A Review of Modern Sail Theory (1981) [pdf] | Hacker News Reader

A Review of Modern Sail Theory (1981) [pdf]

ljjensen.net

167 pointsTomte54 comments

> "As a research aerodynamicist, I was dumbfounded when I first started reading the sailing literature as a beginning sailor and saw what a confused state sail theory was in. Even the basic explanations of how a sail generates lift were wrong. This I could understand, since even the popular aviation books were wrong in their attempts to explain lift. It is difficult to explain the generation of lift for laymen. The simplifications devised in attempts to do this seemed logical but usually turned out to be wrong"

He has a very readable section where he goes through the wrong explanations (e.g. Bernoulli effect) and explains why they are incorrect, and then corrects them with helpful diagrams.

I'm somewhat disconcerted.

There was an article on HN yesterday [1] in which one of the replies to the top comment about AG Bell's attempts at flight said: "The difference is that Bell didn't understand aerodynamics and didn't seem to do a thing about it."

That reminded me about an article I'd read some time back that suggested that even modern aviation experts don't correctly understand lift.

Today, that article is on the front page of HN.

[1]1 https://news.ycombinator.com/item?id=13817912

When i was a kid i had a windsurfer, one of the first "no nose" boards that could barely keep itself afloat. You have to keep these moving if you want to stay up. And they like to move. Comming from racing lasers and hobies, one day I attached a telltale atop the mast. The wind is 20knots, im tacking slightly upwind at 25knots (call it 70* across the wind) and the telltale is pointing strait back. Wtf. So i put one on my hand and hold it away from the sail... still strait back. How it is possible for me to be, from my perspective, racing directly into a headwind. I get the basic physics, but as a teenager on a board it's pretty much magic.

If you really want perspective on sail and wing theory, forget the ropes and booms. Get an airfoil you can hold in your own hands. Feel the effects yourself. Be warned, windsurfing has an evil learning curve imho probably moreso than kiteboarding.

It's interesting to me that even in this theory keel is not mentioned at all, though it is essential for sailing, together with the sails. Few things:

- sailboat without the keel, will not sail at all. It will just move sideways and back

- sailboat driven by a motor in a lull, with sails up and perfectly trimmed, will go slower than without the sails

- plane and air gliders comparison is always wrong, because plane is driven, and both operate in the same medium - air

Sails alone cannot drive boat forward. Only with interaction from the keel on the boundary of two medium, water and air with different density.

Do you have more information on that? I assumed the keel was for balancing the torque that the forces on the sail generate so one doesn't capsize. In land sailing or ice yachting you don't have two medium with different densities and still achieve forward movement using a sail.

Do you have more information on that? I assumed the keel was for balancing the torque that the forces on the sail generate so one doesn't capsize.

I guess this is part of the reason.

As for why it isn't the full reason, think if the boat was a hovercraft or somehow otherwise had zero friction. When the wind hit the sails it would just move the vehicle in the direction of the wind (possibly turning it sideways in the process).

In land sailing or ice yachting you don't have two medium with different densities and still achieve forward movement using a sail.

In land sailing or ice sailing you can steer against a hard surface.

I'm not a specialist in fluid dynamics but this should count for something, IMO it should even get better results.

almost. a sail also deflect the force all by itself, being at an angle from the wind, so there's that - a boat can move two three times faster than the wind given the right conditions

The ice or earth still provides the same function vis-a-vis whatever part of the vehicle is touching it; it's that contact that allows for the force differential between the air & the medium to be exploited.

If you want to sail downwind you don't need a keel except for balance. In land sailing and ice yachting the wheels or runners serve the same purpose as a sailboats keel and provide resistance to the vessel moving sideways in a crosswind.

A boat's keel is weighted with ballast (on larger boats) for the balance reason, but it's fin shape is for the purpose of providing lateral surface area.

This is not quite correct.

