right side up), symmetric foils (like those on very high performance aircraft, ie fighters, as well as fins of sailing vessels) must have a small angle of attack to produce lift.

You may notice that your boat pushes sideways pretty easily when not moving forward. But with a couple of knots of boat speed flowing past the CB, the boat resists lateral forces quite well.

SK: Adrift (sailor, pilot, and engineer)

[adam aunins]

This is hard to do without pictures but the way I'm trying to make sence out of it now is this. with the new board shape there's already a degree or two of angle already built into the forward part of the the foils shape now if the sails force is centered forward of the CB I can picture it giveing the CB a slite angle as it goes through the water and with the added angle built into the forward part of the new board shape it would act like there was more angle of attack thus more righting force than a stock CB. Am I on track

[algonquin]

I like the way Adrift puts it "You may notice that your boat pushes sideways pretty easily when not moving forward. But with a couple of knots of boat speed flowing past the CB, the boat resists lateral forces quite well. "


The CB does have a foil like edge to cut through the water but its function is to stay inlign with the boat and counter the forces of the wind and sail trim. It does this by giving sideway resistence which results in the boat moving forward. Basically the CB remains inlign. I'm not saying it won't have a degree or two of movement because by not being a fixed keel it will but by design it aids in overcoming lateral forces on the boat and allows you to move forward.

[adam aunins]

I understand about lateral resistance and why it's needed on a sailboat,and I can see how a better foil shape will provide less forward resistance as it goes through the water. I just can't picture how it would have any more "force" (I think that's a better word than lift) in any other direction, unless it goes through the water skewed or at an angle to the direction of the boat's travel .

[Adrift]

...unless it goes through the water skewed or at an angle to the direction of the boat's travel .

But it is skewed! Unless there are no lateral forces, the boat does not go where you point it. If your destination is due north, and the wind is from the west, you need to steer 5 or 10 degrees west of north or you won't get to your destination, you get some place down wind (east) of there. When you plot your course on a chart, you take the true heading, adjust for magnetic deviation, add a factor for currents/tides, add a factor for wind drift, and steer that.

If you are doing it visually, you need to steer a few degrees upwind from your destination, from the start, or else all of your courses over the ground will resemble an inverted letter "J" with a progressively increasing turn into the wind turn as you get closer to your target.

The physics underneath...

Imagine the boat, and the board, are pointing due north. The wind is from the west. The direction of the boat's motion through the water is maybe 5 degrees (5 degrees east of north). {Or maybe it's 10 degrees. Somebody go measure it some day.}

That difference between the direction you are pointing, and the direction you are moving, gives a 5 degree angle of attack {or however much} and that provides the resistance to moving more sideways.

[adam aunins]

Thanks Adrift

That lets me "see" it in my head. I knew it wasn't to hard of a concept but for some reason I just wasn't able to picture.

P.S. HAPPY HALLOWEEN to all who read this tonight.

[Roger]

picture your hand out the window of your car at road speed...

your hand is the keel

the air moving past it is water

you slant your hand... creates lift

keel slants windward, water hits leeward side of keel and pushes boat windward...

result... lift to windward, increasing lateral resistance thereby increased pointing to windward

just like a wing though... too much slant and it will stall!

[algonquin]

I understand about lateral resistance and why it's needed on a sailboat,and I can see how a better foil shape will provide less forward resistance as it goes through the water. I just can't picture how it would have any more "force" (I think that's a better word than lift) in any other direction, unless it goes through the water skewed or at an angle to the direction of the boat's travel .

Roger - nice clear example and I'm sure most of us have had our hand out the window as you describe (until Dad told us to knock it off).

Sorry about this but I'm getting a little confused here. The original question had to do with lift and the CB.

I don't see how the center board can be skewed as you guys are putting it. The CB is not skewed. The rudder may be skewed as you are adjusting it to compensate for the wind and the desired direction that you want to travel in. The CB is traveling with the boat and is inlign with the boat and aids in providing force to enhance the foward motion of the boat versus lateral motion. The CB may move slightly because it is not a fixed keel and does have some inherent slop as a result but its function is to be inlign with the boat. The rudder is at an angle of attack (skewed) to adjust for weather and lee helm.

Did I get this wrong or am I just not understanding the question or is this apples and oranges ?

[Roger]

sorry that I am still unclear algonquin!

