# Wave pattern due to hydrofoil motion???

Discussion in 'Boat Design' started by tuanshipland, Dec 15, 2010.

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### tuanshiplandYacht Designer

Hi all,

Shall we share about knowledge of wave pattern due to hydrofoil motion? How to define heigh of fully-submerged foil or surface-piercing foil?

In the original ship, we have the critical speed-length ratio (Thomas C. Gillmer and Bruce Johnson, 1982, Introduction to Naval Architecture, Naval Institute Press, Maryland, USA). How about hydrofoil?

TuanShipLand

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Im not sure what it is you're really refering to?

Any moving body through the water surface creates a pressure field around the body. Thus, any moving body can be regarded as a moving pressure field. Lord Kelvin pointed this out in 1888 and showed the wave patterns built up of two systems: transverse and divergent. It forms the basis of hydrodynamic and more importantly residuary or wave-making resistance.

There is nothing "new" as such to discuss. It is all there in text books.

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### tuanshiplandYacht Designer

I know about residuary and wave making resistance, as you said, this is basic hydrodynamics. However, I talk how to calculate the depth of submerged foil in hydrofoil boat. Does the depth of submerged foil belong the wave pattern? the hull of hydrofoil ship can hit surface of water and lose the lift due to wave hollow.

Best regards,

Tuan Shipland

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The depth is a simple relationship between the speed of the advancing water (vessel) over the foil, the Cl of the foil section, the Area of the foil and the dihedral angle, in a nut shell. (Simple Aerofoil theory). All this dictates how much lift is produced. This lift must equal the mass of the vessel. The deeper the foil, the more area, the more area the greater the lift...once the lift equals the mass, the running depth is in equilibrium. If you know the mass of the vessel, you can calculated how much lift is produced at a given depth/speed of when the lift equals the mass.

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### tuanshiplandYacht Designer

Can you make some explanations more when we know Cl, so you can know depth of foil ( Strut in hydrofoil) in foilborne condition? In Cl equation, I don't see characteristic including depth of foil or height of foil (height of Strut).
Lift = Cl*1/2*rho*v^2*A, there by v is velocity of ship (free stream), A is planform area of foil.

As you said, we can have easily lift ( = weight of ship). I just define lift to be suitable to weight of ship, then I get characteristic of foil, but I cannot randomly choose depth of foil (Strut - connect foil to hull). Because, the foil system will face to wave hollow, so they can be stall. Hence, the lift is lost and we have negative angle of attack.
(Reference: http://web.mit.edu/2.972/www/reports/hydrofoil/hydrofoil.html )

Best regards,

Tuan Shipland

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### tuanshiplandYacht Designer

My first idea is that I will calculate wave height due to hydrofoil motion, then I will get height of strut and distance betweeen aft foil and fwd foil to prevent hydrofoil from stall condition. Is that right way?

Thank a lot,

Tuan Shipland

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Tuan

There is no "idea" to share. All the questions you are asking are well known and exit in text books and technical papers.

All Hydrofoils are either 'fully submerged' or 'surface peircing'. You need to decide which you are designing. Then all hydrofoil motion is characterised as
1) Platforming. This requires adjeustment of the foil to maintain the level 'trim' attitude.
2) Contouring. Where the vessel follows the waves profile.
3) Intermeadiate....which is a combination of the two.

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### tuanshiplandYacht Designer

I intend to design Surface-piercing with non-split, fully submerged rear foil.

I know those questions is well-known, however I don't see any equations discuss the height of strut or deeply topic of platforming and contouring condition in hydrofoil operation. Can you give me some references? Thanks in advance.

Thanks again for you replies.

TuanShipland

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That is for you to decide.

Once you know the weight of your boat, and you know what speed she runs at...you have the lift. This lift, as I noted above, will then dictate what depth the foils run at...why..because, see above again.

Contouring/platforming etc...these are well well known and understood in any good reference book. Go to your library, not Google!

"Mechanics of Marine Vehicles" by Clayton & Bishop....this has all you need to know.

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