The Wind Powered Sail-less Boat

You have not got Slide 2 quite right yet.

If you replaced the propeller with a spinnaker the cart might achieve 2m/s in a 3m/s wind. We will still assume the rolling drag is constant at 9N. The wind is powering the vehicle and in this case providing 2 x 9 = 18W.

The propeller is already able to push the vehicle at 1m/s through the air against a rolling drag of 9N. I stipulated that the drag does not increase with speed. As the tail wind slowly increase the drag does not change so the prop is always able to keep the vehicle moving at 1m/s through the air. The vehicle speed is tail wind speed plus the 1m/s. The vehicle requires 36W to achieve this and 27W comes from the wind. (IT IS ESSENTIAL YOU GET THIS)

If you like, you can envisage the prop as an active spinnaker that is capable of moving 1m/s faster than the wind providing the rolling drag is constant at 9N. Of course if the drag changes then this is not true but I have stipulated that the rolling drag is independent of speed to make the analysis easier.

I will introduce Slide 3 but do not go on if you do not get that the propeller does not need more power for the vehicle to do 4m/s in a 3m/s tail wind. In Slide 3 there is a similar vehicle but I have ditched the prop and electric motor and installed a small generator driven from the rear axle. The questions are:
1. Can I generate power to charge the on-board battery if I push this vehicle along the roadway with another vehicle. (Think of something like a bicycle dynamo only gear drive to improve efficiency)
2. If you agree it is possible are the efficiencies reasonable and power calculations correct.
3. Is the reduction in the wheel friction to equivalent of 4N at 4m/s reasonable given my vehicle design with cycle wheels and lets say total mass of 30kg? (you might want to check the bike calculator linked previously.)
4. What force will the pushing car be required to apply?

Rick W

 

I have revised the Slides based on discussions so far. I now have the drag components separated and changing with speed. The questions would be slightly different now.

As far as Slide 2 goes you must grasp that the operating conditions for the propeller have not changed although the vehicle is now moving at 4m/s. The propeller is still producing 9N of thrust in an apparent wind of 1m/s. The power required to do this at 60% efficiency is still 15W. The wind is providing the majority of the power required to overcome the losses. In the conditions now shown the vehicle would still be accelerating in Slide 2 because thrust is much greater than total drag.

If you do not get this then you cannot be convinced of DDWFTTW.

Rick W

 

I think that might be an over-statement. Different folks respond to different explanations. There are several valid approaches.

 

I am not referring to the Slides. I am referring to the notion of a propeller exerting thrust on a tail wind and moving over the ground at tail wind plus the apparent wind. The conditions that the propeller see are exactly the same as it sees in the same apparent wind without any tail wind.

To me this is a key. It is clearly where Guillermo got stuck because he could never get beyond the propeller being a spinnaker.

There does not seem to be argument that a propeller driven vehicle can move forward through the air. But then introduce a tail wind and for some reason they cannot see that it will speed up even if the rolling drag is constant with speed.

Rick W

 

Understood. But I imagine there are people that are willing to accept that ice-boats can achieve and sustain a 45 degree downwind course with a downwind VMG of 3X the true wind speed. They might not understand the aerodynamics, but they accept the GPS and the testimony of the ice-boat racers that do it all the time.

From there they can accept that two such ice-boats can do this all day long, side-by-side, on opposite tacks. Finally, they can accept that we could attach those two ice-boats with a telescoping pole to form a single vehicle that goes DDWFTTW.

I'm certainly not claiming everyone will accept this argument (although I can't for the life of me understand how you could deny it). But it is one way to accept DDWFTTW without understanding even the basic aero stuff.

 

But your cart has a propeller. You can mount a convincing argument about the two ice boats to prove that DDWFTTW is possible but then you build a cart with a propeller.

There is a leap of faith in accepting the propeller can do the same job if one cannot accept the point I make about a propeller in a tail wind. If you want a cart to demonstrate DDWFTTW using a propeller then people need to understand how a propeller works.

Rick W

 

Well, there's a two part answer to that.

1) I only aim to convince them that DDWFTTW is possible. If the two ice-boats on alternating tacks does the job - I'm happy. If they want to understand the cart, I'm happy to explain it as well. If simply seeing it's possible for the cart on the treadmill does the trick - I'm still good (although I don't think that's ever happened).

2) If they don't get the propeller in a tailwind, but they do accept that an ice-boat actually DOES do as advertized, I can take them through the development of two ice-boats on continuous 45 degree downwind tacks, spiraling along a figurative cylindrical earth. Just shrink that earth (very considerably) and shrink the ice-boat sails to the size of a prop blade and you have our prop. Now you just use the cart's frame, wheels, and transmission to provide the kinematic constraint previously provided by the ice-boat blades. Again, you can get there by understanding that an ice-boat DOES do it, even if you don't understand HOW it does it.

