Leading edges, trailing edges...

[Externet]

A sailboat keel has a roundish leading edge and a somewhat sharp trailing edge.

 

If both edges are made as sharp as a razor blade, setting aside the fragility of such edges; will the sharper keel have less drag ?

[studiot]

The reason that 'streamlining' an object produces a relatively blunt nose but a long thin tail is that the nose and tail perform different functions in the fluid.

 

The purpose of the nose is to part the fluid to the max width of the object. The rounding helps do this smoothly a needle shape does not improve this.

 

The tail however is to reduce or avoid creating turbulence in the 'wake' which goes to remove energy from the object and is felt as 'drag'.

 

So no, a sharper nose will not reduce drag.

 

A further practical consideration is that since the nose is moving forwards it encounters (bumps into) objects and a razor adge would not last very long compared to a more robust one.

 

go well

Edited May 1, 2012 by studiot

[Joatmon]

The reason that 'streamlining' an object produces a relatively blunt nose but a long thin tail is that the nose and tail perform different functions in the fluid.

 

The purpose of the nose is to part the fluid to the max width of the object. The rounding helps do this smoothly a needle shape does not improve this.

 

The tail however is to reduce or avoid creating turbulence in the 'wake' which goes to remove energy from the object and is felt as 'drag'.

 

So no, a sharper nose will not reduce drag.

 

A further practical consideration is that since the nose is moving forwards it encounters (bumps into) objects and a razor adge would not last very long compared to a more robust one.

 

go well

 

The same principles apply to aircraft wings.

 

 

[J.C.MacSwell]

A sailboat keel has a roundish leading edge and a somewhat sharp trailing edge.

 

If both edges are made as sharp as a razor blade, setting aside the fragility of such edges; will the sharper keel have less drag ?

 

Most keels would have slightly less drag at 0 degrees of attack if sharpened somewhat. They are generally optimized (most lift for least drag) for the amount of leeway the sailboat might have while going upwind in the typical conditions the sailboat is designed for.

 

Razor sharp is not an advantage even at 0 degrees as it requires more surface area for the same form drag (therefore more drag overall due to higher skin friction). It also creates turbulence (more drag, less lift) at very small angles of attack, and will stall sooner (turbulence created will not reattach) leading to high drag and poor lift.

[imatfaal]

Is this concept related to the "bulbous bow" design that is now employed on large cargo ships?

[J.C.MacSwell]

Is this concept related to the "bulbous bow" design that is now employed on large cargo ships?

 

Bulbous bows are used to reduce wave making drag by getting the leading portion of the form deeper below the surface.

[Enthalpy]

The rounded leading edge is one more common misconception in fluid dynamics. A sharp leading edge performs better, and the swordfish has one, as well as the wing profiles used at not-very-old air gliders.

 

I suggest to look at an air glider not older than 40 years: its wing differs radically from the old "water drop" tale. In addition to its sharp leading edge, it has its maximum thickness behing the half chord, its lower face is very convex at half chord and gets concave only behind - which also means that the lower face is longer than the upper face.

 

Rounded leading edges are necessary (and aren't very bad) when the fluid's direction isn't precisely known, which is the case of a boat's keel which drifts unless its profile can be made unsymmetric.

 

Unless you have much freedom (much length) to design a smooth trailing edge, you better cut it completely flat instead of making it too steep. This is done at many cars (for which lift and stability is one more worry). You get a clear dead zone after the flat cut and a not-too-bad stream around this zone, while a too steep trailing edge would induce irregular turbulence, noisier and lossier.

[studiot]

There is a big difference between the hydrodynamics of a sailboat and a swordfish (and an aircraft for that matter). The disposition and balance of forces are all different.

 

The saiboat is actually the most complicated since the driving force is rarely directly along the main axis.

 

JC Mcswell has already hinted at this in post#4.

[MigL]

Don't know much about boat keels, but have a keen interest in jet fighter design/aerodynamics.

 

The leading edge of a wing , and keel I would imagine, have to be able to deal with differing angles of incidence and present the same face to the airflow/waterflow while the trailing dge not so much ( due to flow laminarity ). A rounded profile helps with this and is used, at least at low speeds, at hi speed the reduced drag of a sharp leading edge takes precedence.

