Sailboat sailing upstream with no wind
December 13, 2018 9:10 AM   Subscribe

A few years ago, I had my mind blown by the Blackbird land yacht, which was able to "sail" directly downwind faster than the wind using only wind power. At first glance it appears to violate some law of physics, but in the end it turns out that it doesn't; energy is ultimately being provided by the wind, just in a way that you wouldn't expect it to be able to. I have now encountered a new aerodynamics brain twister. Artemis Racing, a perennial America's Cup contender, have said that it is possible to build a sailboat which can sail upstream in a river even if there's no wind, using the "apparent wind" created by the... uh... by the river water pushing the boat downstream, I guess? Maybe? Could someone explain how this might work in a way that I might be able to understand?
posted by clawsoon to Science & Nature (25 answers total) 15 users marked this as a favorite
 
Apparent wind is totally a thing - it helps to think of "wind" as something that is relative rather than something that is external to your situation. If you're moving (due to a river pushing your boat or whatever) yet the ambient air is still, then there is wind - relative to you. If "you" includes a sail, which is really an airfoil, producing lift, then this wind can act upon the sail, create lift, and transfer that lift into forward motion. I hope that is not more confusing.
posted by gyusan at 9:28 AM on December 13, 2018 [1 favorite]


OK, but doesn't the apparent wind vanish as soon as the boat's forward motion cancels out the downstream movement from the river's current?
posted by Anticipation Of A New Lover's Arrival, The at 9:40 AM on December 13, 2018 [2 favorites]


As this boat turns from downstream to upstream, the current is passing by the underwater foils at 20mph. The underwater foils can be effectively used just like the sails.
posted by bdc34 at 9:57 AM on December 13, 2018 [2 favorites]


Ahhh, underwater foils. That answers that. Yes, that would work.
posted by Anticipation Of A New Lover's Arrival, The at 10:28 AM on December 13, 2018


The underwater foils can be effectively used just like the sails.

I thought the underwater foils traditionally (hah!) generated lift, to bring the hull out of the water and reduce drag, not forward motion. Is the boat constantly moving between upstream and downstream? I don't get this at all.
posted by The Bellman at 11:31 AM on December 13, 2018


They do produce lift perpendicular to the centerline of the boat as their primary job, but apparently they also produce some lift parallel to the centerline.
posted by humboldt32 at 12:08 PM on December 13, 2018 [1 favorite]


It would depend on how they were designed. One could easily (well, for certain definitions of easy) design a foil that would function just like a sail, but for underwater currents. It would still be appropriate to call it a foil—a sail is a foil, as is an airplane's wing. The more common hydrofoils which lift a boat out of the water are just one specific application.
posted by Anticipation Of A New Lover's Arrival, The at 12:35 PM on December 13, 2018 [1 favorite]


If you accept that a boat can sail downwind faster than the wind while you're in still water, then a boat that can sail upstream shouldn't come as a surprise. Here's how to look at it:

Suppose you're in a boat traveling with the current at 10 mph, on a river flowing north. In your reference frame, there is a wind blowing to the south at 10 mph, and the water is still. Now you deploy your sail that can propel you downwind faster than the wind. You are now moving south at 15 mph (say) relative to the river water, which is moving north relative to the land at 10 mph. So relative to the land, you would be moving south (upstream) at 15 mph - 10 mph = 5 mph.

Now, accepting that a sail can propel you faster than the wind in still water is a bit hard to wrap your head around; but once you've done that, viewing things in the reference frame of the river water should convince you that the situations are equivalent.
posted by Johnny Assay at 1:13 PM on December 13, 2018 [5 favorites]


I think it's possible. We'll treat current as if it were wind.

We start with our point of sail at 0 degrees, i.e. heading directly into the wind, directly downstream. The sails are flapping; they don't work until we turn to about 45 degrees. The telltales start flying and we begin accelerating downstream.

Now we keep turning downwind to 110 degrees (about 4 o'clock). These boats can go 3x apparent wind speed downwind. The current/wind is 10 knots, and the boat is going 30 knots.