Even while sailing downwind, the sails are still providing "lift" (if we are calling it that here). There are opposing forces for sailing - the Center of Effort (CE) and Center of Lateral Resistance (CLR). These two need to be balanced in order for the boat to sail a proper course - of course if you are just letting the wind push you directly downwind, you could remove your keel/daggerboard, but in practice, you will want to sail on an angle to the wind - on a reach or by the lee depending on the type of boat - so as to create flow over the sails and have a greater pressure gradient than if you just let the wind push directly against the sail.

The hull of the sailboat contributes to the CLR, so it is common in dinghy racing to pull your centerboard/daggerboard up partially so as to balance these forces and minimize drag. If you watch Olympic laser sailing, you'll see while sailing downwind, they will heel the boats to windward with their daggerboards about halfway up so as to move the CE closer to being above the CLR, allowing for less rudder drag to counteract the forces and steer straight.

Every time there's anything on HN or Reddit about sailing, there's a _ton_ of misinformation and blanket statements that might apply to some types of boats, but are far from general rules. I'm happy to answer any other questions too!

I was trying not to overcomplicate the explanation, but when I said "downwind" I meant dead downwind, not broad reach. In this situation your sail plan does not need to generate lift: You can fly a symmetrical spinnaker and be propelled entirely through its drag forces, at the cost of not being able to match or exceed wind speed the way you can with a mainsail generating lift.

More information? Please don't blame me for my next heretic opinions about sailing (upwind) :-)

In a land sailing, glider has potential energy (height), while the boat doesn't have any potential energy. This pot. energy makes glider moving forward (and down).

Consider glider on a airfield, and a strong wind to the nose - glider doesn't fly up and forward, it can only lift up and turn back.

But at the same time, when you have docked sailboat, and a close hauled wind (approx. 30 degrees from nose), the boat will move forward (lift) and sideways (drag). As faster it is, there will be a less drag.

Keel doesn't have to be heavy flat area or 300kg torpedo 2 m below hull, it can be just shallow longitudinal slot of meter - two.

Again, if there will be no water (neglect weight of the sailboat), just air, the sailboat will not have any tendency to move forward, otherwise this will be fantastic.

A center/daggerboard does not balance the capsizing moment, it contributes to it. You have aerodynamic forces towards the lee centered somewhere above the waterline, and hydrodynamic forces towards windward centered somewhere below it.

Aside from the very few boats with hydrofoils, this torque is offset by that generated by the interplay of weight and buoyancy (for iceboats, the rigidity of ice substitutes for buoyancy.)

I feel that you are being intentionally obtuse. The article is about sail theory, and as such it is entirely reasonable to consider the sail in isolation.

I'm not sure why you were being downvoted. There's a huge amount of theory relevant to sailing any boat and no special reason one would need to consider keels above any other interaction of a sail with a boat. I do a bit of kite skiing and I'm hardly likely to complain that the article doesn't consider uphill and downwind sailing enough.

The keel is essential in determining the boat's direction, it's just out of the scope of this paper, which concentrates on how the sail generates lift. That's the specific topic a lot of textbooks get wrong.

I agree it would be cool if the paper also had a quick higher-level overview including how the forces from the hull/keel and sail interact, right now it feels like it lacks a bit of context! Thankfully this higher level is not so prone to error and I believe textbooks generally get it right, in a "spherical cows on a frictionless plane" sort of way. For example:

http://www.real-world-physics-problems.com/physics-of-sailin...

The article is about sail theory.

Keels provide lift upwind and in fact most displacement mode sailboats use NACA foil templates for their keels. Downwind, keels are drag and in some boats, you pull up the centerboard to reduce that drag.

You can always make things more complex.

>in some boats, you pull up the centerboard to reduce that drag.

Oddly enough I'm writing from such a boat right now :). Though in our case the main purpose of having it retractable is to allow navigating shallow waters (like smaller marinas).

Is it still called a keel then? In dutch, keels are also about being a counter weight.

Common usage would have you believe that all keels need a keel weight, but many catamarans have sharper structures on the bottom of their hulls that are called keels. Generally a keel is not retractable, a centreboard is.

Though we do use the term "swing keel" for a large heavy centerboard.

