Surface piercing hydrofoil

Discussion in 'Multihulls' started by Tinkersailor, Jun 10, 2010.

  1. Tinkersailor
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    Tinkersailor Junior Member

    Some questions:

    If someone wanted to outfit their beach cat with V foils mounted outboard the hulls (perhaps to a wing frame) and lift a 250lb boat with 2 crew, How much foil would be required?

    What would be a good foil section (less prone to ventilate) and would fences be required?

    Thanks for any help.

    Leigh
     
  2. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    Surface piercer

    Check out post 947, under Foiler Design here: http://www.boatdesign.net/forums/sailboats/foiler-design-2447-64.html

    This post is part of a series in Foiler design of the actual specs of successful foilers. Post 947 is Doug Halsey's 18' surface piercer and is very detailed. Doug is a member here and you can PM him with additional questions. He generously provided these details to help answer questions just like yours.
     

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  3. Tinkersailor
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    Tinkersailor Junior Member

    Thank you,

    There must be something better than a NACA0012. Do we know what the early (small) version of L'hydroptere used on its main foils?
     
  4. Doug Halsey
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    Doug Halsey Senior Member

    Tinkersailor :

    An initial foil design might start with the dimensions of Broomstick's foils & just scale them up in proportion to the boats' total weights. If we say that Broomstick weighs ~350 lbs & yours is ~250+300=550 lbs, then your foils would be about 1.6 X as large as mine in each dimension. However, large
    departures from these dimensions might still be reasonable, depending on how fast your basic boat is compared to Broomstick (without foils), how much
    wind you expect to sail in, whether you want to foil in the lightest conditions posssible, or whether you're most interested in how fast you can go in
    heavy air, etc.

    For comparison, Broomstick takes off at about 10 knots boatspeed (or a little higher), which takes about 10 knots of windspeed on a reach & 16 knots or
    more close-hauled.

    When I designed Broomstick, I wanted a very basic, uncomplicated boat that would be within my capabilities as an amateur builder, not necessarily the
    optimum design in any sense. That's why I chose surface-piercing foils, V foils, uncambered, untapered, untwisted foils, conventional sections,etc.

    Having said that, the NACA 0012 isn't a bad shape. It has reasonable drag coefficients over a broad range of Reynolds' numbers & it's stall is relatively gentle. The presence of the air/water interface & the actual piercing of the surface change the flow patterns in complicated ways, so a more modern foil (meaning more narrowly targeting a specific range of conditions) would be more of a risk.

    The major change that I would advise making to the foil design (compared to Broomstick's) would be to use a cambered section (maybe NACA 4412, instead of NACA 0012). I have experienced the foils stalling at lower CL vaues than I would have expected & I think I understand why. Going back to basics a little : A 3-D horizontal untapered wing will have a larger local lift coefficient at the mid-point than its average. In other words, the ratio cl_Mid/CL_Total >1. Add some dihedral & the ratio becomes larger. Throw in the free-surface boundary condition & the ratio is larger still. Analyzing foils in a 3-D Vortex-Lattice flow code confirms the above.

    So, the sections near the apex of the V, especially, need a section more appropriate to higher lift. My next set of foils
    (if I ever get them done) will have camber at the apex of the V, but no camber at the top (where cl values are much lower) & will have some physical
    twist in incidence to cancel the fluid-dynamic twist that the variation in camber would produce. This is probably more complication than you would want to consider, so I'd probably recommend you to use a constant section over the whole span.

    One other thing I would strongly recommend is that you make your foil attachments in such a way that you can experiment with finding the correct
    incidence angles. For the main foils it's good enough to be able to adjust them on the beach, but for the aft foil(s), I think it's very important that
    they be adjustable while sailing.

    I apologize if this all sounds a little vague; it was written somewhat hastily. I'm always interested in helping people build similar boats, so I can
    certainly comment more later. It would help if you could give some more specifics : what kind of catamaran you are talking about & what it's general dimensions are (length, beam, sail area, etc.) ?

    Doug
     
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  5. Tinkersailor
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    Tinkersailor Junior Member

    Thanks Doug!