Note that at this angle, our downstream component has pretty much been canceled out! So we're not really going upstream or downstream anymore, but we're going cross-stream at 28 knots. Still generating plenty of lift for our sails, propelling the boat forward.

Now let's keep turning to 135 degrees. Our downstream vector is -21 knots, so we're going about 11 knots upstream. We're still going 21 knots in the cross-stream direction, and still generating lift, still moving forward.

We have to keep turning or we'll hit the bank of the river. The sails work best in the range of about 45-135 degrees, so we'll slow down. But when the boat crosses the wind vector and points in the other direction (a "jibe") we'll begin accelerating again. (Whether the river is wide enough to do this practically is another question)
posted by RobotVoodooPower at 1:49 PM on December 13, 2018 [1 favorite]


So the answer to my question is yes, it's two steps upstream and one step down, crisscrossing the whole way? That makes some sense, I guess.
posted by The Bellman at 2:02 PM on December 13, 2018


If you go back to the Blackbird land yacht, and accept that a similar device can work on water, then it's obvious that the "Waterbird" could make some headway up a stream that was slower than its progress in still water. The Artimis craft works pretty much the same except all the parts have been rearranged.
posted by SemiSalt at 2:14 PM on December 13, 2018


This would work even if you never pointed downstream, as long as the current is moving fast enough for the air to fill your sails and generate lift. Like Johnny Assay pointed out, to the boat it looks like wind, so this is just regular sailing.
posted by RobotVoodooPower at 2:27 PM on December 13, 2018


Would you go straight up the river, or would you zigzag?
posted by clawsoon at 2:31 PM on December 13, 2018


You pretty much have to zigzag (or "jibe") because the sail doesn't generate lift when you're going either directly with or directly against the wind, it collapses or turns into a parachute. A non-sailboat like Blackbird would have different constraints.

BTW I found this cool animation demonstrating the faster-than-wind concept with gears.
posted by RobotVoodooPower at 2:56 PM on December 13, 2018


So, here's the easiest way I have to describe how (I think) this works:

First, you need to understand how a keelboat sails upwind—not directly upwind, which it can't do, but "close hauled"—and in particular how it uses the keel, underwater, to balance the forces of the wind on the sail. There are lots of good explanations online. This one is pretty good. This one has less math-y stuff in it, if you prefer.

Okay, so we get that a boat can sail upwind, by tacking (or gybing, but typically tacking) back and forth "across" the wind. And that it can do this if the water is not moving. This seems like a trivial point, but just to be very clear: you can sail upwind in a still pond. You're not depending on some current in the water to push you along, it's just the water being there for the keel to push against. (If the water is moving against you, you obviously move slower in terms of speed over ground. But the boat doesn't really care or know what the speed over ground is, it's just making some speed through the water. I.e. all this happens in a reference frame where the water is stationary.)

What they are doing is basically turning a sailboat upside down. Including inverting the role of the wind and the water.

Instead of having the wind push against the sails and the keel balance the forces out, such that the net force moves the boat upwind, instead they are using the moving water, and balancing it against the air.

My assumption is that they have to "tack" across the water current, just like you would normally tack through the wind; they can't sail (swim?) directly into the current. But by positioning themselves at an angle to the current, and then arranging their sails properly, there's a net force that pushes the boat upstream, exactly like the wind would do in the normal case.

It's probably worth noting that the Amazon has some of the strongest river currents in the world, and this might not work elsewhere; they also have one of the most sophisticated and capable (meaning, able to sail very close to the wind) sailboats in the world. For most boats, the hydrodynamic drag would be too much for this trick to work. But I could imagine it maybe working for a small catamaran or maybe a specially designed sailboard or something.

