Sailing faster than the wind?

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philip
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Re: Sailing faster than the wind?

Post by philip » Thu Jul 01, 2021 9:30 am

jaap wrote:
Thu Jul 01, 2021 8:34 am
To be more precise, a sailboat's velocity component along the wind direction cannot be faster than the wind, but its sideways velocity component is only limited by drag and not by windspeed. Those two components together can easily result in a speed faster than the wind, though not in the wind direction even when tacking.
That might be true for 'conventional' yachts but only because they are inefficient. There is no physics law that prevents boats going faster. Reduce the drag using hydrofoils, or ice yachts, improve the sail with a double skin, or wing, and yachts go downwind faster than the wind. The recent AC75 America's Cup using single hull hydrofoils and double skin sails is an example.
Boustrophedon wrote:
Wed Jun 30, 2021 8:35 pm
To take the discussion back to sailing, could someone draw a vector diagram that shows how a sailboat can make greater progress in the downwind direction faster than the wind blows?
See these diagrams from discussion about AC72 boats, the America's Cup a few years ago, they used multihull hydrofoils and rigid wing sails: https://www.nauticed.org/sailing-blog/a ... rent-wind/

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Re: Sailing faster than the wind?

Post by Gfamily » Thu Jul 01, 2021 9:46 am

Woodchopper wrote:
Thu Jul 01, 2021 9:21 am
Martin Y wrote:
Thu Jul 01, 2021 8:55 am
That's what I presumed they meant but when I asked on the original JREF thread I was told that no, they really do mean that a tacking yacht can have a downwind component of its velocity which is greater than the windspeed.

A yacht cannot run directly downwind faster than the wind but it can achieve it if it is tacking.

I don't have a pleasing explanation for how it could work but it's something to do with sails working like aircraft wings, which act to divert the air flowing past them downward. The sail diverts the air upwind, filling the void, so to speak, between itself and the wind it's advancing away from.
Its a long time since I sailed, but I don't think so.

The effect you mention occurs when the sailboat is sailing toward the direction of the wind - most optimally at an angle of about 45 degrees - and the sails generate lift.
Is the 'wind' you are thinking of a combination of the air speed and the boat's speed? The problem I have is working out how to translate from a static frame to a boat based frame. Particularly while on a work call.
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Re: Sailing faster than the wind?

Post by Woodchopper » Thu Jul 01, 2021 9:55 am

Gfamily wrote:
Thu Jul 01, 2021 9:46 am
Woodchopper wrote:
Thu Jul 01, 2021 9:21 am
Martin Y wrote:
Thu Jul 01, 2021 8:55 am
That's what I presumed they meant but when I asked on the original JREF thread I was told that no, they really do mean that a tacking yacht can have a downwind component of its velocity which is greater than the windspeed.

A yacht cannot run directly downwind faster than the wind but it can achieve it if it is tacking.

I don't have a pleasing explanation for how it could work but it's something to do with sails working like aircraft wings, which act to divert the air flowing past them downward. The sail diverts the air upwind, filling the void, so to speak, between itself and the wind it's advancing away from.
Its a long time since I sailed, but I don't think so.

The effect you mention occurs when the sailboat is sailing toward the direction of the wind - most optimally at an angle of about 45 degrees - and the sails generate lift.
Is the 'wind' you are thinking of a combination of the air speed and the boat's speed? The problem I have is working out how to translate from a static frame to a boat based frame. Particularly while on a work call.
Yes, indeed. When sailing into the wind, the speed of the air flowing over the sails is due to the wind speed and the forward speed of the boat. So the sails generate lots of lift and the boat moves faster (until there’s too much drag).

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Re: Sailing faster than the wind?

Post by IvanV » Thu Jul 01, 2021 10:15 am

We need to be careful about the distinction between the component of the sailboat's velocity in the direction of the wind exceeding wind speed, and the sailboat speed exceeding the component of the wind's velocity in the direction of the sailboat. Clearly many posts demonstrate an understanding of his distinction, but occasionally people have got a bit mixed up about these.

There is no trouble with the latter, as a sailboat sailing at right angles to the wind demonstrates. The problems I describe in the next paragraph do not occur.