    This is the stuff I am looking for. If I understand you, the foil profile should change over the span because the part used during takeoff will need to be different than the tip which will be used for high speed. Makes perfect sense!
    This also means that the main foils should be tapered so that the surface area is bigger at low speed when the V squared component is not helping so much.

    The starting platform would be a SC17. It is light and easily adapted. The forward beam is only 7 ft from the bow and the tubes are big and round.

    More later.

    Cheers.

    Leigh
     
  6. Tinkersailor
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    Tinkersailor Junior Member

    The other possible platform is a mid 90's A cat. I figure this could yield an all up weight around 200lbs foils included.

    The A is already in the shop having the heavy bits removed from her.

    Leigh
     
  7. tspeer
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    tspeer Senior Member

    I'm not sure which way you're talking about tapering the foils, whether conventional taper with the tips smaller than the root, or inverse taper with fatter tips. Inverse taper in a V foil can cause a huge increase in the induced drag due to lift. Like 80% more than the minimum drag planform.

    Conventionally tapered foils will avoid producing a lot of lift near the surface, where the induced drag is much higher than for the same lift produced at depth.

    You will need to use pitch trim to get the right amount of foil immersed for minimum drag. If you fly high, the wetted area is small, but the span is also small, which increases the drag due to lift. If you fly low, you have lots of foil in the water but the greatest span for low induced drag. There will be a different pitch trim at each speed that avoids both these extremes. The optimum will still have the craft flying higher as it goes faster, but not as much higher as would be the case if it operated at a constant pitch attitude.
     
  8. Tinkersailor
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    Tinkersailor Junior Member

    Yes, conventional taper is what I was thinking.

    Question: is this induced drag/submerged span issue the reason for the non lift producing winglets on the L'droptere and syz foils?

    So, I need tapered twisted and variable profile foils. Do we have other section suggestions? I think it would probably be easiest to have 3 sections and transition slowly between shapes down the foil. This however would mean that I would never have a true section.... Is this a big problem?

    Leigh
     
  9. tspeer
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    tspeer Senior Member

    I haven't had any contact with their designers, so I don't know their reasons. However, a sailboat still has to produce large side force regardless of its flying height. Having a vertical panel at the tip allows the foil to still produce that side force as the portion of the foil that also provides vertical lift is reduced. As the speed increases, the ratio of vertical lift to horizontal lift decreases, because the weight of the craft stays the same while the apparent wind speed goes up.

    So it makes sense for the foil to have a more vertical average dihedral angle as it flies higher. The average dihedral angle is the slope of a line connecting the tip of the foil with the chord at the surface. A vertical panel would have the effect of increasing that angle with flying height.

    I suppose that would depend on how different the sections were, and how critical the section shape was for the performance. However, by its nature, a surface piercing foil needs a tolerant section because it will sometimes be operating deeply submerged and other times near the surface. You might be able to use the same section throughout.
     
  10. Doug Halsey
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    Doug Halsey Senior Member

    Actually, that is not at all what I was trying to say. Let me try again:

    1-At any given flying height & with any CL, foils like Broomstick's (large dihedral, untapered, uncambered, & untwisted) tend to get more highly loaded near the apex than near the tip.

    2-At fairly moderate values of CL, there can be flow separation near the apex.

    3-To try to combat this effect, I'm planning on cambering the foils more at the apex than higher up the foils.

    4-To try to get about the same lift-curve & spanwise loading, I'm planning on twisting the foils.

    I'm not advocating any taper. If you taper to make the apex(root) smaller than the tips, that makes the separation problem worse. If you taper the other way, that increases the induced drag badly, according to Tom (who always seems to know what he's talking about).

    I've commented on this question before in another thread (I'll try to find the reference later).

    The boats you mentioned do not use V foils. Each foil is a single, slanted (or cantilevered) element. Let me just call them slanted foils.

