Easter project 2009: Hydrofoil!

Discussion in 'Boat Design' started by lmfoils, Apr 19, 2009.

  1. lmfoils
    Joined: Apr 2009
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    lmfoils Junior Member

    This is a short write up of our 2009 Easter project. We had been thinking about the realisation of a hydrofoil boat for a few years but never really began to design or build one. The fact that we study at different universities separated by a 7 hour drive did also not help.
    This Easter however we would be in the same place for a week so we went for it!

    The aim was to construct some kind of craft with hydrofoils, the only goal was building something that could carry one person and that would be able to lift out of the water.

    Our largest constraint was time, this implied that our craft would not be autonomous but towed by a motor boat.

    Day 1

    The first task was to find out how high (or low) the top speed of grandpa's motorboat is. A mobile phone with GPS was used for the speed tests. We floored the 9.9hp Yamaha four stroke outboard and after several tests found the top speed to be around 3.5 m/s. With an oar in the water oriented perpendicular to the flow this speed sank to about 2.8m/s, the drag caused by the oar was very high and we hoped our craft would slow grandpa's boat less.
    Second on the list was finding something that would float, on which we would attach the foils. We found an old abandoned surfboard at the local yacht club that looked promising.
    The next step was to create a base which would make it possible to attach the foils to the surfboard. We armed ourselves with carbon and glass fiber, epoxy resin, brushes and so on. Some cellophane was put on the surfboard to stop the laminate from sticking to it. The layup is the following: cellophane, glass and carbon fiber, peel ply and household paper to absorb excess resin. As a final touch cellophane was wrapped around the whole surfboard.
    In the evening we started doing a few calculations, it was clear form the beginning that we would use a H 105 cross section designed by Tom Speer. With xfoil a few simulations were run to have an idea of what Cl to expect at the Re numbers we would have.

    Day 2

    More calculations and a few sketches of possible foil layouts were done. To keep things simple we would use two identical set of foils. In the beginning a V foil configuration looked best, however particularly at higher speeds things did not look good. In the end we decided to use a combination of T and V foil. A central horizontal foil with two struts at each end with two small foils put at a 30 degree angle on each external side of the struts. Have a look at the pictures of the finished craft, it is quite simple really.
    At this point it was also decided that we would shoot for 1400N of lift at 2m/s with a Cl of 0.8 (which is easily developed by the H 105). At 3 m/s the central part of the foils alone would provide enough lift to keep the craft foil borne. The 30 degree figure mentioned earlier was chosen because it looked nice and because it should provide a good balance between stability and lift.
    In the afternoon we took cellophane and peel ply off and admired the base of our supports. The peel ply leaves a rough surface on which other parts, or layers, can be bonded successfully. Notice the difference between the surface left by the cellophane and the peel ply.
    In the evening a design meeting was held in a local bar and all decisions concerning the dimensions of the foils were taken.

    Day 3
    It was now time to start work on the foils. We fired up the CNC hot wire cutter and cut the six necessary pieces. This sounds short but it is not, partly because four of the six pieces needed to be cut to 30 degrees on one side and partly because hot wire cutting is always slow and requires a lot of trial and error to find optimal speeds and heat for each sheet of foam.
    Supports for the struts were CNC milled from 9mm plywood. Everything glued together already looked really nice, because it looked so nice the foils immediately got some heavy carbon fiber which would act as spar. We decided to use a spar less design to save time. Next the foils received some glass fiber. Even tough the colour is darker it is not carbon, is some strange gray glass fiber which we had on hand in some quantity. The parts for the struts were now CNC milled, aligned and glued together.
     

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    Last edited: Apr 19, 2009
  2. lmfoils
    Joined: Apr 2009
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    lmfoils Junior Member

    Day 4

    The supports where the struts would be attached were now aligned and glued to the carbon parts we made on the surf earlier. Five minute epoxy resin and some particular maroon micro balloons (which are really expensive we found out later) were used.
    In the afternoon the foils received a layer of carbon and some glass fiber also on the other side. The supports also got their share of carbon fiber. Last operation of the day was the leading edge of the foils.