There are two issues when the sailboat tries to travel at a speed such that its component of speed in the direction of the wind exceeds the windspeed. First, at this speed the sail experiences wind resistance rather than wind assistance, whatever the shape of the sail: the sail is travelling through the air, in the direction of the wind, at a positive velocity relative to the air's motion in that direction. So, whatever the shape of the sail, it is experiencing wind resistance. Second, when this happens, a traditional sail cannot fill, and so loses efficiency in harvesting even the momentum from the wind that it was harvesting at a lower speed.

As philip correctly states, neither of these issues relates to energetic impossibility, but rather to the inefficiency of the mechanism of extracting energy from the wind. So a rigid-sailed sailboat does not suffer the problem of its sail failing to fill, which removes that inefficiency issue. The boat is extracting energy from the wind to overcome various air and wind resistances, and so there is no reason it cannot additionally overcome the air resistance of travelling such that its component of speed in the direction of the wind exceeds the windspeed. All we need is a mechanism for extracting energy from the wind that allows it to exceed that. There is no reason we cannot continue to harvest energy from the wind at right angles to the direction of travel with a suitable energy extraction device.

It is no more problematic than the existence of a glider. A glider has to overcome air resistance in its direction of travel, and typically considerably exceeds wind speed in its direction of travel. I think a glider has a minimum speed relative to the air to avoid stalling: so that would imply it must travel faster than the component of the wind in its direction of travel. A glider harvests energy from the vertical component of air motion, but is designed to do so only when the vertical component is in an upwards direction. So a glider can carry on flying indefinitely, but only when there is sufficient upwards vertical motion in the air for it to harvest energy from. This can come from travelling near a steep hill where the hill deflects the wind upwards. Or it can come from natural convection of the air which exists only in suitable weather conditions, and also depends on suitable ground conditions to generate the convection. Otherwise the glider comes back to earth before very long.

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Re: Sailing faster than the wind?

Post by shpalman » Thu Jul 01, 2021 10:36 am

A glider obviously conserves energy - in steady flight in still air, the glider needs a small amount of energy input to balance that lost via drag, but this comes from gravity doing work on the glider pulling it down towards the earth. Now if the air were moving upwards because of a ridge or a thermal, well, the glider is still moving forwards and slightly down through that air, but if the air's vertical speed upwards (relative to the ground) is more than the glider's vertical speed downwards (relative to the air) then overall the glider gains height.

A sailing boat is complicated and interesting because of the relative motion of the air and the water.
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Re: Sailing faster than the wind?

Post by jaap » Thu Jul 01, 2021 11:53 am

Martin Y wrote:
Thu Jul 01, 2021 8:55 am
That's what I presumed they meant but when I asked on the original JREF thread I was told that no, they really do mean that a tacking yacht can have a downwind component of its velocity which is greater than the windspeed.

A yacht cannot run directly downwind faster than the wind but it can achieve it if it is tacking.
Actually, after thinking about it this might be true.

In theory this could be possible by using the momentum gained from a sideways movement to coast forward during the tacking turn. It is somewhat similar to one of the objections to the downwind car, where gusty wind was blamed for first accelerating the car and then allowing the car to coast on by inertia faster than the now reduced wind. In both cases the vehicle is temporarily moving faster than the wind but decelerating while doing it. If a sailboat could convert enough of its sideways momentum to forwards momentum during the tacking turns, and make those turns slow enough to overtake the wind that previously gave it that momentum, it is not impossible for its net speed to be faster than the wind. I don't know if this actually happens though.

Most of the time we use a simpler mental model of the situation though in which all the travel is along the same angle against the wind, and tacking is instantaneous and just flips the angle. In this model it is impossible to go faster than the wind in the direction of the wind. The reality is a bit more complicated than that though.

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Re: Sailing faster than the wind?

Post by jaap » Thu Jul 01, 2021 12:25 pm

jaap wrote:
Thu Jul 01, 2021 11:53 am
Martin Y wrote:
Thu Jul 01, 2021 8:55 am
That's what I presumed they meant but when I asked on the original JREF thread I was told that no, they really do mean that a tacking yacht can have a downwind component of its velocity which is greater than the windspeed.

A yacht cannot run directly downwind faster than the wind but it can achieve it if it is tacking.
Actually, after thinking about it this might be true.

In theory this could be possible by using the momentum gained from a sideways movement to coast forward during the tacking turn. It is somewhat similar to one of the objections to the downwind car, where gusty wind was blamed for first accelerating the car and then allowing the car to coast on by inertia faster than the now reduced wind. In both cases the vehicle is temporarily moving faster than the wind but decelerating while doing it. If a sailboat could convert enough of its sideways momentum to forwards momentum during the tacking turns, and make those turns slow enough to overtake the wind that previously gave it that momentum, it is not impossible for its net speed to be faster than the wind. I don't know if this actually happens though.