Are these functionally distinct from normal centreboards? I'm trying to recall the term and what comes to mind is a keel that's hung like a centreboard to reduce grounding damage.

The Catalina 22 is available with a swing keel that is about 550 lbs. Those models have no other keel. By contrast, my Morgan 24 has a full-length shallow keel with 1900 lbs of ballast, plus a fully retractable centerboard that is not particularly heavy. I think the only common distinction between the terms "swing keel" and centerboard is whether it provides appreciable ballast.

In theory, a centreboard is a type of keel, when describing its purpose and action.

However, in practice, nothing that is not a fixed structural element of the hull is ever referred to as a 'keel' [edit:1]. Thus, a reference a keel can be taken to mean a structural keel.

A centreboard will only ever be a pivoting blade that rotates down from (and up into) a housing inside the hull, and such a blade will only be described as a 'centreboard', unless the meaning is otherwise clear. [edit:1 redux, 'swing keel'.]

More rarely, dinghies will have simple 'daggerboards' that slide straight down, and likewise they will only be referred to as 'daggerboards' in any circumstance where ambiguity might occurs.

Keels are usually weighted, of course, but there's no distinct word for an unweighted keel, structural or otherwise.

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1: nicwolff reports, as you might have noticed, that a (deliberately) heavy centreboard is referred to sometimes as a 'swing keel', a term with which I was unfamiliar. So, matters are less cut and dried I'd initially represented them to be. However, I think this leaves the presumption that a keel will be structural largely intact.

> Oddly enough I'm writing from such a boat right now :).

Is there a risk of capsizing? I thought pulling the keel up would be something reserved for racing.

Probably less, as it would go sideways rather than tip.

In fact, lifting the keel/centreboard can be a good technique for manoeuvring towards a peer; often you have to keep the bow pointing a certain direction, but want to move a little sideways.

I believe that the boats where the keel lifts up do not rely on the keel weight to stay balanced. They are also easier to capsize (regardless of whether the keel is lifted) than the ones with a fixed heavy keel that lowers the center of the mass.

If HN consisted entirely of this kind of thing, I would never get anything done.

You might want to dive into the physics of "sailing downwind faster than the wind"[0], then =)

[0] uh, just google it really

An example video of the Downwind Faster than the Wind Challeng (DWFTTW), with windspeed 0: https://www.youtube.com/watch?v=dgHBDESd38M

This: https://www.lhup.edu/~dsimanek/museum/DDWFTTW.htm

This is a great paper. Almost all sailors I've met still believe the Bernoulli effect misconception, even some sailmakers designing carbon fiber sails costing $100k+.

Similar errors are in airplane pilot flight manuals, and believed whole heartedly by pilots.

I think a convincing refutation of the Bernoulli effect as an explanation of flight is the fact that some aircraft have symmetrical airfoils and can fly upside down. This is a less convincing argument to sailors whose boats don't usually sail very well upside down :)

Isn't sailing into the wind the equivalent?

Though I learned in my "Physics of Sailing" course that it would be technically possible to sail with the current instead of the wind by putting the boat upside down in the water.

I had the impression that this was the epitome of the way of thinking thought in a physics degree.

I really enjoyed it too. As a sailor, it's an such an incredibly interesting topic. It only gets more challenging when you consider the twist in the sail (change in angle of attack from top to bottom), varied sail shapes and sizes, and oscillating wind patterns.

I also love how even though a technical knowledge of aerodynamics can help you trim a sail, most sailors can do a pretty good job by feel alone.

I thought the point of the twist was simply to account for the wind higher up going faster.

One of a couple of reasons. It's not likely to be as well developed in a review from 81 but an important part of modern high performance sailing is gust and wave response. That is, when you get hit by a gust or a wave, how does your rig and sail flex in response to help you manage. Generally, having the top of your sail twisting off can help with designing for gust response.

Back in the days of square sails, standard practice was that sails higher up on the masts were trimmed looser , in order to give the helmsman time to react when the wind direction changed in a gust (particularly when sailing close-hauled, of course).

