Design and Build of a Surface Piercing Hydrofoil System$Revision: 1.21 $
$Date: 2005/08/28 13:40:22 $
- Cheap (Home build)
- Easy (not too much time investment)
- Maybe even Work!
The Ultimate Moth Hydrofoil SystemI think the best systems will be electronically controlled active submerged systems. This may not be legal at the moment. (I think after the Euro 2004 meeting it is). I think it is a sensible direction to go in. Fly by wire will make the boats easy to sail and need not be too expensive. Chris Millers control system paper is a excellent piece of work that I think will be the catalyst for such systems. However active control systems, (electronic or mechanical) are ruled out of this project because the time investment to get them working properly is too large.
If you want the Ultimate system now then buy it. It is very good and this project says big up and Re Spect (Alli-G style). However the beauty of the Moth class is that there is always some silly bastard with a silly idea, that sometimes works and changes the sport of sailing! (Mostly however it is just a silly bastard idea.)
The Diamond Hydrofoil System.This is a design that I think is well worth a try. The concept is to stick a tapered surface piercing V foil on the dagger board that will require no active control system. The faster you go the higher you ride. A T foil rudder will provide longitudinal stability. The design has two Vs that form a diamond that can be glued on to the dagger board.
The tactics of Hydrofoil racing mean that you have to get out of the water earlier (in less wind) than your competitors. Therefore we need bigger foils than the active systems. The drag of these big foils will be reduced once the ride height increases and the top of the diamond is out the water.
The boat will need to fly all the time to be competitive. If the wind is so light that you can not fly then the race will probably be canceled. This system will sail with the hull down but it will probably not be competitive. Only active systems that can feather foils will have a chance against conventional Moths in non foiling winds. This design may feather its foils if the sailor sits forward for a traditional bow down trim. The wind range under 7 knots will be a problem, but who wants to sail in this wind.
Ventilation (air attaching to the foils) may be a problem but we do not know until we try. I think the pressures are low so it may be OK. We can always put small ventilation barriers on the foils to help. Maybe the sailor can bounce the boat to clear the air? I think I have to solve this when it happens and not before.
The mail foil will generate most of the lift, so its of attack is critical. The deck fastening and the shape of its root need to allow it to be fastened at different angles. With sea trials it will become clear where it should be fastened and this position may vary in different wind strengths.
The rear T foil will generate lift if the sailor sits back in the boat. The forward foil may jump out of the water in this case, which may or may not be a problem. The proportions of the foils and the sailing technique will evolve. I will try to make a good first guess and develop from there. Therefore is is a waste of time to build expensive high quality foils because after the first sails the hack saw may come out. The build system must therefore be cheap and easy. Once the proportions are settled then high quality foils can be made.
When this system works we do not need much of a boat. Maybe the boat does not need to float much. In this case it can be very small and light and easy to build. Its shape and fairness will be irrelevant the important factors will be weight, strength and windage. This may mean that the expense of female molds will be consigned to history..
My conversion plan consists of a new rudder and new centerboard with a deck fastening system on the boat. I will keep my old foils in tact so that the boat can be sailed normally. Therefore the cost and risk of conversion is small and I can continue sailing my boat.
The first design will go for control to allow the boat to be driven hard. T-Foils on the rudder dramatically increased the speed of Moths not because of hydrodynamic reasons but because you could sheet in and drive the boat down wind rather than tickle it to the leeward mark. Therefore again the ultimate low drag system is to come later. What we need now is a stable system that lifts early and can be driven hard while you look up to see which way the fleet is going up the beet or where the camera boat is on the reach.
Basic CalculationsIf you think these calculations and assumptions are ridicules then you are probably right, so mail me and I can include your thoughts in this project.
Get out the water first.The foil system needs to generate more lift that the competition a low speed, which means low wind. This is a bit tricky because the speed is important for lift and lift is related to drag. Therefore if we have a huge foil system we will have more drag and not go as fast as the competition. If we do not go as fast then the velocity squared thing means we should have stayed in the bar. Therefore in this critical condition we need to have more area and/or a larger angle of attack, and not too much more drag. If we can do this then we are out the water and the drag drops, speed increases, and we wave good bye.
Area of competition =================== The aussie system data I have is: "Main foil is 90cm wide with a chord of 12cm The rudder is 52cm wide with the same chord." => 0.9 * 0.12 + 0.52 * 0.12 = 0.1704 sqm.This is the projected area. If we have have 45 degree foils then we need sqrt(2) 1.414 times more physical area.
Fixed foils have no flaps so the angle of attack is related to boat trim. Therefore we want to change our trim with the use of our ballast (where we sit). That means that is we sit back in the boat the main foil will ride up and increase the angle of attack of the system. We can then sit forward a bit to put pressure on the main foil and lift the boat out the water.
Assuming: 1. We can not generate as much lift coefficient (angle of attack) as active systems. assume 75 % of the lift coefficient. 2. We have more wetted area and more drag therefore we go 10% slower. Which is 20% less lift. SameArea = 1.2 * 0.1704 / 0.75 = 0.273 Our design area = more then present systems (because we want to get out the water first) = 0.3 msq Therefore the submerged system should have a projected area of about 0.3 msq.
Stable Full speedIn the full speed condition the lift is the weight of the boat and sailor. The area of the submerged system is constant and the therefore the coefficient of lift is very small to compensate the velocity squared effect.
At full speed (what ever that is) we need a bit of main foil in the water for stability and the aft foil must remain submerged. Ventilation is our problem so the pressure on the main foil should be low to reduce this problem. Our area is now low because most of the diamond is out of the water. The boat ideally should not jump when it hits a wave because of ventilation and control reasons. Therefore I think that the main foil should lift the boat and the rear foil should be an always submerged stabilizer. This keeps some pressure on the main foil to stop it jumping. It also means that the the rudder has more draft than the main foil to keep it wet.