Most of the time we use a simpler mental model of the situation though in which all the travel is along the same angle against the wind, and tacking is instantaneous and just flips the angle. In this model it is impossible to go faster than the wind in the direction of the wind. The reality is a bit more complicated than that though.
Thinking about it a bit more, I'm now beginning to be convinced that it is possible, even in a steady state, for a sailboat to go faster than the wind in the wind direction. It would be hard to get to that state because you'd have to build up to it to get past that point where the boat is moving at the same speed as the wind. So first you'd have to tack across to get a sideways speed much faster than the wind, which is no problem. Then you'd have to turn, without losing too much of that speed, so that you are now overtaking that wind, in effect going against the wind. Then set up your sail as if you are going against a headwind. The boat angle will be different to a normal headwind, but I think it should be possible if the boat is efficient enough.

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Re: Sailing faster than the wind?

Post by shpalman » Thu Jul 01, 2021 12:31 pm

If you're sailing downwind you're not tacking, you're jibing.

According to the wikipedia I've been reading this morning.
Many sailboats are significantly faster sailing on a broad reach than running (sailing straight downwind). Thus the increased speed of a zig-zag course of alternating broad reaches can more than make up for the extra distance it takes over a straight downwind course.
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Re: Sailing faster than the wind?

Post by Martin Y » Thu Jul 01, 2021 4:08 pm

Again, in the JFEF thread a decade ago it was speculated that a yacht's momentum might let it run ahead of the wind in turns while jibing* but the Blackbird guys and others again said no, they really did mean that in a steady state running on a broad reach a yacht really can outrun the wind which is powering it.

One way of thinking about the Blackbird's propeller blades which helped me was to imagine standing behind it and pushing forward on the prop blade as if your finger was a single streamline of the wind. If the prop was a turbine you would expect the blade to turn aside as your advancing finger moved, with your fingertip sliding across the surface as the blade swung out of the way. But it can't do that as the prop is chain linked to the wheels. What happens is that the whole craft moves forward as you push, and the turning wheels force the prop to turn in the opposite direction. So your point of contact with the blade does indeed slide across its surface but the pitch of the blade means that it climbs away from you. By the time the whole blade width has slid across your fingertip the Blackbird has moved forward by the distance your finger moved plus the depth of the pitch of the prop blade.

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Re: Sailing faster than the wind?

Post by basementer » Thu Jul 01, 2021 4:23 pm

I realised that I find it very easy to imagine that, with a suitable angle for the sails and appropriate gearing, one could put a windmill on a car that propels it directly upwind. The tricky thing was convincing myself that this is also applicable when the car starts by moving downwind rather than from rest.
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Re: Sailing faster than the wind?

Post by Boustrophedon » Sun Jul 04, 2021 5:01 pm

More Youtubers have a go at attempting to explain it. Rosie does a better job than steve Mould who misses the point somewhat.

Engineering with Rosie https://www.youtube.com/watch?v=CkhBsko ... gwithRosie

Steve Mould https://www.youtube.com/watch?v=u5InZ6i ... SteveMould
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Re: Sailing faster than the wind?

Post by Allo V Psycho » Wed Jul 07, 2021 4:38 pm

So if it was a boat, and the propeller on the superstructure was connected to a propeller below the surface, rather than to wheels, would that work?

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Re: Sailing faster than the wind?

Post by monkey » Wed Jul 07, 2021 4:49 pm

Allo V Psycho wrote:
Wed Jul 07, 2021 4:38 pm
So if it was a boat, and the propeller on the superstructure was connected to a propeller below the surface, rather than to wheels, would that work?
The guy who built it reckons so (theoretically).

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Re: Sailing faster than the wind?

Post by Allo V Psycho » Thu Jul 08, 2021 7:52 am

monkey wrote:
Wed Jul 07, 2021 4:49 pm
Allo V Psycho wrote:
Wed Jul 07, 2021 4:38 pm
So if it was a boat, and the propeller on the superstructure was connected to a propeller below the surface, rather than to wheels, would that work?
The guy who built it reckons so (theoretically).
Cool! I'm imagining it as a hydrofoil, and with a lateen sail forward of the propeller rig. I wish I could draw...

Edit: or a catamaran hydrofoil...

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