# recovering thrust with a tfoil

Discussion in 'Hydrodynamics and Aerodynamics' started by philSweet, Jan 7, 2013.

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### philSweetSenior Member

The idea of placing a foil in the upsweeping portion of a boat's wake has me a bit puzzled. I have seen photos of people surfing on ship's wakes, so obviously at some scale this is possible under some conditions. But I question whether it is reasonable as a design feature in an unrestricted design. I would think there must be a specific limit for such recovery.

It has been suggested that there is a best location for recovering the energy and that involves finding a steep area of wake and getting the lift vector leaned forward and running the foil shallow since the upsweep angle will be greater. That makes perfect sense but I think one can only get a little bit of recovery with a little foil this way. As you try to recover more, It would appear the idea breaks down.

If you want to recover as much energy as possible, you are fighting the circulation of the wave orbital itself. The particles of water in the wake are all circulating opposite to the circulation that you need to induce upward lift around the foil.

Or put another way, I suppose the object of the game is to cancel the wave orbitals with the foil. That gets rid of the wake (locally), but it doesn't produce any lift at all if done well. It just reduces the circulation to zero.

This view leads me to a bunch of questions. I think I'll begin with these two-

1. Is the wake of a hydrofoil fundamentally different from the wake of a displacement ship? I don't see the wave orbitals as being produced in the same manner.

2. If a foil is towed though calm water and set to zero lift, does it produce upforce when towed through a head sea and downforce in a following sea?

My suspicion is that it doesn't matter where you put the foil longitudinally if you are going large. I think the fallicy in the argument for recovering energy from the wake in this manner lies in the assumptions made in modelling wakes. The wake is not modelled as a wave orbital, so there is no circulation in the wake models. Anyone else see a problem with that?

Last edited: Jan 7, 2013
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### jehardimanSenior Member

A vessels, or foils, wake is not a periodic wave, it is an impluse wave. It does not have a regular orbital (and a foils 'circulation" is a fictitiuos mathematical construct anyway). So in answer to your questions...

1) No, there is no real difference between the wake of a vessel and the wake of a foil, both are pressure generated impluse waves that appear to progress due to the body velocity.

2) This is a meaningless question, i.e. if the water is calm you can't have a head sea or following sea. But to answer the question I think you wanted asked: In general, if a foil is towed, it will generate lift depending on its angle of attack and relative inflow speed. In a periodic seaway, the wave orbital will generate a AoA and relative inflow depending on the relationship between tow speed and wave orbital velocity. If a foil is towed at the same speed and direction as the celerity of the wave train (i.e. the foil remains in the same relative location of the wave face), then there will be a fixed AoA and lift produced proportional to the inflow (think surfboard on a wave face...it can stay in the same position relative to the crest and be carried along by the wave).

In reality, what you are supposing is totally incorrect, i.e. it is not possible to "generate" propulsive force by using the lost impluse energy in the same direction. All you can do is manipulate the existing pressure distribution over the body to lower your pressure drag. What wave propulsion systems and sufboards do is use the vertical distribution of energy in the periodic waves to extract energy for propulsion (i.e. create a drag force in the direction they want to go).

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Phil, here is an article by dinghy designer Kevin Ellway that may help along with a sketch of an International 14 by Paul Bieker showing the lift from the T-foil and how it allows the crew to move aft upwind.T-foil with or without energy recovery pretty much proven to work on International 14's, National 12's and the occaisonal Cherub. Energy recovery proven to work, according to Paul Bieker, with a properly placed foil.
--
There may be some tidbits of interest here,posts 33,34,35: http://www.boatdesign.net/forums/hy...r-design-foil-assist-full-flying-40894-3.html

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### markdrelaSenior Member

Circulation as it relates to lift is defined instantaneously, around an instantaneous circuit. The "orbital" velocity field of a single wave or multiple waves is irrotational -- it has zero circulation for any circuit.

Your unconventional definition of circulation is for a particle track in time, which has no relation to lift in an unsteady flow like a passing wave.