Assuming: 1. Main foil generates 75% of the lift. 2. We need about a 1/4 of the area of the submerged system (that is now dragging huge foils around) for the same lift. 3. 1.414 times more area for the main foil is counted because it is 45 degrees to the water. NeededArea = 0.1704 * 0.25 = 0.0426 msq RearFoilArea = 0.0426 msq At a 10cm cord that makes a ~45 cm span foil. Going back to the "Get out the water first" case. Main foil total projected area = 0.3 - 0.0426 = 0.2574 msq At 45 degrees this makes a physical area of: 0.2574 * 1.414 = 0.364 msq Starting at an 8cm cord and finishing at a 30 cm cord the foil needs to be: ( 0.08 + 0.3 ) / 2 = AverageCord = 0.19 0.364 / 0.19 = 1.916 m long EachSideOfDiamond = 2 / 4 = 0.50 Therefore our main foil dimensions are. Main foil Diamond. Top of diamond cord = 30 cm Bottom of Diamond cord = 8cm Each side of diamond length = 50 cm
What would happen if this works?Assuming this design works and we are out of the water and sailing lets do a reality check calculation for a sailing at 15 Knots condition.
V = 15 Knots is aprox 8 m /s^2 L = Required lift = 100 KG = 1000 d = Density of water. CL = 0.2 This is a nice CL Number. L = ½ d V^2 S CL 1000 = 0.5 * 1000 * 8*8 * S * 0.2 S = 0.156 This is about half the main foils projected area. Assuming 75% of the lift comes from the Diamond. Projected area from main foil is 0.156 * 0.75 = 0.117 sqm The projected area of the lower half = 0.7 * (0.08 + 0.19) / 2 = 0.0945 Therefore at 15 Knots we would be running with the diamond about 75% submerged.The 15 knot case assumes a lot but it is realistic, so we can conclude our reality check is OK. The CL gives us a lot of room for adjustment. We can adjust the CL by raking the main foil forward. Increasing the CL may make ventilation more of a problem, but would reduce our wetter area and help us get out of the water earlier. This is a subject that is very complex but it could be examined further with maths and computer modeling. The control of the boat and its behavior can never be fully modeled, as it is far to complex. I feel is cheaper and quicker to use reality rather than virtual reality from this point forward.
My feeling is that the main foil will need to be raked forward about 3 degrees to help us get out of the water. So it is important that the mounting on the boat allows this adjustment.
The angle of attack at the top of the diamond could be more than that of the bottom. We could assume that the bottom should run level with and the top should have a small angle of attack. Our mail foil will have a flat underside to make it easy to shape and asymmetric. Therefore the bottom of the diamond could be parallel to the static water line and the top could have a slight angle of attack to make it generate loads of lift at low speed. This however means there is no chance to feather the foils at non foiling speed and it should also generate so much drag that the boat can never get out the water. Therefore the bottom of the foils will be flat and parallel to the static waterline of the boat. This also makes the build easy.
We have our basic dimensions which fit with practical considerations. We might have loads more lift than the competition at low speed so we can ride out the water first and are therefore faster. We might also have a stable hight control system at high speed so we can drive the boat harder with less wetted area in strong winds. Sounds cool but will it work? I have no idea but I think there is only one way to find out.
Diamond Foils SectionsI will use the NACA 64 series foil section because I have data for it. I is also a nice section with the max thickness far back for good laminar flow. This also helps with the even pressure distribution required for surface percing foils, (to help prevent ventilation).
To make the foils easy to build the slight concave aft will be run straight with a sanding board. I also have to adjust the section so that it is 20mm thick. This makes is too fat at the bottom and too thin at the top but easy to shape out of a 20mm thick sheet of foam.
|NACA 64-012 Section offsets and modified for 20mm thick foam.|
|X %||Y 12%||20mm Thick||100mm Cord||150mm Cord||280mm Cord|
It would be of great benefit to be able to adjust the angle of rake, and therefore the angle attack, of the diamond foil. This can be done with a pivot on the underside of the hull and a simple control system in the boat. I rope and jammer system to tie the top of the main foil back (and therefore rake forward) will be enough.
Build DesignFoils are to be shaped out of high density foam and covered in one layer of UD and one layer of cloth. More layers of UD to be added where needed.
Due to the fact that the Diamond is effectively a fully supported beam and only 50cm long I will same some money an use Glass fiber for this not Carbon.
The first sail started as the South wind came in. This meant I launched in no wind and then the breeze increase steadily to a force 3-4. The boat behaved just like a normal low rider but obviously was a bit slower due to the extra wetted area of the foils. As the wind increased she started to lift on a reach. The boat lifted better with a lower angle of attack (4 degrees). The larger angle (8 degrees) caused too much drag and as lift is the square root of velocity so speed not area or angle of attack are important. Therefore less drag is much more important than I thought so smaller foils, at less angle of attack could be better at getting the boat out of the water!?...
I decided to repair the Diamond that had failed at the junction at its top, and fit ventilation fences to the upper foil to stop the drop into the water as soon as the diamond top cleared the water.
The next sail started in a force 3 and the same drop into the water occurred. On studying the water more closely I got the impression that it was not just a ventilation stall, but a complex vortex that was being generated. This vortex was a result of the 3 foil surfaces and the water air interface. It produced a big drag that caused the boat to slow and drop and air to be sucked down into the water. As the wind increased there was enough power to get thought this vortex drag hump and start flying with the diamond behaving as a surface piercing V foil. Then the problem was ride height control I got the impression that this was mainly solvable with technique but did not get the hang of it before the next crack in the top Diamond junction got to a critical length and I had to stop. At this point I feathered the foil for safety and sailed the boat back the the shore and it behaved like a normal low rider
ConclusionI did manage to get the Diamond nearly clear of the water and had some proper crashes from high altitudes, although I was not in control is was fun. I could also switch off the diamond and make the boat behave like a normal Moth. With development of sailing technique and the diamond dimensions a stable ride height may be possible. The drag hump problem would need to be understood better and solved as it is a speed killer and will make the design uncompetative.
Cheap?The project cost me about 600 Euro and half of this was the high density foam. Therefore if a more accurate and cheaper build technique is well worth investigating. Hot wiring cheap low density foam could half the cost of the project and produce more accurate sections. I am not sure if I achieved the cheap design requirement.
Fun?The project was a lot of fun and cost me about 100 hours of build time. Therefore I feel it was worth while, and I archived the fun requirement.
Fast?If is not easy to build a prototype that can compete with 3 years of development. The other hydrofoil boats at the front of the fleet are a lot faster. This system is not too far off working but still needs development to make it competitive with low riders. I do not think it will ever be competitive with the modern hydrofoilers. Therefore I think that it did not achieve the fast design requirement.