    Day 5
    The resin had not had time to cure completely so most components of the craft passed a pleasant morning in the sunshine. As a final touch the struts received a leading and a trailing edge of EPP foam which was also CNC cut. A NACA 0033 section was used and cut in two at max thickness. A few tests with xfoil indicated that the boundary layer would stop being laminar before 30% of the chord, max thickness is also located at 30%. For higher Re numbers the code had convergence problems. Flow would be horrible anyway so maybe turbulating the boundary layer before the straight part of the strut could have its advantages.
    We now moved to the local yacht club and mounted everything to the surfboard. Some of the material used to stop carpets from slipping was put between the supports and the surfboard.
    Notice that the struts can pivot on the supports, with the screws taught and the rigging things become very solid however.
    The front foil received an angle of attack of about 6 degrees, the rear of about 4 degrees. The craft would be piloted with the aid of a variable center of gravity, that is a fancy name for making the person on the craft move about.
    With all the rigging taught the moment of truth came, a static test was in order. With the foils on the ground we both in turn sat on the surfboard. Fortunately everything proved solid.
    Now came the real moment of truth, with grandpa's boat ready to go the towline was laid out, we slowly accelerated and... at about 1.5m/s the surfboard lifted out of the water! Just like that! Stability proved exceptional, standing on this board is surely easier than doing some real surfing! The dihedral of the foils makes it almost impossible to loose balance: if the craft rolls or pitches more area will be immersed resulting in higher lift which in turn has a stabilizing effect.

    Test day 2

    A few detail pictures... and most importantly the video!

    http://www.youtube.com/watch?v=ybCtzm_O4Tk&feature=channel_page

    Nice weather isn't it?
    Notice in the pictures that you can actually corner this thing and how flow changes at different speeds and loads.
     

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  3. Doug Lord

    Doug Lord Guest

    Lift off!

    Congratulations! I assume the system is designed to use the surface as an altitude control-am I correct? Were you able to measure drag? I'd like to know your total foil area and the area of the center section only if you don't mind.....
    Great job-now what's next?
     
  4. Lt. Holden
    Joined: Sep 2007
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    Location: Western Massachusetts

    Lt. Holden Senior Member

    Great Job!

    The low speed take-off and inherent stability are truly amazing! Please share more with us. I am sure that there are many others (like myself) that have always dreamed of this but lacked the knowledge or capability to pull it off successfully, especially on the first try. granted most of us don't have access to the high tech materials and machinery that you do but you still have proven that it can be done.

    Do you think this could be replicated using aircraft type frame and ply skin construction, i.e. do the foils have to be foam-cored or do they rely exclusively on their hydrodynamic shape for successful lift?
     
  5. Rangerspeedboat
    Joined: Apr 2009
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    Rangerspeedboat Senior Member

    Thats Cool!

    I have also wanted to build one of those and put a little motor an the back :)
     
  6. lmfoils
    Joined: Apr 2009
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    lmfoils Junior Member

    Thanks for the comments guys!

    Doug:
    Yes you are correct the surface is used as altitude control. The centre section spans 80cm, the two tips each 45cm. Chord is constant at 28cm on the whole foil.
    What is next? Good question... Human power combined with an electric motor would be very nice as propulsion. We would however invest a lot more time in the design phase for such a craft. Fully immersed T foils and so on. Full composite construction form moulds would also be nice for the foils.

    Lt. Holden
    As long as the foils have the correct cross section you can build them in any way you like. Having access to CNC machines and making heavy use of composite materials is not compulsory, just saves a lot of time.

    Ranger
    That should not be a problem ;-)

    We did a quick measurement of drag, only a 100N dynamometer was at hand, this implied that drag would be measured of the empty craft. Have a look at the following video

    http://www.youtube.com/watch?v=r_U09Eo02bw&feature=channel_page

    sadly speed was not measured.

    An extreme test was also carried out, and as often happens during such tests, something broke. During a tight left hand turn the craft was rolled as much as possible and the rear left foil supported at least halve of the total weight. We expected it would stall. It did not! The upper skin of the foil broke in compression. Have a look at the pictures. The craft became a bit more tricky to surf but we still managed a long parade lap without problems.
     

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  7. rickypigazzini
    Joined: Mar 2009
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    rickypigazzini New Member

    Dynamic Stability Test Results

    I've just uploaded a short clip of the dynamic stability test made the second day of tests.

