Heeling Explained

Sam Salter · Jul 29, 2012

The bit about 2 particles having to arrive at the trailing edge together isn't true!
One particle is going to be long gone when the other gets there. There is no physical reason they have to arrive together.
While the bit about the heeled keel sounds plausible (It's late at night and I'm having trouble seeing it in my mind) wouldn't that just give you lift to windward. I'm not seeing any force to give you extra speed?
sam

Joe · Jul 30, 2012

While heeling can increase the speed of some sailoat designs.... I would suggest that an increase in waterline length is the major reason...... not an increase in lift from the keel.

The fin keel provides lift when the boat doesn't move exactly straight ahead (crabbing, if you will) changing the angle of attack due to side pressure from the sails... the benoulli effect seems plausible here... but... is it a result of heeling the boat????? well, uh, Don... you're article hasn't exactly convinced me after I did some quick research on the subject and couldn't find anything to support the premise.

The consensus was that lift from the keel was a result of its angle of attack from the boat's slightly sideways motion through the water.... and that heeling changed a boat's hull from symetric at the waterline, to a more asymetric shape which would induce lift. Also, in an attempt to circumvent class restrictions, some sailboat's designs incorporated large overhanging bow and stern portions that extended waterline length, and thus potential top speed, when the vessel was heeling....Think 12 meter AC boats.

weinie · Jul 30, 2012

Sam Salter said:
The bit about 2 particles having to arrive at the trailing edge together isn't true!
One particle is going to be long gone when the other gets there. There is no physical reason they have to arrive together.
While the bit about the heeled keel sounds plausible (It's late at night and I'm having trouble seeing it in my mind) wouldn't that just give you lift to windward. I'm not seeing any force to give you extra speed?
sam

quantum entanglement?

Rob2435 · Jul 30, 2012

A never ending argument! But my 2 cents...doesn't the foil created by the sails also lose lift as you get closer and closer to the water, as you are not only trying to make the air flow take a longer path, thereby increasing speed but now you are compressing the air flow against the water? Think of a boat heeled at 88 degrees(theoretically). If you took a very thin piece of wood (think rectangle here) and measured the force it takes to move it thru the water at say speed X, if you "heeled" it 10 degrees I don't think the force needed to move it a speed X would be any different.

The next time your boat is out of the water take a string and measure the difference from the normal waterline and a 20 degree heeled waterline, which I think would be longer. I agree with Joe, it's about boat shape. I also agree with weinie!!!!

anchorclanker · Jul 30, 2012

In physics, as well as proven in the wind tunnel by the Wright Brothers, regarding an airfoil in laminar flow, or in our case a hydrofoil or sail, the flow striking the leading edge and separating to follow both sides of the foil WILL arrive at the trailing edge at the same time. It is because of this that true lift exists at all. On a symmetrical or non symmetrical foil, as the angle of attack changes, flow is altered because the leading edge has changed angle of incidence, forcing the fluid over the two surfaces to take longer and shorter tracks. The fluid taking the longer course over the foil creates a lower pressure than the fluid taking the shorter track, and that force acts to pull the foil in the direction of lower pressure. In fact it was the Wright Brothers we owe for discovering those properties, that allow us to fly, have efficient props, and teach us how sails really work.

It is not the same as dragging a block of wood with squared off edges through the water or air. A foil will have laminar flow until it reaches the critical angle of attack where flow begins to separate away from the longer surface, eddies begin to form, drag climbs higher, and the foil stalls. It should also be pointed out that propellers work the same way. They dont push air or water like a fan, but rather pull via the properties of a proper foil. When a prop stalls, in water the pressure will drop so low on the face as to cause the water to boil, and then we have Cavitation, which should never be confused with Ventilation (air being drawn down into the prop from the surface) Cavitation can destroy a prop by eroding the face.

