What method is best for resistance analysis

DELFTship forum Hull modeling What method is best for resistance analysis

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    • #37274
      Mark
      Participant

      I am trying to analyze what would be a best shape of a hull for hydrofoil surfing.
      A hull is normally supported by hydrofoil, and propelled by windsurfing rig. However before it rises up to hydrofoil it moves some time in displacement mode. Target is to design a hull that creates minimum resistance on pre-foil speeds – 1-6 knots.

      There are three main shapes I want to compare:
      1 – shallow wide flat bottom board, like a regular windsurfing slalom board
      2 – deep narrow flat bottom hull, like a kayak
      3 – deep narrow round bottom hull, like a kayak

      My problem is to choose a proper method that “understands”, that with growing speed, hydrofoils and flat bottom gradually rise hull out of water.
      Also, with “windsurfing board” hull, it may enter to sub-planing and planing on on some speed, that should help to decrease resistance.
      With a deep narrow flat bottom hull, I assume, there should be additional lift force from flat bottom, if it has some angle of attack.
      An analysis method should take these effects into account.
      Which method is best for this kind of analysis?

      I tried all methods from FreeShip, but it looks like displacement methods do not count that hull can be lifted by flat bottom, and planing methods do not count that hull runs in displacement and semi-displacement mode on low pre-planing speeds.
      I am may be wrong, just do not know what method to choose.
      What would you advise?

    • #37275
      giorgio zuppin
      Participant

      I’m a bit confused: maybe is my fault but the use you make of “hydrofoil” term seems improper.
      Anyway, your target seems to be the “hump” zone, where planing hulls slowly build enough speed to increase lift.
      In competition and offshore hulls standard is a deep V shape coped with steps and redans to control and adjust the flow and increase lift.
      Your mention of a wing powered hull instead would suggest the use of a true hydrofoil surfaces: have a look at “Hydropter” Project, maybe could give you some ideas.
      Bye, Jurgen.

    • #37276
      giorgio zuppin
      Participant

      Just to complete the answer, as I know and maybe somebody will not agree, you need a CFD – i.e. computerized flow analysis – it means a CAD to optimize the mesh ( created with DS f.ex. ) and a CFD program: both expensive and demanding a good practice.
      Naval architects usually refine the results with live water tests.
      So…Bye, Jurgen

    • #37277
      Mark
      Participant

      I’m a bit confused: maybe is my fault but the use you make of “hydrofoil” term seems improper…

      Hi Jurgen,
      I was probably not enough clear with explanation.
      I am going to build a windsurfing “board” that is supported by hydrofoil. So on speed the board will be out of water, “flying” on foils.
      But before the required speed acheaved the hull is in water, runs in displacement mode.
      With speed growing, the foil will create more and more lift and finally push the hull out of water.
      So, I need to optimize the hull to accelerate faster while it is still in water.

    • #37279
      giorgio zuppin
      Participant

      Well, a board being what it is, there is not enough flexibility in a project, anyway:

      It’s a simple sketch and I refuse strongly any credit, it is and for me remains an headache!
      Maybe using a model hydrojet you can achieve foiling but under wing propulsion there are so many variables.. beyond my imagination.
      A part from set carefully dimensions, position, wing sections, trim – on both axes – of the foils, you have to cope with dynamic changes due to movements of wind center pressure at any moment of the boat’s course… and taking a jibe…
      Even the most extreme foil sail boat – Vesta’s Sailrocket – has “fixed” trim for a specific angle to the wind.
      But where we would be without the fools?
      Best wishes, with sympathy.
      Jurgen.

    • #37280
      Mark
      Participant

      Hi Jurgen,
      Your model looks cool!
      That how windfoils were started. Often the front foil was made as a surface planing, that worked as a pivot, so the board keeps constant alitude.
      However things have changed from that times and concept of “Flying Chair” was applied to windfoils and kitefoils. Contemporary windfoil boards look like this
      behind the kitefoiling guy you can see Flying Moth.
      Windfoil look similar, but the board is like a slalom WS board, and has WS rig.

      The a slalom WS board is not very efficient in displacement mode, that causes bigger efforts and stronger wind required to rise on foils. The Flying Moth is opposite, it has quite displacement efficient hull, very narrow, like 35sm. But its rig is not WS rig, it is fixed to the boat.
      My idea is to compare both shapes, and may be adjust for best performance in displacement pre-foil mode, then to build and to test one.

    • #37281
      giorgio zuppin
      Participant

      …….!!!! Do not know why, but I’m feeling like a dinosaur on the way to extinction.

      Well it’s a wonderful word where somebody is fool enough to make those thing WORK.
      My tip is simple and maybe stupid, try to stretch the hullform in my sample to the form of the last board in your pics.
      Keep a curved – raised – bow and deepen a kind of keel with gullwing section, I think a 3 points stern could be useful.
      A big thank for the upgrade.
      Jurgen.

    • #37282
      Mark
      Participant

      No problem. 🙂
      I’ve made my models of flat board and “kayak” hulls. I did not attached “fin” and foils because these are going to be same for each hull, and should not make difference. I want to compare resistance of just hulls on speeds from 1 to 8 knots.
      I need to know what resistance analysis method is suitable for flat board and “kayak” hulls.
      The method should take a trim into calculation, preferably. Because acceleration will be with 0 trim, then a rider will increase trim to increase angle of attack on foil to rise up. This trim will create additional dynamic resistance and some lift. All this I want to figure out to select a best shape.
      thanks!

    • #37283
      giorgio zuppin
      Participant

      It’s a slyppery field, but a CFD – computational fluidodynamics – or computerized flow analisys to symplify – program is the right tool for the job, a drawback is the pure cost, AND the theoretical skills it requiers to set a practical result…
      In DS Pro you can manage resistance in variable trims but the algorythm is meant for displacing hull as I know…
      Kayak hulls may not be suitable, though a scale down of racing – planing – hull could be a better solution.
      I suggested a gullwing profile for stability purposes, as it ends in a three point stern it gives the foil a neutral – as possible – attitude when lifting.
      And fins and foils DO give a variable contribution to resistance, since it depends by speed and trim, I suggest to include them in calculations.
      Bye, Jurgen.

    • #37284
      Mark
      Participant

      Thank you Jurgen, I’ll try to look into CFD. Will start with OpenFOAM.

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