Maybe Even Work?For a first go a lot of things worked surprisingly well but I decided to low ride for the European Championship event as the system was not really race ready. I could have turned it of and sailed the boat as low rider and turned it on for the reaches, but I had the opportunity to race competitively against a fleet of Moths so I thought that would be more interesting. I can splash around on my own in Austria at any time. I nearly achieved the Work requirement.
This project was a lot of fun and I have enjoyed it but I will put my future energy into copying the existing systems rather than developing version 2 of the diamond. Thanks a lot to everyone that helped me to do this experiment especialy Adam May that made it possible. This has been a fun project with input and interest from a lot of people and that is what the Moth class is about. This is probably the end of the Diamond story, but I hope you have enjoyed reading about it.
Chris Miller wrote on 06.09.204: hi, read thought your paper, looks good. i tried a similar design, but instead of a diamond i used a V. the problem was the V was not big enough (two 40 cm parts). i used that length since it was what i had. a note on your performance evaluations, when you compare the the drag with the T foil type you say that the drag will be higher, i disagree. There is one thing you don't take into consideration, there is drag from the dagger-board in the T foil design, where as the diamond design has no dagger-board. This means that although the drag from the vertical lifting surfaces is higher, they are also acting as horizontal lifting surfaces. The total immersed areas are similar when you include the dagger-board. However there is an extra surface piecing element which is hard to predict the drag from. overall i really like the design, i think people will like sailing it since you are using body weight position to control the fly height and pitch of the boat. it will be very much like trimming a current boat for pitch. i think i might go and build a set this week. carve them out of wood, then cover in carbon. chris Doug Culnane Replied on 06.09.2004: I think you are right that the horizontal component is complex and needs to be considered. I was thinking on the U-Bahn this morning that this design will work if sailed flat but the traditional heeled to windward state may be a disaster. However this stuff is so experimental that I think it needs to be tested to find out what the real problems are. Unfortunately my boat is in the UK so I can not test it until the Europeans at Garda. If you do test it send me pictures and your thoughts. Thanks for your response Doug Adam May wrote on 07.09.2004 Hi there Doug. Interesting stuff. The really scary thing is that I think it might just work! A few points though: (I know you've made compromises according to your brief, but I'll come at it from the Olympic prep angle and we'll meet in the middle!) I was worried that windward performance would be poor. The conventional (if you can call them that) foilers are not good upwind when first foiling, but in more breeze heel to windward and utilise some of the foil lift to help against leeway, thus achieving good tacking angles in a breeze. You will need to be careful to always stay upright in order to achieve some element of ride height adjustment. But on pondering this further, I've found you actually run very consistent speed upwind, and very little wand or tiller movement is required. I think your system could heel to windward, and you would just have a course, one or two foils in the water lifting system. Body movement, steering, and sheeting would probably be okay. Certainly Graham Vials was fine upwind with Linton's first experimental set of foils. He had no adjustment on the rudder foil, just a big flap on the main foil, but the linkage wasn't very good, so it didn't move much anyway. He could just sail it upwind fine on body movement alone. My main foil was one of the largest, and a more average main foil size is : 85cm / 80cm. 1st in the Euros was 80cm, 2nd 85cm and 3rd, 90cm..... John Ilet is looking at doing all further foil sets at sizes of 85cm main foil and 65cm rudder (spans). Rudder and gantry loads are huge - do not underestimate! The current foilers are very stable once flying, but they have got a 80cm main foil, and 50/60cm rudder foil help dampen the roll. I'm worried you will not be as stable once up at speed. I'd be tempted to start with an over span rudder foil to possibly help with this early on, and then cut down from there. The relative angle of main lifting foil to rudder foil is quite crucial, and an element of adjustability there would help. Possibly not practical for you, but my gantry is bolted on and I have a few different sizes of wedges that I use under the bottom support to fine tune the rudder foil angle relative to the main foil. Currently I'm getting too much lift, and have to sit a long way back. I'll try and measure the angles, and then we can figure out the differences, and get yours close first time! The main centreboard vertical element can be much smaller in chord. The Ilet ones went to a 12cm chord because the boat was getting blown over it once up using a normal size foil. Once up does it fall into the multihull issue that Brett Burvills boat had? A silly point I know but the class needs to encourage foilers of all sorts and a general acceptance of all concepts should be sort. The class doesn't seem to worry about the wand system but I wouldn't be surprised if the rules thing gets brought up at some point. I think we should allow any configs though! The more I think about it, the more I think this could actually work. Good on ya! I'll read it again, and if I come up with anything else I'll give you a shout. Adam Doug Culnane Replied on 07.09.2004: Dear Adam, Thanks for your response it is very encouraging, and interesting. I have been thinking more about this design and I think the diamond needs to be a symmetrical section and run very neutral in angle of attack. I could have a pin pivot that stops the board riding up but allows it to be raked forward. This would allow the foil to be feathered in non-foiling winds and the boat would behave like a conventional Moth with very large foils. As the wind increases then a pull on a rope to rake the foil forward will put an angle of attack of the diamond and it will generate lift. Once airborne then surface piercing properties should keep it stable in heave and the rudder should trail along to keep the boat level like a conventional T-foil rudder. The rake control line could be adjusted like a kicker for wind and course changes. This makes the control of the boat easier because you only have to worry about the tiller, mainsheet, where you sit and where the camera boat is. I do not know how it will handle heeled to windward or with ventilation as these are very complex with may factors to consider. Maybe small angles of attack on the diamond will keep enough of it in the water to grip its way up windward, and reduce the pressure and ventilation problem. This has to be tested but I really think there is a slight chance of getting this working the day before Garda. The multihull thing is a bit strange but as I understand it the wing mounted surface piercing foils made the boat very stable and boring to sail and that is why it was outlawed. I think my system will be suitably tricky to sail, to stay legal. I do however agree that I think it would be wise for the class to not rule out development at this stage. However I also understand that we do not want to sail multihulls. The whole structure thing is going to be a challenge and I think a couple of extra layers everywhere would be a good idea at this stage. I hear Garda is very deep so sinking is a bad idea. I