I believe whats being pointed out in the OP, is that as the boat heels over, a greater angle of attack is taking place along the keel, as well as the rudder, in an attempt to keep the boat pointed. That greater angle of attack is creating greater drag, slowing the boat, while simultaneously the sails lifting force (vector) starts to point downwards rather than forwards. Therefore, as soon as the boat begins to lose forward velocity, the drag has exceeded the forward lifting force. Just as in an airplanes wing, the same four forces are in effect. Force over drag, lift over gravity.

Scott T-Bird · Jul 30, 2012

Theories abound! ...

But I have some simpler thoughts. First of all, hull speed is related to the length of the wave that is created. It's a chord length between waves (the longer the wave length, the greater speed potential of the boat). Any increase in speed due to increasing the length between waves is a result of the boat squatting in the water either when heeled or even squatting when reaching or running. It is the squatting that increases the CHORD of the waterline length, not the heeling. I don't think it is related to the increased length due to curvature of the hull. If that were the case, then all beamy boats would be inherently faster, but they aren't (necessarily).

I assume we are talking strictly about boat speed and not VMG, because VMG is a separate discussion where the hull shape and heeling probably does have a beneficial effect. I'll get into that later.

I was reminded last week (when I was struggling to find a spot on the lake that had some wind), that my boat is faster when reaching. I happened to be on a reach when I finally found a slot that gave me some consistent wind and my boat leaped into a speed that was significantly higher than I can achieve when beating into the wind. With balance and sail trim in good form, I had at most 10 to 15 degrees heel with apparent wind at about 12 knots a little bit aft of beam. At this point of sail, the boat isn't heeling but it is squatting and the sails are standing nice and tall to take full advantage of the wind. Of course I had a full main and a 150 genny pulling with all it had. At this point, I could safely say that my boat was performing at hull speed and probably a little more.

If I had turned around to beat into the wind, apparent wind would have been in the neighborhood of 20 and I would have had more than I want to handle with a full main and 150 genny. I would have trimmed sails to try to maintain heel between 20 to 25 degrees and would probably have been making way at half a knot slower in boat speed compared to my reach. If I had trimmed sails for more power (raised the traveller car) I would probably heel excessively (25+) and with frequent round-ups, the boat speed would slow down significantly. IF I was able to reduce sail to the perfect sail plan where I was able to maximize power and maintain the optimum heel for a balanced helm I doubt that I could achieve the same boat speed that I had on a reach. This is where I agree with people who say that flatter is faster. But it is really just a function of maximizing power from the wind at an angle of heel that is managable. It seems obvious to me that the greater power from the wind you can harness (that includes trimming to allow your boat to sail on her feet), the more speed you will achieve until limiting factors such as displacement hull speed, and losing control of the helm with excessive heel, either limit your speed or slow you down.

When you are beating into the wind with perfect sail trim, but the boat is heeled only 15 degrees, you are not reaching your potential speed for the simple reason that there isn't enough wind to power your sail plan. It seems obvious to me that with more wind, you would harness more speed and heel more. It has very little to do with increasing your water line length or anything hull-shape related.

I agree that hull shape does affect lift, which helps improve VMG to windward. The curvature of the hull shape does cause a boat to climb to windward when heeled to leeward. Windsurfers know that when sailing a board at slow speed (in displacement mode), pressuring the leeward rail makes the board point. The curvature of the board, or the rocker, is the reason for this. A sailboat behaves similarly.

All U Get · Jul 30, 2012

Re: Theories abound! ...

Airplane wing, curve on top. Sailboat wing, curve on bottom more or less. Boat slipping sideways tripping over keel makes mast lean over. It works fine for my understanding level, all I gotta do is make the telltales fly.
All U Get

Ross S · Jul 30, 2012

anchorclanker said:
In physics, as well as proven in the wind tunnel by the Wright Brothers, regarding an airfoil in laminar flow, or in our case a hydrofoil or sail, the flow striking the leading edge and separating to follow both sides of the foil WILL arrive at the trailing edge at the same time.