 

The wing profile described by Enthalpy is commonly known as a supercritical wing and is used subsonically to improve lift/drag ratio.

[studiot]

Two big differences between a keel and a wing.

 

1) The asymmetry of the wing is designed to produce a force at right angles to the flow. If you did this with a keel the vessel would be permanentlt pushed sideways.

 

2) The driving force of an airplane has no intended component at right angles to the direction of thrust and motion.

[MigL]

You may be right about these differences, but consider then the vertical stabilizer on an aircraft. It has a symmetric rounded leading edge/ sharply pointed trailing edge to which all my coments apply.

A lot of aircraft wings are symmetric these days, especially any that spend any time dealing with supersonic flow, they generate lift with leading and trailing edge flaps and other devices, and angle of incidence. Even subsonic airliners or business jets can have locally supersonic flow over the top of the wing under certain conditions.

[studiot]

Yes the vertical fin of the tail plane is more akin to a keel than a wing.

Fish, of course have such devices - they are called fins.

 

However the keel has an additional function. Its large area provides large resistance to the sideways component of force generated by an angled wind, as compared to low resistance offered by the streamlined shape in the forward direction. This is the significance of item 2 on my list.

 

Incidentally in water the criteria are not supersonic/subsonic but super critical/subcritical flow.

 

go well

Edited May 3, 2012 by studiot

[MigL]

You seem quite knowledgeable on the subject. Is the interaction between the keel and the sail responsible for the ability of a sailboat to tack against the wind ? I could never understand how that was possible.

[studiot]

The sail billows out to a curved shape. That is its section is asymmetrical like an aeroplane wing section. The wing is horizontal, but the sail is vertical. In either case the airflow passing either side of the section generates a force at right angles to the airflow direction.

 

For a horizontal wing the force is vertical and is responsible for the lift keeping the aircraft up.

 

Since the sail is vertical the force (some sailors still call it lift) is horizontal.

 

The trick in sailing against the wind is to angle the curved sail against the wind so that the horizontal force is in the direction of the heading of the boat, or at least a component is.

 

Does this help?

[MigL]

Not really.

Using simple vectors you can show that a lifting surface, sail or wing, has no component in the direction of its motion, ie. into the wind. Otherwise you could use a wing for propulsion.

I've been told, by sailors, that it has to do with sail/keel interplay but I've been too lazy to actually investigate the phenomenon.

[studiot]

Not really.

 

 

 

Are you arguing or asking for more information?

 

Using simple vectors you can show that a lifting surface, sail or wing,

 

And by using a simple drawing you can show that although the perpendicular force is always perpendicular to the direction parallel to the sail boom you can show that geometry of the wing and sail are different.

 

Think about what I said before and how the wind passes round the sail or wing.

Edited May 9, 2012 by studiot

[MigL]

You said "Does this help ?" and I said "Not really." , for the reasons that I explaned.

One surface, wing or sail will not have a force component at an angle less than 90deg to the incident airflow no matter what the geometry ( ? ).

 

So, yes, I did think about what you said, but it made no sense. If you aren't prepared to explain, or don't know, just say so and I'll research it myself.

 

 

 

[J.C.MacSwell]

You said "Does this help ?" and I said "Not really." , for the reasons that I explaned.

One surface, wing or sail will not have a force component at an angle less than 90deg to the incident airflow no matter what the geometry ( ? ).

 

So, yes, I did think about what you said, but it made no sense. If you aren't prepared to explain, or don't know, just say so and I'll research it myself.

 

The keel provides the force component in the upwind direction

[studiot]

Hello Mig I am sorry I snapped I did not mean to abandon you, and I see that you did not mean ill either. I was just dealing with another wearisome thread here as well.

 

The key to understanding this is to realise that the force on the boat produced by the sail/wind interaction is perpendicular to the sail.

 

Here is a link to a demonstaration.

 

http://demonstrations.wolfram.com/SailingAgainstTheWind/

 

Note this is a purely horizontal model. We can explore how this perpendicular force arises if you like.

 

go well

[MigL]

No apologies necessary. As you've no doubt noticed, I have bad days too and tend to snap at people. Forgive, forget and move on.