do not think that the angle of attack between the rudder and diamond is critical. The rudder must just keep the transom level and trail along like the other moths. On your system I can imagine that it is very critical. I plan to make this T foil as drawn in the hope I can cut bits off it. The draw foil is very large by conventional standards and we hopefully are going to fly a lot faster. So velocity squared means it is huge and the saw is going to come out. However it is easier to cut off than stick on. Thanks again for your feedback. It is great when someone else thinks this will work because I am in danger of starting to believe it now myself. I do not think this is competition for boats fitted with your system at this stage but it could be good enought for conventioal Moths to be convered to hydrofoil boats and sail with the hydrofoil fleet. Maybe someone will test it for me on their boat? All the best, Doug Olav Arne Nehls wrote on 06.09.2004 Dear Doug, Regarding your diamond foil concept: Does it look similar to Roger Angell's foils at the 2003 Worlds in France (see attached sketch)? Unfortunately it didn't work at least in the waves we had there but it may be great with an additional height control system. Regards, Olav Doug Replied: Dear Olav, I was not aware that Rodger did this because I have not been active in the class for a while (8 Years)... What was the reason why it did not work? All the best, Doug Olav Replied: In the mail you sent me last night you asked why Roger Angell's foils didn't work. Let me first explain how they looked: His hull is based on a Skippy Mk. I - same underwater sections but much less freeboard and no flares (I have included the 4 photos of his boat I took at the Worlds last year to this e-mail). It has a really large rudder blade with a T-foil (elliptical plan form, low aspect ratio) on a removable gantry. The diamond-shaped foils were fitted quite near to the hull bottom (Roger removed them after the first heat) as shown on the sketch I sent you yesterday. AFAIK the biggest problems were ventilation and lack of longitudinal stability. Probably some ventilation fences could fix this (but I have no experience how effective such devices are) and a longer distance between rudder and centreboard would be beneficial. I also think the hydrofoils were too small (if I remember correctly the diamond had a width of maybe 600-700mm max.). After Roger had removed his foils he only competed in one more race in VERY (!!!) light conditions but even then the swell was too much for the low hull. I also have no idea what aerofoil sections he used, they probably had a too big Cl even at low speeds so the boat would have come out of the water too far... All the best, Olav Doug, Doug Replied something along the lines of: Thanks for info what about your foil system? Olav Replied on 07.09.2004: my foil system of course isn't top secret :-) , I didn't even invent it though. As long as I'm not an expert in fluid dynamics and stuff (I have some little knowledge from various textbooks) it'll be some guesswork and experimentation. What I want to build and try out first is the following, based on the design by Rich Miller (www.foils.org/miller.htm): - Canard: My boat has some kind of a 2nd daggercase between the mast and stem. A canard (450*150mm, NACA 0009) on a strut will be fitted into this slot so it runs on the water surface like a water ski. The strut is going to have a free length of around 300mm below the hull which will be the design ride height of the flying boat. After trying this out (I want to start as simple as possible to get some experience - my goal is to have the minimum number of movable parts in the system) I'm sure I'll build a wand-controlled system to operate the canard as Ian Ward did. - Centreboard: Raked back (to increase the distance between the main lifting foil and the canard), 900mm long below the hull, 150mm chord, NACA 0012. The top of the board is 400*225mm/NACA 0012 so it fits into the centrecase. - Main hydrofoil: Sits at the tip of the centreboard, size is 1500*150mm and it features a NACA 4412 aerofoil section. - Rudder: Standard T-foil blade as used in displacement mode, probably with an adjustable T-foil or a trim tab. In foiling mode it'll be attached to a rigid carbon fibre outrigger (similar to the ones John Ilett manufactures, also see my renderings) to increase course stability. As I mentioned I'm sure it won't be the final arrangement but this is what I want to start from. All the best, Olav P.S.: Did you already read the foiler design thread on www.boatdesign.net? Very interesting and lots of useful information! P.P.S.: I use the software "designFOIL" to analyze different foil sections. It includes a virtual wind tunnel, you can add flaps to the section and see what happens. You can download a trial version from www.designfoil.com The only thing is that you can only save 5 times if you want to design your own sections but it can generate the most common NACA 4- and 5-digit sections. I use to export them to SolidWorks but if you have Rhino it's good as well - and exporting them isn't regarded as saving. PHIL STEVENSON wrote on 08.09.2004: Doug, Saw your memo on pommy discussion site which lead me to your diamond foil page. Thought I might help with some of my experience. Last summer I tried a main foil with turned up tips, about 1200 total span with the last 400 each side bent up at around 40 degrees. The section was a hand shaped assymetrical foil, probably higher camber than Illets' NACA section. Down wind it had plenty of lift and I suffered many breakout crashes at pretty good speed. But upwind it was an absolute dog, lucky to beat a scow. My theory is that the leeway angle at which the boat sails upwind, changes the AOA of the sloping foils by such a large angle that one is much higher than optimum and the other is much lower. I have done no calculations. This might be OK maybe with symetrical section foils but was a disaster with mine. So be wary and do some calculations with leway angles included. I then cut off the tips and bonded them on with slight dihedral at span of about 700. With these I got similar performance downwind and good upwind speed without hull flying. Still plenty of downwind crashes though. This season I have an Illet foil with wand. Only one sail so far and some tuning required to get it flying at the right height. I also have a fin raking system working. So I can change the main foil AOA instead of adjusting a rudder foil flap. Phil Stevenson AUS9324 Sydney Doug Replied: Dear Phil, Thanks very much for your input. I was originally planning asymmetrical foils for the diamond but I think this will generate opposing horizontal lifts that do nothing but increase drag. Therefore with my resent discussions I am coming round to the idea of a symmetrical system that rakes forward to generate lift. Therefore it is very interesting to hear you confirm the idea of symmetrical foils. I posted the mail to the pomy list but I will do something similar to the German and Australian list soon because the discussion is very useful and interesting, and I think there are a lot of sailors like me that want to get flying easily and cheaply. I hope to contact Wardi because this project is right up his street. Do you have some pictures of your foils for my collection. Thanks again, Doug Olav Arne Nehls wrote on 08.09.2004: Hi Doug, Your diamond foil concept sounds really interesting and I agree with your basic calculations. Do you have a certain foil section in mind yet? I'm still a bit in doubt with my NACA 4412. IMHO on a foiling sailboat it's necessary to have an aerofoil section with a quite high Cl for low take-off speeds, a good Cl/Cd ratio, forgiving stall characteristics, and ? very important! - a ?stable? Cl that doesn't