Manually hydrofoil on monohull powerboat

Discussion in 'Hydrodynamics and Aerodynamics' started by Sassriverrat, Oct 4, 2017.

  1. PAR
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    PAR Yacht Designer/Builder

    Foil spans can be less than max beam, though there's some advantages to having them longer. Generally the foils, just like aircraft are sized for the lift they need, not arbitrary attributes, such as max beam, particularly on skinny monohulls. The control type is irrelevant; stick, yoke, wheel, whatever, so long as it's effective and you have a comfort level you can live with.

    Low height flight has issues, most having to do with the turbulence at the interface of the two mediums. Just above and below the surface, both the water and air are quite turbulent, so foils tend to operate very poorly. This is one reason you need to maintain depth control, otherwise the foil lifts up, until it hits the turbulence and then loses lift, causing it to flutter, then sink. This rapid and unpredictable isolation has to be avoided, so you need active control. I've seen attempts at non-controlled flight and the boats just porpose, until they fall back down. I've flown, but always with active foil control and the boat still has issues with maintaining sustained flight in some condisions. I agree with Gonzo, you seem to need some understanding of hydrodynamics and basic physics, before a reasonable discussion can continue.
     
  2. BlueBell
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    BlueBell Ahhhhh...

    "I still haven't seen an answer to the following question- has anyone ever seen or had experience with a boat that did not have active flight systems AND the foils did not extend beyond the maximum beam of the vessel? Inherent stability is the topic of concern."

    I think you can safely surmise the answer to be "no" if no one has responded "yes".

    Is it possible? Not likely, or someone would have done it.
    If you're convinced it's possible, then be the first one to do it!
    But "inherent stability" is doesn't have. Simply applied physics.
     
  3. Sassriverrat
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    Sassriverrat Junior Member

    The basic physics makes sense to me; I don't think I'm conveying my thoughts very well-

    Regarding low altitude, I refer to the hull as being near the surface to obtain higher speeds via reduced drag, not necessarily worried about operating through sea conditions. The draft of the foils would be as needed for efficient flight- so the stability questions come from a simply analogy-

    A three-legged stool with vertical legs is perfectly stable and doesn't have toppling issues if the legs are but inches off the floor whereas the same stool with legs 3' tall is obviously going to have some issues toppling over. I'm not sure why similar concepts can't be applied to the foil depth+hull altitude being equal to the leg of the stool.

    The reason for restricting the foils to remain inside the maximum beam of the vessel is simple- docking- whether it be alongside a pier, a bulkhead, or in a slip. Appendages hanging out beyond the beam of the hull are likely going to meet fixed objects when mooring/unmooring.

    The purpose of minimizing flight systems is simple- KISS. It's one less thing to go wrong and one less thing to require time and maintenance.
     
  4. PAR
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    PAR Yacht Designer/Builder

    Again, an understanding of the basics is in order, as this is quite incorrect, when referring to a mono hull flying on foils. You can inaccurately stand on mis-conceived pretence/lack of understanding or perform the study you clearly need.

    Debates like this are always fruitless, as you have to start on a level playing field, which is at the very least an understanding of the principles and concepts involved. I have been on a powerboat design with max beam, fixed foils and the experience was as I've described. You can accept this or mount some fixed foils on something and try it yourself. In fact there are commercially available plans (decades old now) you can pick up. I think the plans cost about 30 bucks.
     
  5. gonzo
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    gonzo Senior Member

    Calculate the center of gravity of the boat. You will see that it will be very high. Very small forces will be enough to capsize it. Foils have been around for over a century. Some basic principles have been worked out. Among them, is that the foils need to provide a righting force. The only way to accomplish that, is that the force from the foil has to be outward of the boat. With foils that are the same width as the boat, they will contribute to the capsizing of the boat. Some designers have used foils with negative lift instead of ballast with mixed results. However, they are also outboard of the boat for increased leverage. Also, the basic principles of fluid flow are quite different between compressible and non-compressible fluids. For example, there will be no cavitation on an airplane wing.
     