    Dynamic Stability test of our self made Hydrofoil. Notice that the floating capacity of the submerged foils is slightly less than the the weight of the board, so the take off speed without loading the board is really low. This test revealed an exceptional dynamic stability and self righting moment offered by the four lateral tilted foils.
     
  8. lmfoils
    Joined: Apr 2009
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    lmfoils Junior Member

    Last week we decided to have a go with a new set of foils. Fully submerged T foils this time, with two flaps per foil. Altitude, roll and pitch are controlled by the "pilot" with an old RC transmitter. The potentiometers of the transmitter are directly wired to a micro controller which processes the three inputs and with the aid of mixing functions generates four outputs.

    The foils are 140mm chord and 1300mm span, airfoil is a Naca 63412. Flap is at 70%.

    Construction started, as usual, with a set of CNC cut foam cores. This time though the negatives were not thrown away but sanded and covered in an adhesive film. This gives some sort of mould. The foil was then put under the mill and a cut out was made for the upper flange of the spar. In there went 10 full length basalt tows and 14 carbon tows. When it had fully cured the part was put in the mill again, this time making the cut outs for the web and the lower flange. The technique worked very well, no deformation of the air foil was noticeable. Special care was taken to saturate the wood and surrounding foam with epoxy resin to avoid the two flanges separating from the web because of shear forces.

    With the spar in place it was time to start work on the skin. The skin is composed of three layers of 220g/m^2 of basalt fiber. Between the first and the second layer there is a ribbon of peelply, which will act as hinge for the flap. The three layers were saturated with resin on one side, the mould was put on, the foil turned and the process repeated. With a bit of care it is no problem to make the basalt follow the leading edge. Finally we had our foils in the moulds, under some weight we let them cure for 20 hours at 27°C.

    As is visible from the pictures the surface quality is far from perfect. We decided to ignore this and have a filler n' sanding party later if we felt like it. Quality was already much better than our last foils so we hoped they would work! After the exciting de-moulding moment the foils were put in the mill once again. The trailing edge and the flap to foil gap was cut on the lower side. There is an interesting detail here: the gap on the lower side is not cut below the hinge line but the centre of cut (with a 3mm mill) 5mm forward. This was guesstimation and worked very well!

    Now the struts were made. Nothing spectacular: some 15mm marine plywood covered with a laver of the 220 basalt on each side.

    At this point it was time to free the flaps. Wit a ruler and a hobby knife the upper skins of basalt were cut a bit, then with a diamond cutting wheel and a dremel the lower gap was extended till the upper skin. Now comes the interesting part: you have to bend the flap, delaminating some of the remaining basalt without breaking the skin off the core. The idea is to only have the peelply and a few delaminated basalt fibers hold the flap. Finding the right amount of violence to do this is not at all easy. In the end it worked even tough we realised we cut a little too much through the skin on the upper side in a few spots.

    Struts and foils were now glued together, epoxy with tome thixotropic agent did the job well.

    Servos and pushrods were added next. We used four Hitec HS-645MG. This sound easy but took quite a bit of time...

    The big moment arrived and everything was mounted to the surfboard, the blocks where the struts are bolted to are the same ones we used at Easter. This time kevlar replaced nylon as guy-wires.
     

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  9. lmfoils
    Joined: Apr 2009
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    lmfoils Junior Member

    I seem to have run out of space for attachments... so here they go directly linked.

    Notice the difference in area between the new and old foils! The first day of tests was quite exciting. On the roll axis the craft was very very twitchy. As a result flying for more than a few meters was a real challenge. We set our software man to work and by the next day the control curve on the roll mix was no longer linear but exponential. The amount of expo is adjustable with a potentiometer.

    During the tests we made the following observations
    - As predicted by xfoil the flap only produces more drag quite soon, only small positive deflections are necessary.
    - The changes in chamber are best kept small, once up to speed the craft can be flown like an aeroplane with the pitch and roll controls.
    - Even though it is possible to take off quite soon, at higher speeds (about 10 km/h) the craft becomes much easier to control on the roll axis because of the smaller deflections of the flaps.
    - Prop wash from the tow boat is felt as a low vibration. The flaps however seem to be solid enough, we had no flutter issues.