increase too violently at even little changes of AOA so the boat doesn't jump out of the water just because you may have moved some millimetres too far back. Does such a section exist at all??? Another factor to consider according to Ian Ward's experience is the (negative) pitching moment an asymmetrical section produces. I guess the main job of the rudder T-foil then is to prevent the boat tripping over its nose, i.e. producing slightly negative lift so the main foil probably has to carry the whole load instead of just 75%. Am I wrong? Looking forward to new thoughts! All the best, Olav Doug Replied: I will spend some U-Bahn time thinking about the foil construction thing. CNC is too expensive if you do not have free access to the gear. Female molds are to much work for one off builds. The hot wire method is very good because you can use the cut out scrap to clamp down the laminate while the resin dries. This makes a hight quality lamination as well as fast accurate shaping. A hotter hot wire may be worth trying.... I think my design is not developed enough to worry about sections. I have ruled out asymmetric (see discussion) and so that leaves the classic NACA rudder sections in my eyes. Perhaps others have ideas they can contribute to the discussion I think that where you sit in the Diamond design determines the lift distribution. The system should be very pitch stable (if it works). The only danger is that the diamond gets negative lift so it has to be raked forward when flying and not be in danger of swinging back or the boat with trip and stop and at 25 knots that could hurt. The hydro dynamic pitching moments of the foils I would say are insignificant compared to the pitching moment the sail has. All the best, Doug Mike Ewart wrote on 9.11.2004: Read with great interest your design brief and the comments, on the point of upwind work surely the use of symmetrical foils will give the effect of a centre board as they are at 45 degrees the same effective area in the vertical and the horizontal plane, and the purpose of symmetrical foils as per tee foil on the rudder is to resist change, thus they are now going to give the effect of a c/b and a lift foil? Doug Replied: Dear Mike, Thank you for your message. I have been thinking a lot about how to analyze the foil system. It is easy to calculate the lift and drag on foils but at 45 degrees I am not so sure. My model uses is a mathematical box around the diamond. These projected areas are the sides of the box. Then you can calculate the forces in the planes we are interested in. I think this model is accurate enough but am can not prove it, only discuss it. There may well be studies out there that use the same approach but I have not searched. This system design I think is good enough to work and deserves to be tried, because if it does work then you are right the foils give the same effect of a centerboard and rudder and also lift the boat out the water. It is easy to fit the system to a conventional boat and has no complex wand control system, therefore it fits my brief. Others seam to have tried something similar but I think their foils were too small. Phil also make what I think is a mistake and one that I initially made and that is to make the foils sections asymmetrical. This turns in the sides of the mathematical box so that the sides generate opposing lifts and drag. I also think that if the diamond pivots and can be adjusted then really could work around the course. I am afraid I have fallen in to the trap of starting to take this seriously, and really believe its going to work! All the best, Doug Mike Ewart Replied: Dear Doug agree with you there is some good stuff about foils and effective area of inclined foils in "High Speed Sailing" and they make no mention of what plane they are referring too, I think it might work as well and am very tempted, I have been playing with some ideas based around a conventional hull shape with angled foils that retract into a recess in the hull, and looking at a system with a scow type hull with curved centre boards but yours is cheaper and simpler. regards Mike Ewart Doug Replied: Dear Mike, A system that retracts into the hull may well be a lot better that the Diamond system, but it will be complicated and expensive to fit, and take time to get it to work. Good luck and please send me detail and pictures of any system you try weather it works or not, because I think there are a lot of Want to Flyers out there like me who are very interested. All the best, Doug Olav Arne Nehls wrote on 22.09.2004: I recently found very interesting instructions on how to make a power device for cutting styrofoam with a hot wire: http://www.nsrca.org/technical/tip_tricks/foam_cutter/foam_cutting_power_supply.htm May be of interest for you? Olav Doug Replied on 1.10.2004: Thanks for the link it is very interesting. There is definitely something to learn about foil building from the model makers.... I think though that it is too complex for me at this stage. I think I will get some high density foam and shape it with woodworking tools. Doug Simon Propper posted to the UK List on 1.10.2004: For all you DIY foil makers I found this site in the US where you can order foam blanks cut to your own profile. May be of use? http://www.compufoamcore.com/ Good luck Simon Doug Replyed to list: Thanks for the link. I think that shaping foam with wood-working tools is very old fashioned and that this kind of service is very interesting. I do however feel that my 2" plane is going to get used waxed up because I am an old fashioned guy. Sooner or later this old dog will have to learn some new tricks. Has anyone else built foils without using a 2" plane? Doug Culnane Chris Miller wrote on 08.10.2004: hi, i've developed a simple mathematical mode of your diamond design. All you have to do is choose a takeoff speed and fly height and max speed (whatever that is!). It produces a tapered twisted blade. Its only hand written at the moment so i cant send it to you, but I'll type it up and forward you a copy. To summarise it, twisting the foils has the same affect as taper and immersion depth, so lift is a function of fly height (well of immersion depth) by way of a cubic, ie L = ax3 + bx2 + cx where x is the foil immersion depth and a, b and c are constants defined by two parameters, the taper and twist ratios and the angle of attack and chord of the tip. I think this is a very desirable characteristic for the system. chris Doug Replied: Dear Chris, Very Cool idea Chris but: What happens when the winds are light and the Diamond is fully immersed? You have a twisted foil that you can not feather to run at zero lift. This means big big drag. (Maybe?) However twisting the foil is a good idea to help with the "lift is a function of fly height (well of immersion depth) by way of a cubic" thing. I originally I thought in terms of asymmetrical foils and having a larger angle of attach on the upper foil but I went away from the idea, because I think it is cooler to be able to feather the foils in light winds. If you can build a case for the Diamond that allows it to be raked forwards when the wind picks up out on the water you can say OK let see if she will fly and rake the board forward to get lots of lift and out of the water. As the wind increases she may be riding too high and the pressure on the Diamond may mean that it is ventilating all the time. You can then reduce the angle of attach to keep more of the diamond in the water and have more control. (Maybe?) I do not know if I am right but I think a straight foil is worth a try first and it is a lot easier to build. You may well be right and small amounts of twist may be useful to refine the design later, because I totally agree that "I think this is a very desirable characteristic for