Challenge: MythBusters and Cruisers Forum

[ThinAirDesigns]

Originally Posted by Tropic Cat

Where's the model data? Heck, where's the data on these already rigged boats?

Try these on for size.

JB

[ThinAirDesigns]

Originally Posted by Ex-Calif

That is rig your propeller powered model boat. Put it in a hydro tank with water flowing 6 knots and blow six knots of air at the back of the propeller.

Ex-Calif, would you mind giving me just a bit more precise detail on your proposed test -- which direction is the water flowing and which way is the air blowing? Thanks.

JB

[Ex-Calif]

Originally Posted by ThinAirDesigns

That's the entire point of Galilean relativity, something established without fail for four centuries now -- of two objects moving relative to each other, one can't tell which is 'moving' and which is 'still' without external reference -- reference wind carts and sailboats don't have.

JB

There is relative motion between the cart and the treadmill belt. No relative motion between the cart and the wind. The cart is essentially stationary.

However there is propulsive force being applied by the propellor which is deriving it's energy from the power grid and being converted via the treadmill motor, belt wheels and gears.

There is definitely relative motion between the propellor and the wind.

Originally Posted by ThinAirDesigns

Ex-Calif, would you mind giving me just a bit more precise detail on your proposed test -- which direction is the water flowing and which way is the air blowing? Thanks.

JB

I am a little surprised you would ask that question.

A hydro tank would take the most controversial part of your rig out of the test. That is the treadmill which is obviously powered and the treadmill which is powering the prop.

If the prop concept were to transfer to the water. You would have a free rotating prop attached to a mast. For the purposes of the first test I would propose the mast not rotate and therefor the prop plane is fixed perpendicular to the centerline of the boat.

If you blow 6 knots of air from astern the propellor should propel the boat at greater than 6 knots.

The hydro tank would have a water current from the front of the boat such that the vessel could stay stationary in the tank. It is simply to counteract the forward motion of the vessel and add the realistic hydrodynamic drag such a vessel would experience.

Alternately a long hydro tank could be used then there is the problem of controlling the airflow.

So far you have demonstrated that a propeller driven, treadmill powered cart can be set up with a total drag coeeficient lower than the propulsive force that can be generated through the energy system.

There are videos of people chasing carts in scientifically uncontrolled circumstances claiming to have land based carts that go downwind faster than the wind.

I am fine with all that.

Now do the work, put it in the water in a scientifically controlled environment and demonstrate it.

This will shut up all the naysayers.

BTW - What you can easily make is a wind turbine powered boat.

Attach the wind turbine through gearing to a prop on the boat. Then you can go just about as fast as you want in any direction.

[Tropic Cat]

Originally Posted by ThinAirDesigns

Try these on for size.

JB

Ummm.........


"...The retarding drag force associated with supporting the weight

W of the vehicle can be characterized by a resistance coefficient Cr. For a wheeled wehicle this Cr would be the rolling-resistance coefficient, and for a hydrofoil boat this would be the drag/lift ratio. For a buoyancy hull, Cr would be some function of the Froude and Reynolds numbers, whose details are not considered here....."

What am I missing? The math is impressive and it shows that the system works, but I have to point out that there is no water resistance number used in the calculations.

If we can assume the demonstation video is accurate, wouldn't it be appropriate to calcuate the system generated energy or "work" and plot it against wind speed? This should be expressed in Newtons, Watts, Joules or Horse Power. Only then can it be determined if enough power is generated to overcome the forces preventing a real world boat from moving through water.

Just a thought.

[ThinAirDesigns]

Originally Posted by Ex-Calif

There is relative motion between the cart and the treadmill belt. No relative motion between the cart and the wind. The cart is essentially stationary.

A: you're talking about the *chassis* of the cart, and not the sails.

B: when you say "No relative motion between the cart and the wind", that is exactly the same condition as going down the street at 10mph in a 10mph tailwind. Identical conditions.

However there is propulsive force being applied by the propellor which is deriving it's energy from the power grid and being converted via the treadmill motor, belt wheels and gears.

Your diverting to the question of what *generates* the wind that the cart sees -- a question that is irrelevant (but perhaps interesting).