With all due respect, the quote above is simply not true. It is a very common misconception that continues to get propogated in schools and even some universities. The myth that two particles striking the leading edge of an airfoil will arrive at the trailing edge at the same time is just that...a myth. There is no physical reason why this will happen. If this were the case then ask yourself how any plane can fly upside down? In fact, this has nothing to do with how a plane flies at all.

You also mentioned laminar flow: Even if the "equal-transit time" myth were true only for laminar flow (it isn't), in reality laminar flow is almost impossible to achieve and certainly never the dominant flow type during any flying or sailing activities. Any property that would hold true only for laminar flow can not be used to describe lift effects on a boat or airplane.

My background is mechanical engineering and I spent some time working in the field of aerodynamics (sadly, this doesn't seem to make me a better sailor!

). The reality of how an airfoil works involves some fairly advanced phsyics and math that would be difficult to duplicate here. However it is the Euler-n equation for forces and momentum along a streamline that is the actual basis for describing lift on an airfoil (or hydrofoil). Bernoulli was a smart guy, but he doesn't make planes fly. Lift is a force that is primarily perpendicular to the direction of fluid flow. Bernoulli's equation explains pressure differences in directions parallel to the direction of fluid flow making in an inappropriate equation to describe lifting forces on an airfoil!

Off soap box now. Sail on!

Oliverhg · Jul 30, 2012

simple answer

I am assuming that if the boat is not healing to the point where part of the keel is out of the water (much more than 20 degrees) than there is no effect of air foil on the keel.

The dominant effect would be the righting moment caused by the sails being more and more off the vertical centerline of the boat. As the center of effort of the sails move away from the vertical centre of effort in the keel, this creats a turning moment to turn the boat to windward.

This is what causes weather helm in a boat that is otherwise balanced in its sailplan when the boat is not healing.

The more healing there is, the more the rudder has to dig into the water to keep the boat on course and this creats drag in the water.

Heal is inevitable as the sail catches the wind but too much heal means too much drag from the rudder. Therefore, it is possible that in heavier winds, a reefed sail can actually give the same or even higher boat speed with less heel.

Would this simple answer make more sense than the air foil?

Oliver....

LaColla · Jul 30, 2012

Ross, do you attribute the lifting of a wing (airplane, keel, rudder, sail) to Newton's third law of motion over the more common explanation of the bernoulli effect?

tkanzler · Jul 30, 2012

Oliverhg said:
I am assuming that if the boat is not healing to the point where part of the keel is out of the water (much more than 20 degrees) than there is no effect of air foil on the keel.

The dominant effect would be the righting moment caused by the sails being more and more off the vertical centerline of the boat. As the center of effort of the sails move away from the vertical centre of effort in the keel, this creats a turning moment to turn the boat to windward.

This is what causes weather helm in a boat that is otherwise balanced in its sailplan when the boat is not healing.

The more healing there is, the more the rudder has to dig into the water to keep the boat on course and this creats drag in the water.

Heal is inevitable as the sail catches the wind but too much heal means too much drag from the rudder. Therefore, it is possible that in heavier winds, a reefed sail can actually give the same or even higher boat speed with less heel.

Would this simple answer make more sense than the air foil?

Oliver....

Which is precisely how my H340 sailed. The fastest I've ever had it was with two reefs in the main, the working jib furled, and a balanced helm. Much flatter, much faster, much more fun than putting the rail in the water while hanging onto the wheel trying to keep it from rounding up, and I'm not talking about sailing in a gale, either. :dance:

Stu Jackson · Jul 30, 2012

Heal - to feel better

Heel - the back of your shoe or boat tilt

anchorclanker · Jul 30, 2012

Ross S said:
If this were the case then ask yourself how any plane can fly upside down? In fact, this has nothing to do with how a plane flies at all.