the system." All the best and thanks a lot for your input, Doug Chris Replied, hey, I wasn't really designing the system for non-foiling, but you are right to consider it. i also considered having a speed sensor or wand to alter the diamond angle (all mechanical). However it may be easier to do it manually. The twisted foil produces a much drag as a larger foil (twist equivalent to taper - i think!). Just out of interest what would be the minimum speed for a moth? i.e. how fast does it go in say a force 1 or 2? another think you could do with twist and taper would be to design the system so it self damped, but i don't know how to do this yet - i think that's well hard core. lets get it flying first! chris Doug Replied: I did not start this design with the non foiling in mind either but it occurred to me that it could be considered, and not written off with some stupid comment about "Who wants to sail in light winds anyway" (quote Doug Culnane). I did not think of a wand sensor controlling the diamond angle that is VERY interesting and something to think about..... Doug Chris Miller wrote: hey see below, i've added comments to your points On 9 Oct 2004, at 13:33, Doug Culnane wrote: >> I think you could be right Chris but it is a long time since I did my >> 2 dimensional foil theory. I think that there is a CD (surface drag) >> part and induced (related to lift) drag part, and you have to add the >> two together. This is going back 10 years... Maybe Adam can remember >> he studied airplanes 8 years ago and has drunk a lot less beer so his >> brain may work better. If he can not remember I could dig my books >> out but I would have to remember where I put them... Yep your right, there is a surface drag component, but i don't know how to model it. i never did that at uni! may need to read up on it. the induced drag component comes from the hight pressure fluid on the lower surface of the foil wanting to go round the end of the foil to the low pressure high side. the creates a vortex of the foil tips. However the diamond foil does not have ends, so the only extra drag will be from the surface piecing. >> >> It did occur to me that a little lift in light winds could be >> beneficial but I thought that I should design a system that did not >> have negative lift and test to see what the best position for the foil >> rake was. I expect this to vary in different wind strengths. >> Therefore the idea of being able to have no lift or lots of lift by >> the pull of a string makes the design very flexible (and therefore >> more likely to work). No lift is also good when the rudder breaks and >> and you do not want to Type D all the way home. Yep, an adjustable foil angle i think will be very important for fine tuning the system, and maybe as another control. You have to remember that you get about 5 degrees adjustment by varying the angle of the boat. i''m not quite sure how the diamond foil interacts with the T foil rudder, does the rudder contribute any lift, or is it set a zero so it simply stabilises? >> >> I am sure that this is complex and when the real world problems like >> "remembering to sheet in to force the bow down when the boat get too >> hi...?!! will not make it more simple to understand. My theory and >> mathematical modeling is crude and we could study this in a more >> mature way (if someone measured accurate speed data of existing >> systems) but this is very complex and a bit too hard. I think it is >> easier to build cheap foils and test to see what works best, however >> this discussion is very nice and fun because it help me understand all >> the details better. >> >> Your idea of a sensor and actuator is very interesting and could make >> the system much much more heave stable, (which is the biggest problem >> with the other systems) I do not have the option of developing a wand >> system because my system has to work out of the box. I may well try >> this out next winter. I am very interested in the details of your >> experiments. Some sort of heave velocity sensor could be employed, for example a small vane that aligns itself to the flow direction. When the flow is going up or down the foil this vane could actuate a small control flap or something like that. This could also be done on the fastacraft foils that would make the craft much more heave stable. It would be in effect adding differential control, such the D in PID control (proportion, integral, differential). However the foils currently do this already to a certain extent, due to when the foils move up or down through the water you get an apparent flow effect (such as you get apparent wind while sailing downwind). The larger area the diamond design has will increase this affect. Do we really want sensors and the like on the foils? its only adding cost and complication to the design. It would be better to avoid active systems if we can. >> >> I thought I would study the videos of the Australian surface piercing >> Moth which seamed to work very well, until one foil ventilates and >> that side drops. The sizes of it foils are in the same ball park as >> the Diamond and so I think it is good evidence that a surface piercing >> system could work if the headache of ventilation can be overcome. At >> least the Diamond has more chance of dropping level when it does >> ventilate. >> Each foil was 0.5m long, which gives your design twice as much surface area. Also with twisted tapered foils huge amounts of lift should be available in comparison. The biggest problem wind rush was waves. Sailing downwind when one foil was on the crest and the other in a trough caused the boat to be very unstable. The diamond design should be much better due the foils being closer together (windrush's foils were wing tip mounted). >> All the best, >> >> Doug >> chris Doug Posted to the UK Moth List: Dear Moth sailors that may be interested in hydrofoil design calculations and what I can not really remember, I could not find my books but I looked at my Moth design paper which does not display the formulas correctly but I think I am correct when saying there are two components of drag when calculating wing performance. Component of Drag from the section: Profile drag is related to the section shape and how it performs at angles of attack. (This is what people get over interested in but basically every smooth foil is the same. Some smooth foils do behave slightly differently to others.) This is very important if you are designing the next Boeing 747 that will have to carry some large foils around for a billion miles and each mile costs expensive fuel. A 0.02% increase in performance vs. drag will be worth spending $50,000 on research to get. However if are a low rider and want to be a DIY high flyer then I think that any smooth plank will do because the design problem is control and the fact that it must work at 0.01 knots of boat speed and 30? knots. (speed squared means 0 to (30? x 30? =) loads and loads.) Component of Drag from lift: Induced drag is related to lift and influenced by the plan form and aspect ratio for the foil. End plates like T foils and water air interfaces (Surface Pricing :-) ) extend the effective aspect ratio which helps. Therefore this is complex and not worth getting to uptight about (when is is more important to remember to sheet in to go down?!..) I think that it is true to say that you do not get something for nothing. You can not get lift without extra overall drag. So in a Moth you have to carry your foils around in no wind and so they should do nothing. However they are good when they do something and reduce the wetted area of the hull or better still, get the thing out the water completely and also reduce wave drag as well as wetted area drag. I love the expression "Low Rider" it sounds cool and if the diamond foil