Okay, assume the boat (and cb) are slicing straight ahead through the water. in this case there is no lift and the boat moves slightly to leeward as it moves forward. To compensate this leeway, you move the rudder so that the boat (and cb) now point more directly towards the wind. The board is now no longer slicing straight through the water but is 'skewed' so that like your hand there is a force of water hitting the leeward side of the board causing lift to windward, thereby decreasing leeway, improving pointing, bringing apparant wind further forward, and .... increasing speed.

[Phill]

The CB is almost always moving through the water 'skewed'.

When you are sailing you may experience the slight, or not so slight, pull on the tiller in one direction or another. When the tiller tries to pull to leeward, toward the side where the boom is, this is called 'Weather Helm", ie the boat is trying to turn to weather if you let go of the tiller. If the pull is away from the boom, then it is called 'Lee hlem', ie the boat wants to turn away from the wind.

Both of these helm conditions indicate that the boat, (hull, sails, CB and Rudder is out of balance. You may also notice that this helm pull increases or decreases with more or less heel.

Lee helm is undesirable. Slight weather helm is desirable, especially upwind. Sailboats are designed and tuned by sailors to acheive this slight imbalance.

This helm feel is telling you that the boat is actually side slipping, moving through the water 'skewed' or as we sailors call it, "making leeway". That is to say you are not really traveling exactly where you are pointing.

Finally the answer to your quest. For this example the wind is NW 315 degrees, and we are pointing N at 0 degrees. With the weather helm and leeway, we are actually traveling through the water at 3 degrees. This side slipping, skewing, leeway is what causes the symetrical foil shape to create true 'lift'. The CB no longer looks symetrical to the water, but looks asymetrical. The 3 degree difference in our path is creating an angle of attack, or incidance, that is not in line with the centerline of the CB or hull. So the CB becomes a lifting wing due to this asymetrical, off center flow. This Lift is perpendicular to the plane of the CB. Just as an airplane's horizontal wing lifts vertically, our veritcal CB's create lift horizantaly.

For leeway example, try sailing with the CB most of the way up on a beam reach and sail in a straight line. You should feel fairly strong weather helm. (strong in my opinion, subjective feeling). After sailing a bit look at your wake you will notice it is not a straight line directly behind the boat, but a staight line aiming a few degrees toward the wind. direction.

None of this works if the boat is not moving, like an airplane sitting with no airflow over the wing. Faster movement = higher lift for both wing and CB.

ADRIFT explained this same thing, just differently. Only difference might be that he ends with the phrase "resistance to moving more sidways".

This may be confusing: the only time that pure sideways resistance is occuring, is when we are stock still, no movement. As soon as we start to move, the CB and rudder are 'flying', either creating more or less lift based on angle of attack (incidance) and velocity of the flow over the foil, or not flying with a stalled flow over the foil. This causes airplanes to fall and sailboats to move very badly through the water.

This whole "physics" thing with sailboat CB's and balance is the sum of many varied vectors:
lift and drag of the sails
lift and drag of the hull
lift and drag of the CB
lift and drag of the rudder
degree of heel ? more heel perpendicular CB lift actually vectoring slightly down making the boat seem heavier, adding more hull drag.....
etc..

My 'final answer'...
Each of the vectors are part of the magic of a boat moving majestically over the jeweled surface of our favorite lake or bay.

Enjoy the magic !

phill

[algonquin]

Phill - Roger - Adrift

Excellent help on this issue. I finally have a clear picture here. Many, many thanks !!

[Peter McMinn]

I'm into another one-design these days, but still love to browse this site for all the excellent discussions.

My naval architect bro forwards the following on this topic (writer identified only as Thom):

I was browsing and relearning why both the "Newton" camp (the angle of
the foil forces air/water momentum change down so the foil goes up by
reaction) and the "Bernouli" camp (air/water going over the top surface
has to go faster because the top surface is longer so the pressure
drops up there so the foil lifts) were wrong about what causes lift
(both sails and foils) and I ran across this cool simulator that uses
simplified Euler calculations (pretty accurate for non compressible
gasses (or water) and low speed air (such as sails)).

Thought you might enjoy. BTW, I used to teach folks the bernouli lift
theory before I knew better in explaining why coming to periscope depth
often caused the submarine to broach (go to the surface when you wanted
to stay submerged). To be honest, I don't know a simple explanation
anymore--how do you explain viscosity in a fluid and how that relates
to conservation of momentum and mass? ANyway...

http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html