I have to admit to being taught wrong for over 40 years to the idea the molecules met at the trailing edge. I have been studying aviation and aerodynamics since I was a kid and thats what has always been taught. Regardless of that, however, how airfoils create lift and drag is still basic knowledge. Thus, changing angle of attack changes lift and drag. The harder you heel over, the greater the angle of attack of fin and rudder, the greater lift and drag produced. At some point drag overcomes sail power and speed is lost.

How aircraft fly inverted is equally basic.

Whats fascinating is how much physics plays a part in sailing. Unlike flying, where all the flight surfaces are equal to one another and work in symmetry with one another, we have completely different foils at opposite ends all fighting against each other. To roll the plane both wings create equal and opposite forces and round you go. On the sailboat, heeling is sort of like anti rolling. The sails going one way while the fin is working against it and the rudder is back there trying to steer the butt end around. And all that drag is enormous! Imagine if it could be reduced.

There was a documentary on Ken Warby of Australia, who broke the water speed record in 1978 with a boat he built in his back yard using a surplus jet engine. He couldnt get the boat to go over 235 MPH or somewhere around there, and built a homemade afterburner in an attempt to increase speed but it wasnt getting him anywhere and he couldnt figure out what was holding him back. He got some engineer to start working with him and the engineer started looking at the rudder and did some calculations on. IIRC he cut some 9 inches off the rudder as that 9 inches was calculated to be generating some 3800 pounds of drag at 300 MPH. Once removed, the boat rocketed to 288, and then to 317 MPH. Record still stands, though two have been killed trying to beat it. Sadly, one of those Art Afrons nephew.

anchorclanker · Jul 30, 2012

Stu Jackson said:
Heal - to feel better

Heel - the back of your shoe or boat tilt

That dead horse will never heal if you dont stop hitting it on its heels with that stick.

anchorclanker · Jul 30, 2012

Oliverhg said:
I am assuming that if the boat is not healing to the point where part of the keel is out of the water (much more than 20 degrees) than there is no effect of air foil on the keel.

The more healing there is, the more the rudder has to dig into the water to keep the boat on course and this creats drag in the water.

Would this simple answer make more sense than the air foil?

Oliver....

If the boat were going straight forward through the water, in a level upright orientation, there would be equal lift and drag forces on either side of keel fin and rudder.

As the wind heels the boat over by side pressure aloft, and thrusts it forward, the boat also wants to point away from its true heading through the water (yaw) and we correct yaw with opposite rudder, which then changes the angle of attack of the keel fin. The boat is no longer going straight through the water, but is yawed to its true heading, sort of like an airplane doing a side slip of sorts. So not only is the rudder creating additional lift and drag, but the keel fin also but in the opposing direction. And that drag is opposite to the thrust generated by the sails. Just as in an airplane, if thrust overcomes drag, we accelerate. When the forces are equal, we maintain speed. When drag overcomes thrust we slow down. Anything that reduces drag will increase speed.

There is another force, that as the the boat heels over, and with the fin and rudder working harder to keep the boat upright, the boat yawed from its heading, it is also angling the bow down deeper in the water while the rudder is acting to lift up on the stern, so now the bow and rudder are creating even more drag than is necessary simply to control heel.

I suppose with sail trim you could take some load off the rudder, but that would cause the boat not want to round up which could be dangerous. Better to run out of rudder authority so it rounds up before it becomes critical. Better yet to reduce heel by backing off some sail before it gets that far.

So yes, reefing could create more speed by creating less pressure aloft, creating less yaw tendency, hence less correcting rudder trim, and hence less drag.

Ross S · Jul 30, 2012

LaColla said:
Ross, do you attribute the lifting of a wing (airplane, keel, rudder, sail) to Newton's third law of motion over the more common explanation of the bernoulli effect?

Newton's third law plays a role but probably not in the way that you're thinking. Yes, there's an "equal/opposite" force relationship, but it's not strictly the force of the air hitting the bottom of the wing forcing it up. Although this effect is real, if it were the primary mode of lift generation then wings wouldn't need any curvature at all much to the relief of those designing them.