Personally I find their explanation of how it works with the 'apparent wind' confusing as well; the 'apparent wind' is best thought of (IMO) as the difference in velocity between the air and water (and boat, once it gets going—although it's always easiest for me to start off with the stationary-in-the-water case). Any time you have two fluids moving at two different velocities, it's possible to extract energy out of the area where they meet. This is fundamentally how all upwind sailing (and boundary layer tricks like downwind-faster-than-the-wind) works.
posted by Kadin2048 at 3:12 PM on December 13, 2018 [2 favorites]


It’s not the current that’s providing the lift — it’s the apparent wind the boat generates by moving forward. With no wind the boat drifts downstream until it generates enough speed to lift on the foils (to reduce drag), and keeps going until the apparent wind is strong enough to sail through a tack 180 degrees and continue upstream, sailing on the apparent wind generated by its own forward motion.

No sailboat can head directly into the wind, but a boat as efficient as this one can sail as high as 12 or 15 degrees off the apparent wind when close hauled (a good keelboat can manage maybe 30 degrees). These ultra high performance yachts generate so much speed that on downwind legs the apparent wind comes all the way around from aft to fore and they sail close hauled, just as if beating upwind.

Still, you have to think that at some point the current and drag are going to reduce the boat’s speed such that it won’t be generating enough apparent wind to keep going, and it’ll be forced to head downstream for another boost. I think. It’s hard to get your head around!

(I guess one way to think of the forces at work is to think of them the same way we might think of ships in space. When a rocket engine propels a rocket the rocket accelerates and continues to accelerate as long as force is applied and as long as there is no drag to reduce acceleration. For these super efficient boats the wind is the rocket engine and while the wind pushes, the boat keeps accelerating (until drag etc). )
posted by notyou at 5:26 PM on December 13, 2018


Also note: Once the boat is moving with the current, the current is not applying additional force to the boat (curves in the river aside). There is apparent wind due to current, but no significant hydrodynamic force due to current. Any lift and drag or other forces on the hull and keel are from the boat's velocity relative to the current.

I think there are a number of racing boats developed in the last 15 years that could pull this off, since the key is going about 1.5x wind speed downwind. But it'd be kind of like driving an Indy car down a narrow alley while the pit crew optimizes the tire inflation.
posted by RobotVoodooPower at 6:10 PM on December 13, 2018


There are what appear to be a few contradictory answers here. As I try to wrap my brain around this, I see the following, for which I'd love corrections:

1) The sailboat cannot gain speed while sailing directly upstream (i.e. at an exactly 180 degree angle to the direction of flow).
2) The Blackbird land yacht can gain speed directly against the wind. I believe the main difference here is that it employs a propeller with a mechanical connection to the drive wheels while a sailboat does not have a mechanism to directly translate forces in that way.
3) The sailboat can sail upstream with tacking and jibing.
4) The sailboat can sail upstream with tacking and jibing indefinitely, without needing to turn around and go with the wind occasionally. (This is the part that contradicts what I see in some of the answers above, though I may misunderstand them.)

FWIW, the key to my gaining this understanding (which I hope is correct), was Kadin2048's suggestion to look at this as an "upside-down" sailboat. The underwater foils are acting as sails, while the large sail in the air plays the role a keel normally does. The foils redirect water in such a way that there is a force on the foil with a component pushing in the forward direction of the boat, and the sail pushes against the air to cancel out the component of that force pushing the boat sideways. The net is a force pushing the boat forward, upstream.

So now I want a version where the foils and the sails are all horizontal, parallel to the ground. The foils could tack and jibe through the water with small vertical velocity components (moving the vessel up and down instead of side-to-side), while the large horizontal sail would push against some of that, giving a net forward force. Is that possible? I think so, but my understanding of all of this is tenuous.
posted by whatnotever at 7:34 PM on December 13, 2018


Except for the case where the boat is sailing dead downwind such that the sails are just bags of wind and not acting as airfoils, the underwater foils are always acting as sails.