If you question that above conclusion, let's go back post #58 and to the regatta on the deck of the aircraft carrier -- do the sailors and sailboats in that regatta know, care, or behave differently depending on whether that wind across the deck is generated through solar (answers A, C), nuclear (B), or electric (D, E, F)? Of course not -- the answer is apparently so obvious that no one will actually take the time to answer it.

Just for fun, I'm going to add one more option to that list of possible answers: "G": I lied --there isn't an aircraft carrier deck at all ... I built a carrier movie set, placing it on a giant and very long treadmill belt that is moving (or not moving) through the fog.

So tell me Ex-Calif ... I'm again asking you to either stipulate that every scenario in #58 (A through G) result in a successful regatta on 'deck', or to engage me on which scenarios result in something other.

Thanks in advance.

JB

[ThinAirDesigns]

Me:

Ex-Calif, would you mind giving me just a bit more precise detail on your proposed test -- which direction is the water flowing and which way is the air blowing? Thanks.

Ex-Calif:

I am a little surprised you would ask that question.

First, please do understand that I placed this thread in the "Brain teasers, and brain storming" section of the forum on purpose. Though there are others around with other intentions, I have none when it comes to building a water craft that will travel DDWFTTW. I've done it on land and though happy to share what I've learned, will let others take to the water.

Any questions regarding why this is on a sailing forum at all are answered in the OP.

A hydro tank would take the most controversial part of your rig out of the test. That is the treadmill which is obviously powered and the treadmill which is powering the prop.

And not being a boat designer, that's why I'm asking you for the particulars of your proposed test -- I'm not familiar with the terms that would be used in a hydro tank so I simply asked for flow and wind direction so I could imagine it and respond.

If the prop concept were to transfer to the water. You would have a free rotating prop attached to a mast.

No, no, no and no!!!! If the prop concept were transfered to the water, the air prop would be geared to a water turbine. ****At no time and under no circumstances will a "free rotating prop" relate to the DDWFTTW cart****.

The cart on the ground has it's prop geared to it's wheels. This gearing provides the 'keel constraint' of a traditional sailing rig. A water version of this device must have it's prop constrained to a turbine beneath the surface to do the same.

JB

[ThinAirDesigns]

Originally Posted by Ex-Calif

If you blow 6 knots of air from astern the propellor should propel the boat at greater than 6 knots.

The hydro tank would have a water current from the front of the boat such that the vessel could stay stationary in the tank. It is simply to counteract the forward motion of the vessel and add the realistic hydrodynamic drag such a vessel would experience.

Ok, Calif ... this is why I wanted you to clearly state your test.

You wish to have 6 knots of air blown from astern and you wish the water speed to be set such that the boat stays stationary in the tank. Let's say for sake of discussion that if the boat travels 1.1x the speed of the wind, it will have proven to you that it can be done.

From the above assumptions, I will give you the velocities and directions for wind, water and boat in your theoretical test, going from the wind of 1 knot and working up to your requested 6 knots. all directions and velocities are relative to the observer standing next to the hydro tank.

***********************
Test #1:
Air = 1 knot --->
Boat = 0 knots
Water = 1.1 knots <---

Test #2:
Air = 2 knots --->
Boat = 0 knots
Water = 2.2 knots <---

Test #3:
Air = 3 knots --->
Boat = 0 knots
Water = 3.3 knots <---

Test #4:
Air = 4 knots --->
Boat = 0 knots
Water = 4.4 knots <---

Test #5:
Air = 5 knots --->
Boat = 0 knots
Water = 5.5 knots <---

Test #6:
Air = 6 knots --->
Boat = 0 knots
Water = 6.6 knots <---

************************

Ex-Calif:

Now do the work, put it in the water in a scientifically controlled environment and demonstrate it.

This will shut up all the naysayers.

Ex-Calif, have I properly understood your test, and if done in a scientifically controlled environment and properly documented(say at an independent University lab) and returning the above results, do you feel this test is a valid DDWFTTW test?

Thanks.

JB

[Ex-Calif]

Originally Posted by ThinAirDesigns

Ex-Calif, have I properly understood your test, and if done in a scientifically controlled environment and properly documented(say at an independent University lab) and returning the above results, do you feel this test is a valid DDWFTTW test?