    In boats with slanted foils & no vertical fin, the force on each foil acts in only 1 direction, normal to the span. The difference in direction of slant of the windward foil & leeward foil allows any combination of vertical force & side force to be balanced by the forces on the foils. But when the boat heels far enough for the windward foil to be out of the water, this balance is no longer possible. Any imbalance in side force would then have to be taken by the rudder, so you would get large variations in weather helm or lee helm.

    If the slanted foils have vertical fins & the windward foil comes out of the water, the forces on the slanted portion & the vertical portion of the leeward foil will readjust, but there need not be any major changes in the forces on the rudder, so the boat will be more controllable.

    Boats with V foils do not need vertical fins (at least not for this reason), since the windward segment of the leeward foil acts analogously to the windward foil on the boat with slanted foils.
     
  11. tspeer
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    tspeer Senior Member

    True, but you can also control this through the pitch attitude of the craft. Even though the loading, in terms of force per unit area, goes up, the lift coefficient can go down if you operate at a lower angle of attack with speed. This also has the benefit of immersing more foil in the water, increasing the span and reducing the induced drag.

    At any speed and loading, there is an optimum immersion that results in the best combination of parasite drag and induced drag. Operating a V foil at constant angle of attack will probably not get you this optimum.

    You may be making the problem worse if, through adding camber, you are operating at a higher lift coefficient. I can think of a a number of possible reasons for separated flow at the apex - stall, interference effects, and surface effects.

    Although the foil loading increases as the speed goes up and the area shrinks, the product of area*dynamic pressure should stay the same (or be decreasing via pitch trim). So stall should not be an issue.

    At the apex the high velocities on the suction side of the two panels are superimposed, leading to significantly higher velocities in the junction. This also means stronger velocity gradients, which lead to boundary layer separation. If, by cambering the section and operating at a negative angle of attack you can reduce the pressure gradients, then the camber may work for you. However, I don't get the impression this is what you had in mind.

    I can't help you much with regard to nonlinear surface effects that can lead to excessive spray or ventilation. Obviously the suction pressures will be greater, and if there is separation then ventilation is a real possibility. However, I suspect these can also be improved by reducing the angle of attack and running deeper.

    Which way are you twisting - wash in or wash out?

    ("Seems" being the operative verb!)

    I think you may have misunderstood me. Small chord at the apex is what is bad for induced drag.

    However, you have to take into account the whole design. You can always reduce induced drag by increasing the wetted span. There may be other reasons to do something that will increase the induced drag for a given span, if that is what is best for the whole boat.

    [/QUOTE]...In boats with slanted foils & no vertical fin, the force on each foil acts in only 1 direction, normal to the span. The difference in direction of slant of the windward foil & leeward foil allows any combination of vertical force & side force to be balanced by the forces on the foils. But when the boat heels far enough for the windward foil to be out of the water, this balance is no longer possible. Any imbalance in side force would then have to be taken by the rudder, so you would get large variations in weather helm or lee helm. [/QUOTE]

    I don't think this is entirely true. There are several degrees of freedom at work, here. A large load on the rudder would result in yawing moments. I think the equilibrium would work differently. I think it's worth asking, "Why would it heel?"

    It's true that the ratio of side force to vertical force depends on the dihedral angle of the foil in the water. However, I think the equilibrium would be reached by changing the heel angle, thus changing the effective dihedral angle. The leeway angle will vary to match the side force. The heave will vary to match the total force (vector sum of lift and side force), according to the pitch attitude, which sets the angle of attack.

    But this doesn't necessarily equilibrate the rolling moments, either. So depending on the beam and the height of the rig's center of effort, the windward foil may still be involved. I think you have to solve all six degrees of freedom simultaneously to get the trim solution for a given configuration. And there will be conditions beyond which it cannot trim.

    I think the vertical fins will change the heel angle at which equilibrium is achieved as the boat is driven harder. The weight will remain the same, but as the apparent wind and speed pick up, the side force will increase, so the ratio of side force to vertical lift needs to change.
     
  12. Doug Halsey
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    Doug Halsey Senior Member

    I'm afraid I only have time for a short reply, but I did want to address some of Tom's comments.