Carbon and kevlar tapes are used in sails to minimize their stretch along their stressed areas and allow for optimized sail shape. I don't think anyone selling or making sails tries to convince of any other reason.

Telltales on sails were a Gentry invention and they were long known as Gentry tufts. He's also known for disproving the 'slot effect' theory. The more complex theory is the Kutta condition covered in the article. He went to Cal (class of 55), as did Lowell North, Paul Bieker and Tom Blackaller.

This is a really interesting topic that I know little about. This paper was published 36 years ago—is it still current with modern understanding of sailing? Or has anything changed or been refined significantly since?

In the last couple decades, there's been a lot of innovation with hydrofoiling, which significantly minimize friction between the hull and water.

The America's Cup AC35s [1] and Flying Moth Class Dinghies [2] can achieve incredible speeds thanks to being elevated out of the water.

[1]: https://www.youtube.com/watch?v=H3uxAi3z8WU

[2]: https://www.youtube.com/watch?v=frAkDEszgZc

Thank you!

Guilty as charged, I always thought that the Newtonian impact theory explained in Figure 4 was how it worked.

This comment prompted me to read TFA, thanks! Viscosity! That's why my wing paddle feels and functions so much better than the old plop-and-tug paddles I used to use...

A fun little demonstration to do when teaching sailing to kids is to fill a baking dish around half full with chocolate pudding and level it, then drag spoons through at a slight angle to simulate a sail. It's far from scientific and isn't really working on quite the same principle, but if your pudding is the right consistency, it does a decent job of showing the pressure gradients. Best of all, you've got a bunch of chocolate pudding to eat after.

Arvel Gentry, the author, wrote plenty of fascinating articles on sail theory for laymen (and more technical work as well). I suspect his seminal work was How Sails Really Work (1973)-- https://web.archive.org/web/20120308100740/http://www.arvelg... .

You can find many more of his other articles at his archived website/blog[1] or this website[2] that was created posthumously.

[1] https://web.archive.org/web/20130925151347/http://www.arvelg... [2] w/ short bio- https://arvelgentry.jimdo.com/who-is-arvel-gentry/

Physics Today had a nice article on sailing a few years back. I think it's well done: The physics of sailing - http://physicstoday.scitation.org/doi/full/10.1063/1.2883908

As an outsider, their explanation of keels seems awful:

A keel works only if the motion of the boat is not exactly in the direction in which it’s pointed. The boat must be moving somewhat sideways. In that “crabbing” motion, the keel moves through the water with an angle of attack. Just as for the sails in the wind, that causes the water on the “high” (more downstream) side of the keel to move faster and create a lower pressure. Again, the net lift force on the keel is due to the combination of that decreased pressure on the high side and increased pressure on the other (low) side.

The first sentence is a bit silly, since if the boat is moving in the direction of the keel (rather than the direction of the wind), one could also say that the keel is working quite well.

But the real problem is the explanation of lift based on pressure differences caused by the speed of the water on each side. It's not exactly false, but isn't this the same classically unhelpful explanation that implies a symmetric wing (such as an angled barn door) produces no lift?

Isn't it clearer to think about the angled keel pushing the water out of the way as it moves through the water? https://www.av8n.com/how/htm/airfoils.html. Or is there some difference about water and air that makes the Physics Today explanation more useful for keels on boats than for wings on planes? But then way do they say "just as for sails in the wind"?

I read this several years ago and the next time I went sailing I could see the flow effects in the behaviour of the sail. It helped me trim the sails more effectively.

So lift is due to pressure differentials on the top side of a foil? It's just that the traditional reasoning as to why those those differentials formed (air moving at a faster speed over the top) was not accurate?

Not quite. The air does move faster over the top of the airfoil, and this does result in lower pressure there. However, explanations related to this being due to the difference in length of path are completely wrong. The air molecule that goes over the top gets to the trailing edge long before a similar air molecule that goes under the airfoil. The explanation in the paper is that this difference is primarily due to the circulation flow around the airfoil, and he goes into a lot of detail about this flow.

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