Thanks.

JB

I have no idea.

As of 2 posts ago I discovered we were talking about 2 completely different vehicles.

I have seen your math before and I have seen many threads about this.

Unfortunately, I suspect that it is impossible to nail down the boundary conditions, the vehicle in question or the goal to be achieved.

Any science requires specificity.

You seem to consistently refute that the cart on land, the gear cart on a treadmill and a propeller driven vessel in water are in any way different yet your conclusion remains that DDWFTTW is possible.

Specify more clearly in design terms what it is you are attempting to prove or disprove and a more convergent discussion can be had.

So far, I have avoided endorsing your claim or refuting it because even after all this reading your claim is ambiguous to me.

Just for the record I do not doubt the results of the geared, treadmill powered cart at all.

And now that you have stated you don't care to "take it to the water" I am much clearer on your point.

However, each time you post this on a sailing forum people assume (maybe incorrectly) that you proscribe a theory that a free propeller (analgous to an autorotating helicopter) will propel a sailing vessel DDWFTTW.

All I am saying is the science, math and experiment have not confirmed this theory.

So as I am not interested in carts and you are not interested in sailing vessels, I will rest.

[skookum]

Something to keep in mind. Ice boats and wheeled carts pick up little if any added drag from their supporting surface as they speed up since friction on ice or on wheels is not proportional to speed. It's a different story on water however. The vessel's drag will increase with the square of its speed through the water. I'd guess this would limit the top speed of such a vessel as compared to a land or ice vehicle of the same sail or disc area. Though I suspect with the right combination of hull shape, propeller shape and vessel weight, it could still achieve DDWFTW.

[ThinAirDesigns]

Originally Posted by Ex-Calif

However, each time you post this on a sailing forum people assume (maybe incorrectly) that you proscribe a theory that a free propeller (analgous to an autorotating helicopter) will propel a sailing vessel DDWFTTW.

Why would *anyone* assume that I am proscribing a theory involving a "free propeller" when not a single picture or video I produce nor display, not a single description I give, not a single drawing or formula nor a single post I write involve a "free propeller"?

You've got a severe case of "free propeller" tunnel vision going there.

But on the upside, at least you now know there is no free spinning propeller involved in DDWFTTW -- water OR land.

All I am saying is the science, math and experiment have not confirmed this theory.

On the contrary, they all three confirm that a "free propeller" will not propel any vehicle DDWFTTW ... land, water or any other medium. By the laws of physics, that method will not even get you to windspeed DDW.

JB

[ThinAirDesigns]

Originally Posted by skookum

Something to keep in mind. Ice boats and wheeled carts pick up little if any added drag from their supporting surface as they speed up since friction on ice or on wheels is not proportional to speed. It's a different story on water however. The vessel's drag will increase with the square of its speed through the water. I'd guess this would limit the top speed of such a vessel as compared to a land or ice vehicle of the same sail or disc area. Though I suspect with the right combination of hull shape, propeller shape and vessel weight, it could still achieve DDWFTW.

Absolutely right skookum. For reasons you've stated, I don't believe someone will use $40 bucks worth of of the shelf parts and create a water version as we did for land.

We're in the design/build process of a 3meter prop for a ride-along version of our cart and I've already been approached about recycling that prop once we're done with it onto a specialized water craft built for purpose.

I believe someone will make it happen, but I also suspect that it will be by only a small margin rather than by the multiples seen on land.

JB

[Tropic Cat]

A totally useless thread for a sailing forum. Let us know when you buy a boat. Until then....

Cya

[Ex-Calif]

Originally Posted by Tropic Cat

A totally useless thread for a sailing forum. Let us know when you buy a boat. Until then....

Cya

I am not so sure.

At least in my case and maybe ThinAirs we share a "lot of wind" in common.

LOL...

I really feel like an idiot. I have been reading these cart and propellor threads for a long time and it took me until this thread that we weren't ever going to talk about boats - LOL...

[ThinAirDesigns]

http://sailmagazine.com/

http://sailmagazine.com/racing/running_faster_than_the_wind/


JB

[Bash]

Ok. Let me be the first to admit. I'm a believer.