    What I was trying to point out was that the loading at the apex is much greater than the average loading on the foil, so that stall is a problem at lower values of CL than you would expect. This is why I'm advocating a cambered section at the apex; one that can acheive a higher cl before it stalls.

    The purpose of the twist that I was referring to was to make the cl of the cambered section the same as the cl of the uncambered section. In fact, the spanwise load distribution for the cambered & twisted design is the same as for the uncambered & untwisted design (at least according to my vortex-lattice code).

    References to the tip & root of V foils always confuse me. That's why I prefer to talk about the apex.

    Yes, if you taper the foil, its wetted area is reduced (for a given span), but my VPP code still says the net effect of the taper is bad.

    On the question about the vertical fins on slanted-foil designs, I still believe their main benefit is to the directional control & that their effect on induced drag is of less importance. I would love to continue discussing them, but it will have to be at a later time.

    Returning to the original question starting this thread, I would highly recommend that Tinkersailor read the book on Icarus, by James Grogano, if he hasn't already done so. Also, do a little research on Sam Bradfield's foiling catamaran that has been discussed in another thread recently (I'll try to dig up the reference).

    Doug Halsey
     
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  13. Doug Lord
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    Doug Lord Flight Ready

    C-Fly

    Here is another multifoiler under development in the UK using surface piercing foils-and a lot of them. The commentary is from The Daily Sail but you won't get any more info unless you subscribe:

    Four wheel drive foiler
    We look at the new British foiling cat, C-Fly developed by a leading aerospace engineer and a genuine rocket scientest

    L’Hydroptere charging around the Solent over the last few days has brought out of hiding another UK hydrofoil project that has managed to survive for 15 years under the radar. On Sunday the Daddy Long Legs foiler cat lined up against the French world sailing speed record holder on the Solent.

    The C-Fly project is the baby of two ex-Tornado sailor/very high powered engineers – Chris Edwards, who’s day job is as Aerospace and Defence Director of Frazer-Nash Consultancy and who’s past jobs have including working for the marine division at Rolls Royce while Richard Varvill is genuinely a rocket scientist in his role as Technical Director of Reaction Engine, a company that designs advanced space transportation and propulsion systems. They have been joined by Peter Whitelaw, a sailor who brings marketing and business skills to the campaign.



    Fundamental to the C-Fly concept is that, unlike L’Hydroptere or Paul Larsen’s Vestas SailRocket, it is NOT a speed sailing boat. The aim has been to produce a foiling multihull with high speed potential certainly but principally to perform well and be manageable in all conditions and points of sail. In particular she has been designed to handle well in waves.

    As Chris Edwards explains: “We didn’t design it to break the sailing speed record. The design speed we set ourselves was 40 knots, but with very good all round performance – that was the key and handling in sea state, a boat that you could push off the beach and go sailing.”

    Daddy Long Legs, the working 26ft long version of C-Fly was launched five years ago and has been under development by the team ever since. It should be stressed that this is no plywood-built, belt and braces affair. Using all their professional engineering know-how Edwards states that around 32,000 hours of R&D time have gone into the project to date and this has included several sessions at the sadly now defunct GKN high speed tank testing facility on the Isle of Wight.

    So what spurred them to do this? “We’ve been a follower of Weymouth Speed Week and we have sailed performance multihulls all our lives, so we’ve enjoyed the faster end of the sailing spectrum,” says Edwards. “When we started to look at the problem of going much faster than the current breed of boats, we decided to go down the hydrofoil route. Since 1995, quite a few more developments have happened, such as the planning Open 60 types with swinging keels, but we felt to make the real step change when you are crossing an ocean say - you need to go to hdyrofoil technology. And if that is the case then we needed to prove it.”

    The results is C-Fly, based on a purpose-built catamaran platform with two straight cross beams and a D35/Alinghi 5-style flying strut running fore and aft to handle rig loads and allowing the overall beam to be increased...
    http://www.thedailysail.com/inshore/10/55683/c-fly-review

    pix from the Daily Sail/C-Fly+ Hydroptere Helena Davelid
     

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