    Video coming soon!

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]
     
  10. rickypigazzini
    Joined: Mar 2009
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    rickypigazzini New Member

    Video!

    Here is the video of our Summer Project 2009. Enjoy!
     
  11. Munter
    Joined: Jul 2007
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    Munter Amateur

    Nice work! How did you find the servos coped with the flap loads?
     
  12. lmfoils
    Joined: Apr 2009
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    lmfoils Junior Member

    Thanks!

    We operated them at 5V, they are rated at 7.7kg.cm (106.93 oz/in.) at 4.8V. Since the distance between hinge line and flap horn is twice the distance between servo rotation point and the hole used in the servo horn we have a lever arm of 2:1. That makes a torque of 14.4kg.cm available at the flap. This seems to be more than sufficient, from the quantity of energy drawn from the battery the servos are operating within their capabilities.

    In winter we hope to do a few tests with a complete telemetry and altitude control based on a pressure sensor. Now we just know that it works and that no component is at the limit. A bit more data is necessary to really understand what is going on and to make the next steps in the right direction.

    By the way I see this thread gets quite a few views but very little replies. Anybody out there building experimental hydrofoils?
     
  13. chengtc
    Joined: Oct 2009
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    chengtc New Member

    flap control

    Hi,
    I am new to designing hydrofoil (for our school project actually..)...and my group and I are actually quite impressed with your design, especially with the results shown in the youtube video. In your youtube video it shows that the flaps at the tail can move up and down. This is essentially what we wnat to achieve as well. how did you guys do that? It looks like there's a wire attached to the top of the flap, and I am assumign the flaps just relax back to their original position by the stiffness of the black layer (metal?),the control is released?

    Ours is actally designing a retrofitable automatic control system for changing Angle of Attack of flap, and our idea is using a motor to directly rotate and drive the flap. We will be using a R/C boat to do our testing as well, so I think what you guys have done certainly help with the designing. If you guys have any ideas/feedback for us, or have any good resources we could use (on sensors too to maintain boat's height above water), that'll help tremendously. Our's is of fully-submerged type too.

    Thanks a lot for your help.
     
  14. lmfoils
    Joined: Apr 2009
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    lmfoils Junior Member

    Hi,
    flap control is done with a spring steel pushrod that moves in a plastic sleeving. Look carefully at this picture http://www.boatdesign.net/forums/at...51800730-easter-project-2009-hydrofoil-17.jpg You will notice that the sleeving is fixed to the strut in several points with glass fibre. The last part of the sleeve is curved because we did not have the servo straight overhead, the rest however is firmly bonded to the strut. The pushrod takes tension loads while the sleeving prevents the pushrod from buckilng under compression loads.
    This kind of technique is used extensively in model aircrafts.
    The black layer is basalt fibre and does not oppose any resistance (on the hinge line only a layer of peel ply holds foil and flap together).

    As for control we are using an Axon microcontroller with four HS-645 RC servos. A member of our team will do a bachelor thesis about an automatic control system next semester.

    Please post your progress! It is always interesting to see what others are up to!
     

  15. chengtc
    Joined: Oct 2009
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    chengtc New Member

    Re: on AOA change

    Hi,
    just some questions below.

    i)
    about the flap, at what stages during the entire boat testing operation did you guys find yourself changing the AOA using the flap? how much of AOA does your design allow? And is the corresponding change in hydrodynamics significant? We are trying to see how a real world control system might work with your pushrod mechanism, or a similiar mechanism you know may apply for a real boat foil?. Our leisure boat selected for the project is around 8m long.

    ii)
    I am guessing, from your photo, http://www.boatdesign.net/forums/att...drofoil-17.jpg that the attached black box is the servo motor right? Did it not submerge in water? Our design requires selection of a motor that's submergible or one with a waterproof enclosure.

    iii)
    We are novice to model building as well, and chances are we will be using parts for model kits to do our testing as well. Did you get most of your parts at a hobby store? (includin the microcontroller). Our budget's around 1000 CAN so I don't know how much we can get lol.

    Thanks a lot for your help again. Your postings here in the forum have been very insightful and helpful to our design consideration process.
     
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