Foil assisted multihull design

Discussion in 'Boat Design' started by groper, Sep 29, 2013.

  1. groper
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    groper Senior Member

    Ive been operating in my local waters for many years - dont know how many miles ive done, but its a shitload... never hit anything, ever...
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    May your luck continue ! :D I sometimes think I prefer a well-raked bow on any kind of fast boat, if only if it reduces the chances of a hitting something flush, rather than a less arresting, glancing blow.
     
  3. groper
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    groper Senior Member

    Found it, courtesy Kolysaev et al.

    The following limitation shall be placed on the separation of front/rear foils for positive interference between foils;

    V*sqrt(s/g) < Lk < 3V*sqrt(s/g)

    Where;
    Lk, is the separation distance between foils
    V is the velocity
    s is the span of the front foil
    g is the acceleration due to gravity

    Matveev et al. derives the optimum spacing so that the rear foil remains in the upwash of the front foil by the following;

    0.5Lh < Lk < Lh
    where
    Lh = 1.95V*(s/g)^0.5

    One more piece of the puzzle reveals itself...

    And a theisis on a sailing catamaran, foil assist design project -> http://www.google.com.au/url?sa=t&r...=XZkZy8oy1gSqZJU5GmVrdQ&bvm=bv.53899372,d.aGc
     
  4. sottorf
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    sottorf member

    Yes you could use a Canard arrangement. At the relatively low speeds you are working - it can work. A Canard variant of the HYSUCAT system was developed 20 years ago already... For example check out US Patent 4,665,583 by Gerdsen et al. It has been applied to a number of vessels larger than yours and works well -especially for speeds just over the displacement hump.

    Stop worrying about the optimum spacing of the foils for positive interference. That is a side show for a hydrofoil supported hull. The main issue is reducing the hull resistance - use the most practical configuration that gives you the correct balance of forces irrespective of the front-rear foil interference.

    Regarding structural strength, using a safety factor of 3.0 is not a bad rule of thumb to allow for cyclic loads without having to undertake a full cyclic load and fatigue analysis which is pretty difficult (but doable and necessary if your vessel is classed). It is overkill in many cases...

    Regarding impact strength, I suggest you consider impact with floating objects carefully. Your foil needs a fail safe connection method to the hull if it does impact something big.

    Secondly consider that for the foil to perform properly you need to keep it clean, that means you need a hard wearing surface that will withstand regular cleaning with a brush or a scraper - depending how fast barnacles grow in your area. Anti-fouling is only partially effective because even a small amount of fouling affects the performance of the foil significantly.

    Also consider that you may experience "singing" of the foil. To correct that (very necessary to avoid fatigue) you may need to make changes to the profile trailing edge later. The structure there should be suitable to be able to do this.

    It sounds like you are struggling to find a good calculation method for hydrofoils. A nice simple method with good accuracy was published in the book by Peter du Cane: High Speed small caft: http://www.amazon.com/High-Speed-Small-Craft-Peter/dp/0715359266 - a very good book to have in your library. Alternatively SNAME Principles of Naval Architecture Volume II also provides a good method for calculating lift and drag of hydrofoils.
     
  5. michael pierzga
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    michael pierzga Senior Member

    Coppercoat... The epoxy, copper antifoul is very hard wearing and a logical choice for retractable foils. Ive seen it used succesfully on monohull center boards and lift keel foils
     
  6. michael pierzga
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    michael pierzga Senior Member

    You will.

    The leading edge of my keel shows much impact damage and deep scar cuts from picking up fishing long lines .

    These cuts are deep into the lead.
     
  7. tspeer
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    tspeer Senior Member

    XFOIL only supplies the Cpr value for Matveev's formulas. It will also report the minimum Cp values (if you have CINC turned on), so it can be used to calculate the incipient cavitation speed.

    Biplane theory goes like this:
    - The surface of the water at rest is flat and has zero vertical velocity.

    - At high speed (infinite Froude number), the foil zips past a given point so fast that the water hasn't had time to deviate from a flat surface until well after the foil has passed, and it's a good approximation to assume the surface is flat. The pressure along the surface is also assumed to be the constant atmospheric pressure

    - The foil causes the surface to have a vertical velocity.

    - If you represent the foil as a system of sources/sinks and vortices, the classic way of dealing with a flat boundary condition like the surface or a solid wall is to assume a mirror image of the same geometry exists on the opposite side of the boundary. Symmetry considerations then generate the desired flow at the boundary.

    - The velocities induced at the boundary by the foil and its mirror image partly cancel each other at the boundary.

    For a solid wall boundary, the boundary condition is there is no vertical velocity (flow across the boundary), but there can be flow along the boundary, which changes the pressure along the boundary. This is the zero Froude number approximation, where gravity is much stronger than the hydrodynamic forces and squashes the surface flat. If the singularities in the mirror system have the opposite sign as the singularities in the foil model - a complete mirror of the foil - then their vertical components (perpendicular to the boundary) will cancel at the boundary and their horizontal components (parallel to the boundary) will add, which makes the velocities twice as large there and there will be pressure differences along the wall.