I looked for a website I could link to but there is so much misinformation regarding this topic that I couldn't find a really good one. As I stated earlier I subscribe to the Euler-n equations as describing the lift of an airfoill. In a very simplified form this equation is:

Pressure across a streamline = (Fluid density * Fluid Velocity^2)/Radius of curvature

So, what makes an airfoil generate lift? This is the area where the widely held Bernoulli myth is actually correct. A lower pressure is generated above the wing resulting in lift. How do I propose to create this low pressure? Please consider two points: one "far" from the airfoil and another "close" to it. Far away from the airfoil the air is flowing in roughly a straight path (Radius of curvature = small) while close to the airfoil the air is following the curved path of the wing (Radius of curvature = large). By inspecting the equation above it becomes clear that increasing the radius of curvature results in lower pressure. As we originally stated lower pressure is the source of lift! We are assuming for now that fluid density and velocity remain constant....generally reasonable assumptions.

Let's relate this to sailing and sail trim, just to stay on topic.

Increased draft depth = increased radius of curvature = lower pressure = more lift = more power

Eventually a point is reached where draft depth is increased to the point that the additional lift does not overcome the additional drag created or the radius of curvature is increased to the point where the flow separates causing turbulence and thus a loss of lift.

This is a very, very, very simplified description of a topic that could fill an entire text book. The interaction of the keel and sail adhere to the same principles, but the relationship between the two working as a system is very complicated. (Hint, it also explains why a small degree of weather helm is desirable. The keel and rudder act as a single airfoil and the position of the rudder with a small amount of weather helm actually increases the radius of curvature of the airfoil resulting in lift).

It's amazing that we ever learned to sail into the wind, let alone build planes!

Just to add to the confusion, the Euler-n equations are actually derived FROM the Bernoulli equations. But they do describe different phenomena.

LaColla · Jul 30, 2012

Ross, thanks for your remarkably understandable explanation. As interesting as the physics are, I think it is equally interesting that there is uncertainty in the explanation of exactly how a foil works, especially given how much they are used in aviation, sailing, wind generators, ect. For instance, those that I have read that use Newton to explain lift use you first example as evidence for it- that you in fact don't need a curved surface of a wing to generate link-think of the balsa wood airplanes with the flat wings by way of example.

Anyway, I find the topic fascinating and I'm really amazed to see that not everyone agrees on exactly how a wing works.

Ross S · Jul 30, 2012

LaColla said:
Ross, thanks for your remarkably understandable explanation. As interesting as the physics are, I think it is equally interesting that there is uncertainty in the explanation of exactly how a foil works, especially given how much they are used in aviation, sailing, wind generators, ect. For instance, those that I have read that use Newton to explain lift use you first example as evidence for it- that you in fact don't need a curved surface of a wing to generate link-think of the balsa wood airplanes with the flat wings by way of example.

Anyway, I find the topic fascinating and I'm really amazed to see that not everyone agrees on exactly how a wing works.

Suffice it to say that I think the folks at NASA, Boeing, Airbus, etc... know EXACTLY how this works. Or if not exactly then awful darn close!

Any attempt to simplify the very complex physics of flight (including my own simplified explanation above), will introduce errors and results that won't hold up to experimental scrutiny. My explanation is a good one, but it's not 100% accurate. To get to that level of accuracy is well beyond me and would take an entire textbook. The theories that I dismissed above, I dismissed because they are not the "primary" means of lift generation. That is not to say that they don't have any effect at all, just a small effect compared to the overall amount of lift generated. I suppose that in theory I could fly a flat barn door at some large angle of attack. It's not very efficient, it's not fully described by my explanation, but it might be possible! (In that case, the Newton's 3rd law argument takes precedence)

So, despite a background in mechanical engineering and a small amount of experience in aerodynamics I still sit in my cockpit with Don's sail guide trying to get my boat going!

Still struggling with that sail twist thing....