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5. ### Number4Previous Member

Hi Phil,
The late Earthrace / Ady Gil's amas were positioned so that under certain conditions it could surf on it's own wake, or a following sea. I suppose this may be true for all of Loomes' wave piercing designs.

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### philSweetSenior Member

Thanks for the responses.

- Mark and others, regarding this-
I realize that wasn't making the jump from particle motions to vector field characteristics properly when I wrote my first post, but I am still inclined to think that for periodic waves with particle orbits there is instantaneous rotational flow within a bounded region. It will tend to average out to zero as the wave pattern progresses past the region in time or the region was expanded laterally to include a longer section of the wave, but regions of circulation could be selected. A small foil could be dragged along at a favorable location (contrary to my initial post).

I'm still chewing on jehardiman's comment that wakes don't have orbitals. That isn't digesting too well at the moment. Once a wave train looks like a moving wave, it is one. If there wasn't orbital motion of particles, you ought to have clapotis, not something that travels. Then again, if the wake isn't transporting energy as it travels, maybe there is some escape from this.

3. Does the wake transport energy in the direction of travel?

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### jehardimanSenior Member

That piece by Mr Ellway refering to Mr. Bieker's interpertation of what a T-foil is doing can be charitably described as the wrong understanding of the right data. Setting aside all the lift benefits of a stern foil, what it is really acomplishing is changing the effective hydrodynamic lwl, making the boat appear longer to the water. This is not new or revolutionary and in keeping with hydrodynamic theory. He his not "extracting" energy from anything, just lowering the Cr by pressure manipulation as I described above. (In fact the drag is shown as greater at low speeds as expected with curciform on that thin of a foil but the fact that the boat is so overpowered renders the greater drag at high speed moot.) In this case it is similiar to the stern wedges fitted to ships that do have limited power to reduce powering at a given speed by manipulating the stern pressure (actually negative relative to SWL). See the attached paper.

http://www.scielo.org.ar/scielo.php?pid=S0327-07932004000400011&script=sci_arttext

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=====================
Thanks for the info. In the I-14 class a few years ago Biekers foil system was
"new and revolutionary". He is one of the most respected sailboat designers and innovators on the planet as well as a naval architect and marine engineer. See the Proboat article-kind of old but still relevant:

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### jehardimanSenior Member

Both a wake and a periodic ocean wave are distributing energy, it is what they are doing with it that differs.

A periodic wave is caused by the shear stress of the wind on the water; i.e. the surface of the water is being pulled by viscious forces in a single direction. This sets up a mass flow in the direction of the wind. Because of the internal viscious forces in the water and the fluid interface, the way that this mass flow (i.e. energy transfer) manifests itself is a peroidic wave. Note that a wind wave orbital is not closed, that there is a second order "mass drift" in the direction of travel. If the wind blows over a finite region of an infinite fluid, these waves move out from the source to distribute this energy throughout the fluid in this mass flow, eventually setting up circulation currents so the transfered energy is "bound" in the moving mass of water. For a good explination of this see Weigel; "Oceanographical Engineering" Chapter 2.

Wakes on the other hand are formed by impluses. As an object moves through a fluid it creates a pressure disturbance. This pressure disturbance, if near the surface manifests itself as a rise in the water free surface, which is the only way the fluid can support that pressure. Think of it this way, lets say I push down on a 1 square meter area of water. The water has to get out of the way and the force required to push the water out of the way is proportional to the velocity of the push squared and the amount of water displaced is equal to the area x distance pushed. So we just added energy to the water...but initally just to the displaced water. This energy causes a rise in the still water level, proportional to the volume, equal to the energy added. This is all the energy that will ever be in that event. Now gravity takes over and attempts to return the surface to level. The wave front spreads out in all directions, getting longer..but the volume and energy must remain (effectively) the same so the wave gets smaller. In an infinite fluid, this wave will spread to the furthest limits getting infintessimaly small...think a rock thrown into a still pool and the single ripple spreading out. What a wake is, is not a single pressure event, but a continious event, like a string of thrown pebbles. And like a string of thrown pebbles, each descrete event interacts with the others giving the appearence of a single wave, but in actuallity it is multitude of individual events. Again, see Weigel; "Oceanographical Engineering" but this time Chapter 4 or you can look in PNA for a description of how wakes are formed and analyzied.

