Outrunning the Wind

The physics of 1. sailing with a velocity made good (VMG) of double windspeed, 2. of sailing directly downwind with a velocity of greater than windspeed, 3. sailing on a river with no true wind but with relative wind, etc, etc. are all fundamentally the same. The landsailer described should work exactly as it does.

The first landsailer that could sail directly downwind at greater than windspeed was demonstrated in the 1960's (+/-). Odd that it took so long, which I suppose might have to do with the seeming implausibility of it, even though it was well-known at the dawn of the 20th century that ice boats could sail at large multiples of windspeed, and could have VMGs directly downwind at small multiples of wind speed.

A "trick" of the windmill-powered landsailer is that the windmill is driven by the wheels, rather than the other way around. So the vehicle starts moving as a result of simple wind drag. That motion causes the prop to move in the direction that propels the vehicle, and which initially increases wind drag, because the prop is still stalled (operating at a far-too-high angle of attack). If one were to cut the chain between the wheels and windmill, the windmill would turn in the opposite direction (from the direction that is turns as the vehicle moves forward).

Some previous discussions:

http://cr4.globalspec.com/comment/574399/Re-Sailing-Newsletter-Challenge-06-01-10

http://cr4.globalspec.com/blogentry/13488/Wind-Powered-Cart-Goes-Faster-Than-the-Wind

The Wikipedia article on sailing faster than the wind.

http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind

On the following page you can scan down through the boat pictures to find the Windrocket, a boat I built to go fast, and which could sail at a VMG of greater than windspeed downwind, while sailing a course about 1.4 times as long as the direct downwind course.

In effect, the propeller is "sailing" across the wind rather than with it, and the wheel/chain business is analogous to a keel or centerboard. That may seem abstract, but it helped me to understand this wannabe paradox.

Definitely.

I like your term "wannabe paradox". It's not a paradox, yet appears paradoxical if you haven't thought about the physics of sailing.

Way back in the 1960s and 1970s I saw small sailing craft do this, it is not anything new, and is a well known phenomenen!

Xanasax

I'm confused, if it is such a well known phenomenon, why the fuss? I mean, apparently well respected physicists and teachers don't believe it can be done.

I know that you can sail across the wind (at varying angles to almost downwind) at higher than the (downwind) wind speed, but I think this is something else.

And it doesn't seem that every sailor is jumping to say they do it all the time...

You're right -- sailing directly downwind at higher than windspeed is not "well-known". In very fast wind-powered craft (ice boats, land yachts, and to a lesser extent soft water boats like my Windrocket) one can sail generally downwind to make a VMG of greater than wind speed. Ice boats (and even the Windrocket, when sailed well) tack downwind -- meaning that the apparent wind passes over the bow, not the stern. (In an ordinary sail boat, one jibes downwind, meaning that the apparent wind passes over the stern -- what you would expect).

An ice boat can sail at a VMG downwind of twice windspeed, by sailing a course about 45 degrees below a beam reach. However that is different than sailing a course directly downwind. On such a course, the best ice boat can only do a (largish) fraction of the wind speed. So on an ice boat, when you turn further downwind (from 45 degrees down from a reach) the boat slows dramatically.

So yes, this is something else. People who follow this stuff have recognized that a windmill-powered boat can sail directly into the wind (in fact they are ideal for that). As such a boat sails into the wind, its speed adds to the windspeed, creating more apparent wind for the windmill, higher windmill rotation speed, higher underwater propeller speed, and therefore more thrust. But many people have assumed that a windmill-powered boat could not go faster than windspeed when sailed on a course directly downwind. No soft sailed (or rigid wing) boat can exceed windspeed when sailed directly down wind.

However, a windmill that is driven by the boat's motion can create thrust (making it a propeller). (This is the reverse of the usually windmill-powered boat, in which the wind drives the windmill, which in turn drives the underwater prop.) The best way to drive a propeller from a craft's motion is to make the craft land based, so that the drive is highly efficient.

Although the arrangement would be less efficient, the idea might be easier to understand if the wheels drove a generator. Aero drag on the craft's structure and the motionless propeller will cause the craft to be pushed by the wind. The wheels rotate and the generator generates. Now drive the propeller with an electric motor in the direction which makes the craft move faster.

My wife wrote an as yet unpublished novel with an incident about this very concept. A smart-cookie princess notices that when stepping on a wet rock, the wedge between her foot and the ground can squeeze the rock into shooting fast off to the side. By putting two and two together, she sees that water-keel and sail-wind interaction can do the same thing with sailboats. The novel is basically a medieval/magical fantasy, so this turns into a brilliant and war-winning discovery.