works I will call it the "Easy Rider". If it does not work I will call it the "**** Rider". However anything is better than a "No Rider". Doug Culnane Low Rider GBR 4019 Tom Edom wrote on 21.10.2004: Doug and other DIY foilers, You may know about this but... at Boatdesign.net there is an extensive thread on dinghy foiler design. Many but not all are moth related with lots of input from Wardi, the Stevos, Andy P and Rohan. There are also some very perceptive technical inputs from Tom Speer and some others (see his website at www.tspeer.com) . There are about 12 pages to the thread and wading through it is almost as good as reading a book about dinghy foilers (if there was one). The URL is http://www.boatdesign.net/forums/forumdisplay.php?f=12 and look for Foiler Design. If you want to air your design you will get an international peer review! Cheers Tom Edom GBR 4064 Doug posted to the UK List on 17.11.2004: I recently spent a week in the UK on holiday from Austria and took the opportunity to visit Adam May and take him up on the offer of a go in his boat. This is a brain dump of my thoughts and experience, with respect to trying to DIY fly. I welcome feedback and discussion. I would class my self as a reasonable Moth sailor from the middle of the fleet. I have not sailed a Moth or any other high performance boat for 7 years, so I am a bit out of practice, but I think my first time experience will be representative for most experienced Moth sailors. Day One: I had great luck that the wind in Weymouth was a steady 10 Knots on flat water. This was enough to fly but in the lulls it was low riding conditions. I got the odd gentle gust to give me a taste of how the boat may behave in stronger winds. This is probably the optimum first fly wind. Launching looked a very wet, cold and uncomfortable activity so it was good that Adam could do this alone. After a quick sailing demonstration Adam landed the windward wing on the pontoon and stepped out allowing me to step in. This was very civilized because swimming in November is not my idea of fun. I quickly remembered what sailing a Moth was like and that it aint that easy. The boat was slow upwind and lifting a bit, but felt like a draggy low rider. I could not really fly it upwind in these marginal foiling conditions but I think unfortunately this was due to bad technique. The boat seamed very unstable and I put this down to the high angle of the wings on Adams boat. It had to go a long way before a wing hit the water and provided some roll damping. I tried a bit of reaching and the boat flew nicely, but it was clear that there was a lot going on. Maintaining flying hight is very complex. I decided to set and forget the rudder trim tab because it was more trouble than it was worth. The mechanism is a bit clunky and you have to remember which way to turn it. The rudder and mainsheet have a large effect on the pitch of the boat, and therefore the heave. I started to re-learn how to use them to control pitch and heave with the forward drive of the sail. This was something that was clearly going to take a few flying hours to get right. The wand did not seam to be any help at all, and dragged thought the water most of the time. I had one nice little gust when broad reached and hiking off the wing bar. After the initial fright from the acceleration. The boat climbed high and would not go back down. I was waiting for the boat to do a ventilation drop and concentrating to try and work out which type it would be (A, B ,C, D, E....) I then realized that the wand was working and controlling the ride hight. I was riding high up in the air and the wand was bouncing along the surface. There were no waves and so it was easy. After a few seconds I started to relax and enjoy the experience (and look for the camera boat). My GPS top speed reading of 14.4 knots what probably at this point. Adams 17.2 Knots reading in the same wind shows how boat handling is the key to control and control is the key to speed. I tried a few gibes on foils for fun and could only laugh at how weirded it was. Again there was a lot to learn to get this right. I managed to deliver the windward wing back to the pontoon and stepped out so that Adam could step in and swim the boat into the beach. I had escaped the first day with no capsizes and no swims. I had a lot of information to process and had the feeling that sailing in a force 3 could be a great deal of challenging fun and also that I was back on a very steep learning curve which is my favorite place to be. The flap controls worked but had lots of development potential. The overriding factor when competing with other boats will be boat handling and that flying time is essential, fun and very very fast. Day Two: The next day was windy. I estimate that there was a good force 4. After a pint of lunch time bravery juice I was ready to have a go. It was clear that we both needed to swim to get the boat launched. I made the beginners mistake of going to the leeward wing (where it was shallower and therefore a little warmer) to hold the boat while Adam boarded it. Adam sheeted in and the leeward wing forced me under the water, and then I was reminded that there are other foils under the water by a painful rap on the angles from the main foils. Adam said that he was not trying to drown me on purpose but I am not so sure. At this point I realized that today my body, and ego was going to get a bashing. Adam handed over by capsizing near the pontoon and I swam out to right the boat. I sailed up wind gently and under control. I then boar off to see if I could get back downwind. The answer was a clear NO FUCKING WAY. I did not have the confidence (or stupidity) to sheet in and drive the boat hard so I tried to nurse it downwind but there was no way this thing was going to play nice. The boat would ride up on the foils and then it was either going to to fall into a broach or capsize. The wings hit the water before the hull and so it was very hard to recover from these crash landings. After a short wrestle I decided that a win would equate to getting the boat and myself back to the beach in one piece. My fitness meant that I had a limited amount to energy and time to get the boat in and a near miss with a concrete jetty confirmed that there was no chance of a rest. I was very relived to get the boat close enough to the pontoon to shout SWIM at Adam. Adam then showed me how the professional do it. My jaw dropped when he drove the boat into a gibe forced the boom across because the apparent wind strength meant the gibe was technically a tack. It all look very impressive until as he tried to get his balance on the new gibe and pressed the ejector seat button by accident. He bounced 4 foot in the air out the back of the boat over the transom. On the second attempt he fell across the boat head first as it capsized at about 8 knots...... I am not saying that sailing these boats are impossible to sail in strong winds, but it is not easy and it can only be done well with lots of determined practice. There is a lot happening and you need to understand a lot of new dynamic stability factors and how they interact, because this is a very exciting new dimension to sailing. If you remember the excitement and challenge of sailing your new narrow low rider (with no T-foil rudder) in a force 5 for the first time, then double it and you got a foiler in a force 4. Random Conclusions: - The idiots that talk about having a top speed to brag about should shut up because it is scary out there and the trick is slowing the thing down as the wind increases. - You got to be fit and on the ball to make a reasonable job of flying. The action never stops. There is always something going on and there is no physical or mental rest from the time you take the sail out the bag until