The Wind Powered Sail-less Boat

Why do you think there has to be some "extra" energy? It seems to me you are not fully understanding the basic physics of sailing, which is based on the velocity difference of water and air NOT just air velocity. You extract energy from that velocity difference indipendently of your own velocity and direction and you do it my making that velocity difference smaller in your wake. Thus you leave behind air and/or water that has lesser kinetic energy than it had before you passed by. The actual formulation depends on your frame of reference.

You are talking about a extracting energy from a "bigger amount of wind". What if you had a huge spinnaker that would extract energy from the same "bigger amount of wind"? Would that too go faster than the wind DDW? Obviosly not, thus the key is not extracting energy from a big amount of wind. It is being able to still extract energy while having VMG above the velocity difference.

Joakim

 

Hello all, back again, fools rush in and all that. I was really taken with the iceboat explanation I read over at Sailing Anarchy. It occurred to me to imagine an iceboat towing an ice skater downwind. The skater would keep his course DDW, while the iceboater was free to tack back and forth downwind. Looong towrope. Having arrived at the other side of the frozen lake (and having arrived before the hot air balloon that set off at the same time going DDW), they all went back to the start point, ascended in the balloon, and were amazed to see that the tracks in the ice perfectly described the motion of a propeller blade tip rotating around the propshaft. My suspicion is that the cart and boat would be pulled forward by the lift of the prop blades, as the skater was pulled forward by the single sail of the iceboat. If this is true, then this lift would be a function of both the motion of the true wind and the motion of the propeller that makes the true wind into the apparent wind. All the wheels and water turbine are doing is spinning the prop. The true wind must be where the energy is coming from because none of these contraptions will start or continue to operate without it, even when exceeding true wind speed DDW. How this happens is out of my field of knowledge to explain, but I am content to know that there are folks here that can. And have. In math I was too busy reading history to learn. Hence my reliance on visualizations. I do hope I have gotten nearer the mark this time.

 

The energy comes from the air movement relative to the water.

I have taken a 6m diameter 3-bladed prop with maximum blade chord of 470mm. It is connected to the turbine through gearing to achieve 40rpm when the boat is doing 7kts (say 3.5m/s).

The performance data for the prop is given in the attached table. It is able to produce a static thrust of 51N with 81W of input power.

The mechanical linkage between prop and turbine is operating at 97% efficiency. Hence the output power from the turbine has to to be 83.5W.

My 800mm diameter water turbine is operating in a nice regime and is achieving 80% efficiency so the power extracted from the water is 104W. The drag on the turbine at 3.5m/s is thus 29.8N.

The hull drag on my long slender boat is only 20N at 7kts. So total drag of turbine and hull is 49.8N. There is no wind drag because apparent wind is now zero. The prop is set clear of the hull such that nearfield air flow does not impact on the hull. But the prop is now producing 51N so the boat is continuing to gain speed.

The system input power is the wind velocity over the water times the force applied to the vessel at the propeller. This is 3.5 x 51N = 178W. The boat drag is absorbing 3.5 x 20N or 70W. The turbine is absorbing 104W. Excess power is 4W available to continue accelerating.

Rick W

 

If you think of wind, you don't care and know what makes the air move and what is the energy path that originally gave wind it's kinetic energy. For a wind tunnel or a treadmill you know it is the motor that makes it move. Do you consider every wind tunnel test invalid?


Joakim

 

Guillermo is just a kook, for reference.

His remark about 'completeness' pretty much demonstrates that. Only mathematicians interested in Foundations of Mathematics and Kooks reference that term in that way. He might be a naval architect or something, and maybe not even a kooky one, but here, he's just a kook.

 

You keep ignoring the *sail* of the cart and continue to focus on the *chassis* of the cart. That won't get you far.

***The sails of the cart are not going DDW, only the chassis is. The sails are on the same broad reach that the ice-boat example is on.***

If you believe I'm wrong on that point, then say so and we'll demonstrate it, but to continue to say you need energy for the cart that you don't need for the ice-boat is just silly when they both get and use their energy the exact same way.

So Guillermo, do you agree that the sail tips on the cart are on the same angled broad reach as the sail tip on an ice-boat or do you believe that the sail tips of the cart are going DDW?

Answering that question directly would go a long ways towards determining where our disconnect is.

JB

 

Propably not quite true. Notice "!" and stalled %. The analysis of JavaProp is not accurate when these appear. According to the link I gave (#461) the static thrust should not be more than ~30 N at 81 W. For 51 N at 81 W the diameter needs probably be over 10 m.

Joakim

 

I've got a dollar that says he won't be answering that question in any way, shape, or form. You feelin' lucky?

 

Joakim
You are right. I only use JavaProp as a quickie analysis which is about all that I am prepared to put into educating some.

I can achieve my stated operating conditions without stall if I go up to a 9m 2-bladed prop. It has a NACA 16-408 section having a maximum chord of 420mm. The near field velocity is 1.28m/s.

I think we had determined earlier in the thread that we needed something like 16m to get a manned boat in operation so the 9m is for a large model. This is consistent with the hull drag I nominated.

As I have pointed out many times; impossible it is NOT, practicality is another matter.

I havee looked at the design conditions for JBs manned cart and its low rolling friction combined with wheels rather than turbine makes a dramatic reduction on the required prop size.

Rick W

 

From the kook

Going back to the basic physics of sailing boats, we can write the equation of movement for a sailing boat (not taking into account heeling) as:

Fs = Fh (vectorial)

Where ‘Fs’ the force of the wind acting on the sails (or whatever device) and ‘Fh’ the force acting on the hull (whatever type and/or appendages).

Simplifying for a dead downwind situation, this can be solved as:

ma = Fs - Fh

where ‘m’ is the mass of the boat and ‘a’ its acceleration.

All the forces acting on the hull go against movement and it doesn’t mind what kind of hull it is or what kind of appendages or devices it has. A propeller/turbine spinning in the wake always takes energy from the boat and slows it.

Period.

 

Of course not. I consider invalid the treadmill experiment as being equivalent to the open air test, as it does not address the equality in the amount of energy applied to the cart.

Cheers.

 

Explain me how, please.
Again: would you please perform the energy balance?

 

Well, there you have it. Everyone else's analyses pale in comparison to the rigor you've brought to bear. Heck, we didn't even end ours with "Period".

Aside from the COMPLETE red herring of attempting to write the equations of motion for a traditional sailboat going direction downwind (which has nothing to do with the problem at hand), it still leaves us with one sticky problem... your analysis and my reality don't match. Shouldn't your analysis be able to describe what any of us can easily observe?

I'm also curious why your analysis doesn't follow the scheme of looking at the energy flux into and out of a control volume during a transient event. Is it because:

A) You wouldn't like where that leads to (<<< obvious sarcasm).
B) That's a silly approach that wouldn't yield anything useful
C) You wouldn't begin to know how to develop such an analysis nor how to interpret such results - despite the fact that you keep demanding exactly that.

Clearly this is a rhetorical question, as we've come to learn that you only ask questions - you don't answer them.

 

It's been explained to you countless times.

Again with the energy balance!? Would you please perform the energy balance you requested for a traditional sailboat on a beam reach?

 

I do not consider those proper demonstrations, because they do not address energy

Of course. It's the mother of all this.


I keep on asking you: Please perform a conservation of mass and conservation of energy analysis, throughout whatever control volume you want, for an standing still cart in an steady wind as initial condition and that cart at speed greater than wind as final condition. Or if you prefer so, just when the cart reaches wind speed.
Thanks.