# Foil for current-driven propulsion?

Discussion in 'Hydrodynamics and Aerodynamics' started by MacktheYounger, Aug 30, 2022.

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### MacktheYoungerJunior Member

Hey all,

With the spread of foil sailboats and e-foil boards, I've been doing a bit of reading on the principles involved, and fear I may have fallen into the dunning-kruger valley. With this in mind, I'd appreciate the assistance of those better-educated than myself with poking some holes in an idea I've just had.

My understanding is as follows: A foil, just like a sail, operates like a wing. A fluid moves over its surface and generates lift, more-or-less perpendicular to the direction of the fluid's flow. The force of this lift is affected by the viscosity of the fluid and the speed of its flow, both having a positive relationship with lift produced.

With this in mind, why haven't we seen foil-driven boats? That is, boats which "sail" using water currents, rather than air currents. Given the viscosity of water compared to air, it would seem that a small foil, oriented correctly in a strong current, would be enough to propel a small dinghy at least.

Are water currents too capricious and hard to predict/react to? Is it simply the case that it would be terribly expensive, with no obvious advantages? Or is there some element of hydrodynamics that I'm ignorant of, that renders those considerations moot?

Thanks in advance to any and all responses.

edit: I wanted to say, I'm familiar with "wave-driven" boats like the Suntory Mermaid II that use the rocking motion of the boat itself for propulsion, but that's a rather different approach than using the currents directly.

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### Doug HalseySenior Member

Sailboats depend on forces acting on the water, not just on the air.

Without a keel or other means to generate hydrodynamic forces, they would not be able to sail closer than 90 degrees to the wind direction.

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### Alan CattelliotSenior Member

Hi MacktheYounger,

Your comprehension of lift generation is ok, but I would say that the polar of this kind of boat would be very narrow and the expected static speed (if reached) would be very small, since lift force is always perpendicular to the incoming flow. You can verify it by yourself, using a simple vector composition.

With a more intuitive approach, It never occured that I was able to sail my A-cat using my rudders only, with my mainsail torn. Anyway, many thanks , Mack, for your reference, that gives me the opportunity to discover this crazy idea of "wave sailing".

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### Doug HalseySenior Member

The total force on a foil includes both lift and drag components and therefore, will always be at a larger angle to the flow than you have shown.

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### Alan CattelliotSenior Member

Yes Doug, I agree with you. Neither is taken into account the hull drag, and the windage and.... My intend was only to put Mack's idea into a simple picture, avoiding too much arrows...

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### FlotationSenior Member

This video explains how the Artemis foiling cat can "sail" up the Amazon river at a speed of 30 mph.
With a wind speed of 0 mph.
Using the current going downriver at speed of 10 mph.

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### Alan CattelliotSenior Member

ahem.... the current only makes the boat create an apparent wind. Then the boat would jib, keeping its speed during manoever (hopefully). Then, it is assumed that the boat could accelerate, due to her capability to create an increasing driving force @AWA = 0 degree (magic ?????). Anyway, she does not use the lift of the foil only. The high aspect ratio of their sailplane allows very small AWA and may be seen as the main contributor to this (very) theorical race.

Honestly, I don't really know if Artemis has really done what is explained in the video, or if it is only wrong calculations, or worth... communication . Amazon waters are not blue... Sails do not produce driving force at zero incidence angle. Does Artemis use false publicity ?

What is true, is that most light multihulls are wind machines (some monohulls too). They create their own wind, allowing to go faster than the TWS. Even without foils. But with sails.

Advertising.... Communication....Media.......It is so nice to see that everyone is contributing to our fresh new world full of very responsible people, taking care of the environnement...How touching...

Last edited: Aug 30, 2022
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### FlotationSenior Member

My take from the video above is that it doesn't really matter if the water-medium is moving relative to the air-medium or vice versa. Which makes it sort of relevant to the original question.

Edit, thread from the first(?) time this video was posted:

Last edited: Aug 31, 2022
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### Alan CattelliotSenior Member

Sorry Flotation, I don't understand what does this particular part of your sentence mean. If you want to create a driving force from a wing shape, say a sailplane or a foil, you will need an incidence angle. You can't create a drive force from the wind of speed only. This video is a fake, and would have put a great shame on Artemis, if only the comments of true aero-hydro dynamicists had not been equally balanced with some of lazy "sunday's admirers", who prefer watching nice animated pictures, rather than performing themselves any corroborating calculations.

if one could just follow a little bit of what Will proposed, in his thread, it would perhaps be of some great help for science, in general, regarding this matter.

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### Alan CattelliotSenior Member

As I understand Mack's thread, the idea may be to try to sail underwater currents, with this kind of boat :

Although I may be mistaken...

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### SkyakSenior Member

There is no clear question so there is no clear answer.
To generate power, a craft must tap two masses with a usable relative velocity. Sailboats tap the air above and the water below. What two flows do you propose tapping, what is their relative velocity, and where on earth does this condition occur?
The only place I know where only current is used is rivers driving river ferries on cables.

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### FlotationSenior Member

Skyjak explains it better in his reply above:

"To generate power, a craft must tap two masses with a usable relative velocity. Sailboats tap the air above and the water below. What two flows do you propose tapping, what is their relative velocity"

From the perspective of a boat it doesn't matter if air moves above the water (10 mph wind with no current) or if water moves under the air. (0 wind condition in a 10 mpg current). A sailboat extracts energy (i hope this wording is correct) from the speed difference between water and air.

This probably doesn't all the questions @MacktheYounger has but is does offer another perspective.

A craft should also be able to extract energy from two bodies of water moving relative from each other. Sometimes water at different depths moves at different speeds for example.

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### Alan CattelliotSenior Member

Okay....Okay...Okay.... Thanks for the explanations (even if I don't see any equations, drawings or VPP result answering the question in this thread....).... But Okay.... Let's tap three ways, if you allow me to sum up with examples to illustrate what you are saying. I hope that it would not be too much of a burden nor an offense for you if these examples are too basics :

1. Dynamic soaring

2. Pumping the wind

3. Pumping the water

So... given these examples, can you show me the mechanic that exploits underwater currents , exactly the way a sailplane exploits wind ?

Extra question : Do you really believe what's being showed in the video from Artemis ? If so, would you please take your VPP and show me more precisely how it works ? With maybe a polar ?

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### Alan CattelliotSenior Member

To me, the question is clear enough, though, again, I may be mistaken. -Errare humanum est -

It is maybe better to wait for Mack to reformulate it, the way you like.

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### Alan CattelliotSenior Member

The perspective of perpetual movement ? Maybe, since Artemis is also a God. - Of Wild Animals ! (and of Chastity ! sic...)

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