The things to keep in mind are:

1. Water has a much greater density than air, so the underwater foils need to be correspondingly smaller so that they balance with the sails.
2. Other than the rudder, underwater foils typically can't be trimmed the way sails are because of complexity and tradition. Some extreme racing yachts can, and some dinghy sailors will use shims to (usually illegally) "trim" the centerboard/daggerboard like a sail.
3. Foiling boats can be pushed to 1.5x wind speed "upwind" and 2.5x "downwind", though those terms become very confusing when apparent wind gets that strong.
4. Foiling boats already need complex control systems to maintain the exact right amount of lift, so trimming the foils to port and starboard (and possibly fore and aft) in addition to up and down makes sense if you are enthusiastic enough.
5. It is pretty hard to reason about even a basic, conventional sailboat without being on one, and playing with the controls in moderate wind.
posted by b1tr0t at 8:51 PM on December 13, 2018 [2 favorites]


b1tr0t: Some extreme racing yachts can

I believe that extreme racing yachts are exactly what they're talking about, so I'm guessing that we can treat this problem as "foil in the air and and foil in the water, both of which you can manipulate any way you'd like".

Thanks for all the thoughtful and detailed responses. I'm still chewing on them. I can almost picture (what I think is) an analogous setup where you make a toy truck travel up a treadmill by holding a yardstick across the treadmill at the right angle, and turn the wheels of the truck at the right angle.
posted by clawsoon at 9:00 AM on December 14, 2018


The thing I left out is what makes sailboats work at all: two different fluids, moving in different directions.

If the wind and water are both moving North at 5 nautical miles per hour, the boat will do "nothing" and float north at 5 kts. As soon as there is a difference between the wind direction (and, implicitly, velocity) in the two fluids, that differential can be exploited. That's what sailboats do.

The thing that makes it possible for sailboats to go faster than the wind is the lifting foils. Once you pull the hull out of the water and support it on tiny supports, you are dealing with levels of friction similar to ice boats and sand racers that have been going faster than the wind for decades. Displacement boats and "merely" planing boats still have too much friction with the water to go faster than the wind.
posted by b1tr0t at 9:00 PM on December 14, 2018 [1 favorite]


It has taken me a few days, but my brain finally clicked into the first step in figuring this out, as explained by Johnny Assay above. Taking the moving water as my frame of reference turns this into a "downwind faster than the wind" problem.

I'm sure I'll understand the rest of the problem as I continue to meditate on more of your answers.
posted by clawsoon at 4:05 PM on December 16, 2018


Between Kadin2048, b1tr0t, and this video, I think I've got the basic intuition. It clicked in for me about right here.

The keel (and the hull itself?) acts as a foil in the water which creates a sideways lift force in this direction: ↗

The sail acts as a foil in the air which creates a sideways lift force in this direction: ↘

The sum of those forces - the resultant force vector - goes in this direction: →

The speed of the boat is not determined by the speed of the wind. It is determined by the speed at which the drag forces match the keel+sail force vector.

It is limited more practically by the fact that the keel lift force and the sail lift force aren't acting in the same plane, which creates torque that threatens to tip the boat over. Hence the use of catamarans for this: They are able to counteract the torque with less surface area in the water, and thus less drag.

Am I close?
posted by clawsoon at 7:16 AM on December 17, 2018


You are thinking in the right direction regarding the differential between fluids.

Whether or not a catamaran has less drag depends on the catamaran and the monohull you are comparing it to. Cats generally have displacement hulls, high performance monohulls from the last few decades have tropically been designed to reduce surface area and plane when heeled (tilted) properly. Of course, foiling multihulls (cats or trimarans) or monohulls will have the least drag, when up on the foils.

In light wind, multihulls generate a ton or drag, and turn very slowly. Keelboats are almost equally happy tacking or jibing. Multis love to jibe and hate tacking. Keep this in mind if you find yourself on a beach cat at a tropical resort. But also keep your head down, as you don’t want to get whacked on the jibe.
posted by b1tr0t at 3:49 AM on December 29, 2018 [1 favorite]


Ice boats have made the forces involved easiest for me to think through, so thanks for that mention as well. It's easy to picture pulling almost perpendicular on an ice boat and making it slide forward on its skates.
posted by clawsoon at 5:34 AM on December 29, 2018


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