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### philSweetSenior Member

Thanks for sorting that for me. The particle motions in the wake are peculiar to the way a wake is formed and must be different to linear plane waves because the energy and momentum transport are different. I probably should have picked up on that before now, but somehow I didn't. It was Elway's paper that started this itch. I was pretty sure that at the end of the day, things didn't really work that way. There seems to be many parallels between the induced drag of wings and the induced drag of hulls. I'm hoping it goes beyond merely using the same modelling conveniences to treat the two cases (standard vortexes for instance).

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### daiquiriEngineering and Design

IMO, placing the foil behind the stern of a boat can have a positive net effect, if the geometry of the foil has been correctly designed. The overall effect on resistance, again imo, can be seen as the combination of three separate actions a foil would perform:
1. the volume of the foil, placed so close to the water surface, would act as a kind of stern-mounted bulb, creating it's own wave system which would be superimposed on the wave system of the hull. Whether the resulting wave train (and hence, wave resistance) of the foil+ship combination would be reduced or increased - it would depend on the speed, hull shape, foil's dimensions, depth and distance from the transom.
2. The foil's camber and angle of attack would create the circulation around it, which means lift. This lift would decrease the vertical force required from the hull, since the sum of lift forces produced by the foil and by the hull must be constant and equal to the ship's weight. Saying that the hull gives less lift (buoyancy + hydrodynamic) equals to saying that it has less displacement, which is clearly beneficial to the resistance. Basically, through this action the foil virtually increases the slenderness ratio of the hull.
3. The lift produced by the foil, having the point of action very far aft, will create a trim-down moment on the ship, which is often (but no always) beneficial for the overall ship resistance. By this mean, the action of the foil is similar to the action of the stern flaps or wedge. Again, there will be a point beyond which the net effect will become unfavorable, and it will depend on the ship speed and hull shape.
Considering the above, there will probably be a sweet spot for the foil's action, around which the net effect would be beneficial for the ship resistance. This sweet spot, for a given hull shape, will likely be speed-dependent, apart the obvious dependence on the foil design. If there's a need for a benefit over a wider range of speeds, the foil should imo be mounted to a variable-geometry (ideally, automatically controlled) supporting structure.

The problem is somewhat akin to the design of airplane winglets. They work, but they have to be carefully designed and matched to the wing they are attached to, for a given cruising speed.

Cheers

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### TackwiseMember

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Hull Vane

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### philSweetSenior Member

That would seem to be the case, but it is confusing. It is not obvious to me why the same technique used to improve a wing's performance by interacting with it's tip vortex should also work to improve a hull's performance by interacting with it's wake. I have just discovered there is a wake drag associated with aircraft and it is described by thin ship theory. I don't know if the solution is identical to hulls as far as the numbers go. A minimum wake airship is the Sears-haack body. Like in ships, the section area is important but the section shape seems to have nearly no effect. The prismatic coefficient of the thing is 0.589 . So I'm still adding ducks to my collection. I'll ask your help getting them lined up in a row later.

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### TackwiseMember

Phil ,

I don't think you should consider them as being the same technique! The winglet of an airplane is mainly designed and put in place to reduce the vortex which is always generated at the wings ends. A smaller vortex is less resistance.

For the Hull Vane or other Stern foils, their main benefits are not directly linkable to a big vortex. There are various possible benefits, and its the combination of some of these benefits which may result in a favorable resistance.

Some benefits that could apply:
1) A more favorable shedding of the water flow at the stern. For example: The pressure field the foil generates is able to keep the water better aligned to the stern shape and shedding of the water takes place at a more favorable position than without the foil.
2) The positive pressure field at the stern is increased therefore creating more forward force.
3) (here we do have a vortex ) The vortex generated by the propeller and/or rudders are counteracted
4) The pressure field around the foil generates a lift vector upwards and forward, which reduces the overall resistance.

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