  6. Sassriverrat
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    Sassriverrat Junior Member

    "Again, an understanding of the basics is in order, as this is quite incorrect, when referring to a mono hull flying on foils. You can inaccurately stand on mis-conceived pretence/lack of understanding or perform the study you clearly need."

    If this concept is so wrong, then please explain to me why foils operating at sufficient depth but with a hull flying very close to the water (let's say 1 foot to give an example) is going to have an issue flying. Yes, if waves and such are coming in contact the hull there is a problem. Yes, if the foils were not at sufficient operating depth then other issues like cavitation and ventilation do arise. The stool analogy isn't wrong on the stool side- that's why I asked about applying.


    For Gonzo- the US military has stated in extensive documentation that "hydrofoil craft are basically similar to those encountered in aircraft," so my analogy there isn't bad....but they do go on to say that manual control of motions and flying level is not practical [for hydrofoils nearing 1000 tons].
     
  7. Sassriverrat
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    Sassriverrat Junior Member

    gonzo- now that makes an interesting point. So there has not been successful implementation of foils that are contained within the maximum beam of the hull and still surface piercing on a monohull. I wonder if foils similar to the latest AC foils could see success on the basis that they provided significant lift for leeway on the boats. Further- this boat has a very low CG and I thinn it could be kept low (in perspective) with the necessary GM. So that's an interesting thought- a pair of AC type foils on a hull. (Again altitude is still controlled primarily by pitch which is a separate discussion with a bow design.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Foils.png Sassriverrat, use the Microsoft "paint" program to draw a rough sketch of what you consider may work, a frontal view will suffice, and post it here. A scheme that is within the beam limits of the boat, I mean. This is my little thought bubble.....:)
     
    Last edited: Oct 7, 2017
  9. Sassriverrat
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    Sassriverrat Junior Member

    THat was my first thought!!! My second thought is the one shown below. The third would be the below design- yet each would be built into centerboard trunks so they came out on a diagonal at the chine. But I like your drawing (my first thought) the most and my below drawing second!
     
  10. Sassriverrat
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    Sassriverrat Junior Member

  11. Mr Efficiency
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    Mr Efficiency Senior Member

    You need to know where the waterline will be, and figure out what righting moment develops when the show tilts. In my diagram, the waterline would intersect where the strut (a foil in itself) from the hull, intersects with the rest of the foil structure. The problem would appear to be instability in the transition on to the foils.
     
  12. Sassriverrat
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    Sassriverrat Junior Member

    Makes sense. I'll have to do some calculations. I've spent the last few days building an excel workbook that calculates a LOT- hopefully done maybe this afternoon or tomorrow and I can start plugging numbers into it. Happy to share it- just PM me. Also- Mr Efficiency- I finished the basic CAD drawing including the bow job but had figured out yet how to easily draw foils. So it's missing the aft foils. Happy to share that privately with you.


    So why do you think your drawing suffers from instability during takeoff? I need to do calculations but my design is such that the waterline is right around the tip of the foils which would be just over 15-18" from the bottom of the hull. Foils would have variable angles of incidence so that as more foil was immersed, it would have exponentially more lift.
     
  13. PAR
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    PAR Yacht Designer/Builder

    You're over thinking the foil shapes. You'd do well to think of them in two aspects, struts and foils. One holds the assembly, while the other does the work, so their shapes will be different. Simply adding winglets or fences to the foils will likely make them tolerable, within max beam restrictions and using a simple dihedral instead of gull or inverted gull or whatever, makes construction and flow dynamics easier to predict and less draggy. Curved foils would be nice, but likely way over your pay grade to get remotely close to right. KISS, a V'd foil with winglets or fences maybe the struts can poke through to serve both purposes.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Certainly way beyond my ken to accurately calculate the lift and drag and structural requirements of a practical full hydrofoil craft. They are as scarce as rocking-horse manure in recreational powerboating for a lot of reasons, if it wasn't devilishly difficult/expensive, they'd be out there.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    Probably because you go through a stage where there is very little boat hull left in the water, and most or all of the foil assembly still is. So what stops it tipping at that point
     
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