Given all that, I have to agree it's good to understand the business about the prop operating in a favorable wind. However, I'm always careful to avoid talking about the prop pushing on a tailwind, because it leads to one of the most common misconceptions among those that finally accept it works. They think there are two air masses converging behind the car. They picture the prop-thrust "bunching up" or pushing off of the wind from behind. The reality of course is that the prop is simply operating within a fluid that has an advantageous velocity relative to the ground.

 

I have done some design work on a manned boat to achieve DDWFTTW.

It is easy to look at numbers and not appreciate what they mean. Something showing the proportions provides greater meaning.

I was intending to show a bike chain drive between the turbine and prop but that would be too unwieldy. I expect a carbon shaft up the prop mast would be the best.

This is not something I would ever contemplate building and I doubt anyone else will. It might be possible to reduce the prop size but it means it will need higher wind to operate.

The exercise of looking at a large model boat might yield something more practical but it will still need quite a large prop.

I have included a performance curve to give an idea what this could achieve. In the current configuration power transfer becomes a serious consideration over 10m/s. I did not go above 15m/s wind because the power is over 10kW. This would require much heavier components than I have allowed for in the displacement.

Rick W

 

commercial airlines regularly exploit this theory by flying in jetstreams to reduce fuel use etc.

if a river boat has a cruise speed of 10kts and makes a return passage in a 3kt current then it is travelling upstream at 7 kts and down at 13kts give or take losses for other external factors. i'm sure most would agree with this, i can't see why it is being disputed in the dwfttw examples?

 

Flying in jetstreams doesn't exploit the energy available at the interface of two air masses, or an air mass and surface, etc. It simply takes advantage of a tailwind to arrive at its destination more quickly. In this case the plane simply adds its own velocity to that of the jetstream to yield a higher velocity relative to the ground.

 

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is flying in a jetstream not the same as rick is saying in the above quote? groundspeed= tailwind+aircraft airspeed?

please feel free to say where my thoughts are mistaken.

 

Yes it is. When you said flying in a jet-stream uses this same principle, I thought you were refering to the method of going DDWFTTW. My apologies.

 

Spork has proposed exactly the scenario (derived from a pair of iceboats) that I came up with yesterday. As he says, each of us will have our own way of proving that DDWFTTW can be achieved. I also found an analysis (on a physics forum that I forgot to bookmark) that makes it clear why the propeller on a cart doesn't violate any laws of physics: at the steady-state velocity, although the force on the wheels and the force on the airscrew are equal and opposite, the airscrew and the wheels are not moving at the same velocity with respect to the medium they are operating on - hence they do not develop/absorb the same power. The difference overcomes the losses.

Back to Rick's Slide 2. OK, if we temporarily delete the active and reactive forces defined in Slide 1, we are left with a wind and a zero resistance (because you said resistance doesn't increase) - so the rig will happily accelerate up to the velocity of the wind. Now add back the forces from Slide 1, which were based on a steady state velocity of 1m/s. Now the cart is doing 4m/s, but we still haven't provided a source for putting as much charge into the battery as we are drawing from it.

Rick, you have already said that, while theoretically possible, a system working on the air/water boundary rather the air/land one isn't really practicable. I agree, partly because the losses in any kind of water drive are going to be far higher than those for wheels on land.

 

Interestingly, this seems to be a common approach that many of us have arrived at independently. I'm pretty sure we each think we're the first to think of it.

I think I know the physics forum you're speaking of. That forum is absolutely shameful in its closed-mindedness, and the incredible lack of understanding most of its members have about basic physics. That said, this particular analysis is right on the money (and has been posted on many other forums as well).

 

Not sure if it's the same one, because the one I looked at today had a 'proof' that DDWFTTW cannot work... Definitely populated by people past their prime, who have lost the ability to abandon preconceived notions. I was never good at applied maths, but I knew that when things didn't work out it was usually because my approach was invalid.

I'd expect the iceboat scheme to have appeared long ago - I'm surprised it isn't used as a way to convince even people with very little education in physics or mechanics. I've done a bit more thinking about it, and come up with some ideas which are also likely to have been worked through by plenty of other people:

1. If you couple two sails on opposite tacks in this manner, there is no longer any need for contact with the ice to provide lateral resistance - the forces normal to direction of movement cancel out. This should mean that such a craft has the potential for a VMG a little greater than that of the equivalent iceboat, because its runners can be aligned with the wind direction and not provide any side force.

2. It's easy to imagine an escapement-like system for gybing the sails as they reach a predetermined offset from the centreline (think of those irrigation sprays). It would be a good idea, however, to use some of the lateral force to charge a battery to provide power to sort the sails out if things stall. This could also be used to sheet in the sails during the initial acceleration period.

3. One sail would have to be slightly behind the other, so that they could cross over as each swings from one lateral extreme to the other.

Logically, the above system is no different from an airscrew, but it doesn't have to be anywhere near so high off the ground so stability is no longer a significant problem.