you get it rolled up again. - If you can not get the boat off the beach and are having trouble launching then you probably should not be out a sea. - Simon Payne says that the boat is alway talking to you. This is total rubbish. The thing is screaming it head of, you have to learn to manage a state of information overload. - Sailing Wizkids beware: Unless you can get round the cans in a force 5 in a low rider, you can not jump out of your 4*0, B*, RS* or R2D2 and expect to fly a Moth. Moth sailing is hard. Flying Moths is Fucking hard. - When you get your new shiny foiler be careful who takes it for a joy ride, because these thing are not Laser dinghies.. - The KA McSail is very like a wind surfer sail. It has an On - Off type of trim. It is either generating loads of low drag power or a stalled out rag of crap. There is a learning curve to get the most out of the modern sails and they are not forgiving. - The Moth class has a challenge to get new sailors good enough and determined enough to sail these boats. Being the hardest boat to sail is good and will alway attract the best sailors, but retaining the best sailors attention is hard. No Flying Moth sailor will want to sail a Laser but not many Laser sailors will want to go thought the ego bash of a Moth. Narrow Moths started this effect and I see that the hydrofoils will exaggerate it. I think that the class has to be careful that the middle and lower third of the fleet are not left behind. But maybe this is crap because I am a middle of the fleet sailor that has been encouraged back to the fleet by these developments. - Lintons female molded foils were very impressive and well made but I was not worried too much about the section and build quality of the foils while I was trying to get back to safety in a force 4. I think we are still in a pioneering period where concept, control system and rider skill is more important that build quality. - I used to be able to sail my low rider around the cans in 30 Knots of brease. I would like to see the video of a top fliers getting to the leeward mark in that wind. - Talking to Adam about Lesson videos the camera boat can destroy your boat if they get too close so you need a big bay an no one chasing you if you are flying. These boats stop very fast and unpredictably so you have to make sure you have got a landing slot and you need to protect it. (When you find out were the camera boat is tell him to piss off). Thanks: - Big thanks to Adam for trusting me with his boat, and for swimming to my rescue. - Thanks to Linton for showing me round his workshop and providing boat launching facilities. - Thanks to Alison and Colin for the hospitalityInt-Moth_UK@yahoogroups.com and traditional English Chinese. What effect does this have on the Diamond foil project? Life is too short not to be involved in this exciting game and I want to Fly more than ever. The Fast Foil / Full Force (4F) system is very well developed and the thing basically works very well, but it is not easy to sail well. The top sailors will be refining these systems and working on boat handling. I think that you can not compete with these guys unless you spend a great deal of time flying. I plan to turn up and fly around lake Garda at the Europeans with an untested home build system and no practice. Since Garda is a traditionally windy location this is a rather stupid plan, but a good opportunity to test a new concept and have a laugh. Therefore I still plan to build the diamond design and take it to Garda. One feature of the diamond design is that it can be potentially turned off by feathering the foils. This could be its most important feature when I have to make the decision weather to use it for the event after the first few test sails. The latest plan is that Adam will make the center board box arrangement that fits to the boat and post it to me. I will glue a diamond to it and make a rudder. The foils will be shaped out of pine and covered in loads of UD carbon. My next task is to go to Ikea and buy a few wooden shelfs and shape them into foils. All the best, Doug Hey, Chris posted to the UK List on 11.12.2004: I've put some stuff about a simulation I've done on the diamond foil. http://www.intmoth.f2s.com/theory/diamond.html chris Doug Replied: Very interesting Chris. Did you model a FastCraft type system as well. I think that it would be interesting to compare the two systems. I also think that it would be interesting to run the simulations with different foil configurations to find out what the effect of a large main foil is or what happens as the rudder T foil gets smaller. This is all very theoretical but I think it may be good enough to help us understand the control effects of the many different factors. I do not pretend to understand control system as well as you but I would have thought that this modeling should be targeted to be able to make control conclusions like. Big main foils are easier to sail but slower for example. Doug Lyndon Beasley wrote on 21.01.2005: I am following your foiler development with interest, would like to have a dabble at making some if I ever get the time. How big would I need to get a Magnum7 airborne? Doug Replied: I have no idea how big the foils should be. But a clearer answer would be "I would say that the mag 7 is 10 to 20 kg heavier (~15% more total weight) so make the foils 15 % bigger." However I have no idea if the dimensions of the original design are good. I have cut out my diamond plan form blank. It is a diagonal cut through an Ikea Traggen? ( pine shelf cost 6.99 EUR :-) ). My carefully calculated dimensions are: 30cm cord at the top of the diamond. ~9cm cord at the bottom and the diamond. The diamond is now 4 x 60cm in length. This is bigger than planed but it allows me some extra length for mistakes... I now need to make some section templates. I think I will ask Chris Miller if he has some symmetrical surface piercing hydrofoil section data or CAD files (please Chris) to save me some work... Once the two sides are shaped I have to get some carbon on them fast before the pine warps too much... I will post some pics of the Ikea catalog on the site soon for reference...... Chris Miller Replied: Hey, I'm about to start building some diamond foils too! The section you should use is an H105 section. It is a modern laminar flow section that also reduces pressure peaks over the foil. This is good for surface piercing foils due to cavitation being reduced. Refer to my papers section of my site for downloads of the paper detailing its section etc.. http://www.intmoth.f2s.com/forum/viewtopic.php?t=20 Its the first link on the page. chris Doug Replied: I had a look at the section and it is exactly the kind of thing I want: "It is a modern laminar flow section that also reduces pressure peaks over the foil. This is good for surface piercing foils due to ventilation being reduced." However it is not symmetrical and I need a better quality drawing of it or some offset data to make a drawing. Do you know if this exists? What are the dimensions of your Ikea shelf or have you got some proper materials? Send me pics if you have them. Doug Posted to the UK List on 14.04.2005 The project is under going a reality check at the moment. If it passes this test I hope to produce a prototype but I am not sure if it will work. I found a better source of materials than Ikea and found CAD drawings of NACA sections in my /home/doug/StuffDoneAndForgotten directory. I am ready to start building but before I spend cash on plastic... The reality check question is am I stupid enough to build something that may not work and costs x when I can buy something that does and costs y. I hope the answer is Yes, but the cost mathematics of materials against lintons price list are more accurate than my foil theory calcs.