    If the mirror system has mirrored geometry but singularities of the same sign as the foil, then the vertical components add together at the boundary and the horizontal components will cancel. The cancellation of the horizontal components means the pressure is the same as freestream pressure everywhere on the surface. The vertical velocity at the surface means that although the surface is flat at the instant of the analysis, it has received an impulsive disturbance and is about to explode vertically. This is the linear (because the surface shape is unchanged) free surface approximation.

    It's called biplane theory because the flow at the boundary is like the flow midway between the wings of a biplane. In this case, the wings of the biplane are the foil and its virtual mirror image.

    The wave drag is implicitly accounted for by the approximation. Because the vertical velocities are twice as great at the surface, the induced drag of a hydrofoil operating at the surface will be twice as much as for a deeply submerged foil operating at the same speed and having the same span loading.

    See NACA Report 1232 for a comparison between biplane theory and experiment for hydrofoils operating at various depths and speeds.

    The chord of a hydrofoil is typically small compared to the length of a boat, and so the boat doesn't have to be going all that fast for biplane theory to be a pretty good approximation (Froude number based on chord > something like 2 - 4)
     
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  8. Ad Hoc
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    Ad Hoc Naval Architect

    I'd be interested to see the results of that, if made public. To compare against the data I have and use.
     
  9. groper
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    groper Senior Member

    Thank you Tom Speer!

    Why bother with further testing or calculation when the link you gave has tank test data for the exact size foil of my full size foil - the chord was the same! It also happens to have the lift and drag data for a submergence of 0.84chords which is very close to my intended running condition. Everything is right there... But there is a trend showing up in most of the experimental results I'm looking at.... With a reasonably high aspect ratio foil, seems easily achievable to reach a L/D ratio of 20:1.

    This means at volumetric froude numbers above about 2, there should be an improvement in resistance. Seems there is a peak in efficiency around the Fnv =~ 2.3 with a dip in the total resistance vs speed plots of many experimental results of hydrofoil assisted cats.

    All this tells me the design is at least feasible in the regime I'm looking to design for. Which kinda sucks as it means I now have a lot more work to do...
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    So what is the target cruising speed now, 25 knots ? And you are not factoring in dynamic lift of the hull at this speed ? The overall resistance is going to be somewhat greater than it was for a foil-less 15/16 knot cruise with your original engine choice, so you will need bigger engines, and if you are sticking with four-strokes, you are getting into the danger zone of excessive weight perched right out the back. Whatever happened to the simple life ?
     
  11. groper
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    groper Senior Member

    No not really... Fnv 2.5 is around 20kts... from other published experimental data, it should see a reasonable improvement in resistance and therefore lower power requirement at this speed. Peak hump resistance should occur around 13kts and the crossover or break even point with foils will be around 15kts. So I should get similar fuel economy at 15kts with or without foils, and improvement at any speed above this. A penalty in drag will be paid for speeds below 15kts, but the fuel burn at low speed is trivial anyway. The boat wasn't designed for speeds below 15kts originally, otherwise it wouldn't have a fully immersed transom and more elliptical waterlines of a full displacement slender hull.
     
  12. petereng
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    petereng Senior Member

    Morning Groper - Been thinking more about this foil and what if we curve it upward and take advantage of membrane effects so its mainly in tension. Then we can have a slender section and try to load/twist couple it to provide high AoA at low speed and twist off as speed increases? This sounds like a good model to do.... Peter
     
  13. groper
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    groper Senior Member

    It gets too shallow Peter, an upward curve would put it very close to the surface with more ventilation problems and Lower lift coefficient...

    As it is, the lift coefficient should be multiplied by 0.85 If it operates at 1 chord length below the surface assuming flat calm water. In reality in waves, it will probably ventilate frequently and the lift coefficient reduces to that of surface running where the lift coefficient reduces 0.5* the deep submergence coefficient. Curving it upwards would just make it ventilate more often I'd say... As opposed to a Downward curve would improve things in this regard
     
  14. petereng
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    petereng Senior Member

    Infusion modelling

    Hi Groper and welcome to 2014!! Over the hols I've been playing with modelling infusion filling. Most people don't need this but for complicated geometry it maybe useful. Heres an AVI of a 600x600mm panel filling with different runner strategies. Green means its filled. The contour is in degrees but ignore that. FE programs usually have a thermal solver. The conduction of heat through a solid has the same principles as flow of liquid through a medium. If you look at Darchy's eqn (flow through porous material ie infusion) it has the same form as the thermal conduction equation. So I have set up a thermal model that mimics the infusion. Once I figured the correct factors for conduction and specific heat it worked out OK. Cheers Peter S
     

    Attached Files:


  15. Ad Hoc
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    Ad Hoc Naval Architect

    Any simulation/modelling is only as good as the assumptions and whether there is any verification/validation of the results.

    To model such flow, is very different from simple thermal conductivity. The accuracy of any flow analysis for infusion will need to take into account that resin viscosity influences the filling time. Porosity of the reinforcement influences both the filling time and flow pattern. And then of course the permeability of the reinforcement influences both the filling time and the flow pattern, which is very dependent upon the porosity.

    Thus any simulation must have verifiable results of such assumptions being made. Otherwise it is just another pretty colour picture/video.
     
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