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I think the stuff about going downwind faster than the wind is all bogus.
The next interesting question is how fast it can go upwind though
I've finally got round to looking at the link in the original post. I think the stuff about going downwind faster than the wind is all bogus. As soon as you reach the speed of the wind you lose all power, and from then on your turbine will do nothing but add to the drag - it's worse than a perpetual motion machine.
Yes but look at the vectors in the lemon pip analogy. The maxium velocity is achieved when the direction of the force (wind) against the immovable, slippery plane (keel) are nearly at right angles. If you are pushing your pip (wind dierection) in the same direction as the slippery plane (keel) you only get the pip going as fast as you can push (as fast as the wind).
Their friendship started as an argument over an aerodynamics riddle that hinged on whether you could know the true direction of the wind while hang gliding without looking at the ground. (You can, though Cavallaro has never fully conceded the point.)
my internet connection is too slow to watch the videos so the answer may be in those.
If it's true that boats moving at perhaps 135 degrees to the wind make actual downwind progress faster than the wind
I had been wondering how the wheels were connected to the turbine, but it appears to be a simple connection without variable gears, which fits in with the extremely slow acceleration: the wheels may initially hold the turbine back.
Once the vehicle starts moving faster than the wind, the turbine is from that point on moving the wrong way to act as a propeller.
Do they flip the blades to a different angle to get round this problem?
I now want to try to understand how a boat or ice/land yacht can make progress downwind faster than the wind,
but it's going to be a lot harder to think through as it's not so easy to see energy being picked up from the water and then thrown out from the sail.
Quote from: David Cooper on 23/03/2013 22:02:03 my internet connection is too slow to watch the videos so the answer may be in those.You should really watch those first. This will spare you a lot of guesswork (pay attention to the blade angle vs. rotation).
Quote from: David Cooper on 23/03/2013 22:02:03Once the vehicle starts moving faster than the wind, the turbine is from that point on moving the wrong way to act as a propeller.The pitch and transmission are chosen such that it always acts as a propeller. Nothing changes at wind-speed about that.Quote from: David Cooper on 23/03/2013 22:02:03 Do they flip the blades to a different angle to get round this problem?No. The prop pitch is always positive. On the prototype the pitch is even fixed, and it still accelerates from zero to >2 windspeed.
You have to go the a reference frame that moves directly downwind faster than the wind, at the same speed as the downwind VMG of a boat going at TWA 135°. In this frame the boat slows down the moving water via keel, and accelerates the air via the sail. This frame is analogous to the rest frame of the DDWFTTW cart.
There is no turbine - it never acts as a turbine, and that's the real reason the acceleration is so slow rather than being down to torque. I had thought that it was acting as a turbine to begin with, but it isn't - the whole vehicle is actually acting as a very ineffective sail, but as it gets blown along faster it turns the propeller faster and faster as a propeller, so instead of having air moving downwind through a turbine there is air being moved upwind through it.
I'm still struggling to visualise it, and I can't see how a keel can slow down the water as an ice runner isn't going to be able to slow down the ice,
In the case of the boat travelling at 135 degrees to the wind, this bar of soap analogy still fits, but because the boat is making downwind progress faster than the wind, it's hard to see how it can still be being squeezed from both sides.
Now try it again though with the water moving while keeping the air still. The water is flowing towards you from ahead while the air is still.
What I'd particularly like to see now though are diagrams of tables showing different angles to the wind and boatspeed, ideally for many different kind of craft (including slow monohulls).
Once you have faster internet you should check the animations. They pretty much show what you are trying to grasp intuitively.
Here is an article explaining this as well:...sorry, you cannot view external links. To see them, please
REGISTER or LOGINThe top two are catamarans, and I particularly wanted to see how well the Tornado (until recently an Olympic class boat) races the wind - the answer is that it just about achieves the speed of a ten-knot wind at 135 degrees and will therefore fall far short overall, though at 130 degrees it goes 10% faster than the wind and actually makes slightly faster progress downwind. The lowest part of the line is the point where progress downwind is greatest.Actually there's a serious flaw with those diagrams which makes them very misleading. If you look at the one for the Soling (Olympic class monohull), it looks as if it can gain by tacking downwind, and the text below claims that all six of these boats do gain by doing that, but if you apply pythagoras/trig to the diagrams you should be puzzled at how a point where the wavy line hits 8 knots on th 135 degree line can be further down the diagram than the point at 6 knots on the 180 degree line. The answer turns out to be that the numbers appear to have been put on the wrong lines with 4 actually being 3, 5 being 4, etc., so either the speeds are wrong or the graph is distorted, and I suspect the former because if it was the latter there would be no advantage for the Soling in tacking downwind. Clearly you need to keep your wits about you when looking at polars.Edit: I've just seen the links you've added. It's interesting to see the Class C ketch in the tall ships one - in 3 knots and 20 knots of wind it should tack downwind, but at 12 knots and 30 knots of wind it should go directly downwind. You really need to know your boat's polars.
The answer turns out to be that the numbers appear to have been put on the wrong lines with 4 actually being 3, 5 being 4, etc., so either the speeds are wrong or the graph is distorted,
The most impressive VMG preformance on water, that I'am aware of:...sorry, you cannot view external links. To see them, please
REGISTER or LOGINAs a child I used to sail Mirror dinghies in a handicap fleet which contained at the top end Nacra 5.2 catamarans (crewed by people who raced at national competition level). We all sailed more or less directly downwind, not knowing any better, though the p.y. handicap ratings for the boats would have been based on the way people sailed back in those days. Things have clearly moved on a lot since then, though I still can't find a polar for the Mirror dinghy. I imagine that it's quite hard to work them out though, and different people measuring speeds and angles in different and varying windspeeds are going to produce different polars. It would take GPS and windspeed and direction measurements to do the job properly by putting such devices an all boats and merging the data to create the most accurate polars, although they'd still vary just through weight and weight distribution of different crews, plus steering and sail handling style, so even then it won't be possible to get anything definitive. It will of course be able to distinguish between the top sailors and the rest, so it would help everyone up their game, indicating where they're doing something wrong.
Things have clearly moved on a lot since then,
Great posts MarkV - thanks
Thanks MarkV, for the detailed clarification and explanations; I have only a simple understanding of the principles, not the details necessary for a clear explanation. Your contribution helped me too