During the Easter holidays we built a hull for our Yamaha 15A. See Easter Project 2010. The plan was to build a set of T-foils this summer. As time passed however, the necessary control system proved difficult to realize: the chances of getting it to work in a week or so were judged rather slim.
The Summer Project started with two days of discussion about what type of foils to use and preliminary calculations. The goal was to have something self stabilizing that required no control input. At first a classic two V-foil configuration seemed the best solution, but we had already tried that and not much can go wrong with such a setup. In the end we opted for a front V-Foil and a rear T-Foil, this choice would bring some excitement to the whole project.
The area of the foils was calculated to enable the boat to take off at about 10 knots, this resulted in a rear foil with a chord of 14cm and a span of 1.4m and front foils with 12cm chord and 60cm span. A special airfoil used on model aircrafts was chosen. We assumed the front V to have an angle between 25 and 30 degrees.
We set off immediately and cut the molds from pink styrofoam, then to be covered in Oracal, waxed multiple times and used like any conventional mold. With this procedure surface quality is not perfect, the cost and construction time is however unbeatable. While the wax cured the boring task of cutting the necessary material for the skins and the spars was tackled. The rear skins are made from three layers of 220g/m^2 basalt cloth in 45° direction, the front skins have 5 layers on top and 4 layers on the bottom. The spars are built with 600g/m^2 unidirectional glass fiber. Shear forces in the rear spar are handled by a double carbon sock over an EPP core, the front spars are rolled 600g/m^2 glass UD. After cutting the material for the vacuum bagging we polished the molds and set off with construction. Skins and spars were laminated into the molds and then let cure at 40°C with a vacuum of 0.4 bar. Once everything had cured the bags were opened and the peelply removed. To calculate the hight of the spar some play doh was used, by closing the two halves it was compressed to the correct height and then measured.
At this point we had obtained the material for the support of the front foils: a broken Fiberfoam A-Class catamaran mast. We sawed off an undamaged section of 176cm. We were quite excited by the accuracy of the “tape around and saw by hand” method.
Work proceeded with the rear foil, the carbon sock was placed over its EPP core and impregnated with resin. Before closing the mold some very micro ballon rich resin was placed on the leading and trailing edges and on the spar.
Next day the mold was opened. We were impressed, to say the least, by the quality of our quick and cheap technique. The wing was not as rigid as we hoped though. We decided to try a very ambitious workaround: insert a second spar into the already closed foil. First static deflection was measured under three point bending (with a big water canister). The maximum deflection was 35mm.
We cut 1.4m from our faithful glass fiber UD and rolled it together, placed a stripe of vacuum film on both sides, tied some string to one end, pulled the UD roll through the wing and pulled the vacuum film stripes out the other way. Quite amazingly all this went so fast and so well that we had finished before we really knew what was happening and the foil went back in the oven.
To later fix the our support boom (A-Cat mast) we needed some kind of sleeve to glue to the struts and the central incidence variation mechanism. The idea was not to glue anything directly to the mast, so we could reuse it for something else. We decided to make the sleeve on the mast and to pull a vacuum to make sure everything stays in shape. To make sure the sleeve comes off the mast two layers of baking paper were used (works just as well as many high tech films). Three layers of 220g/m^2 basalt were used. The vacuum bag itself proved to be quite a challenge, in the end we reached -0.55 bar which is acceptable.
Encouraged by the success of our UD roll method as spar we did the small front foils the same way, this time to avoid any artistic operation two spars were immediately installed before closing the molds. The usual micro-ballon party followed.
At this point everything was in the oven and we headed home early to catch a bit more sleep than usual. The next day was like Christmas, a lot of unpacking to do! All went well: the rear foil now had a maximum deflection of 26mm (as opposed to 35mm without UD roll), the front foils are absolutely rocket proof and the sleeve came off the mast nicely. Under three point bending, supported at both ends and stepped on, the front foils have a deflection of about 3mm with a weight of about 85kg. Not bad for a 14.4mm thick wing.
Now that the composite parts were more or less done the whole support structure was tackled. Because we had no means to calculate the exact angles of attack for our foils, both had to have a system that permitted an easy variation in a given range. To be on the safe side we chose -5 to +10°. The base that goes on the hull is 9mm plywood, the supports of our beam (mast) are 17.7mm plywood. All pivot points were designed with one screw inside a brass sleeve and a second screw to stop them from rotating. In the meantime the sleeve was cut into four pieces and everything was put together to test fitment.
The next step was to cut two openings into the hull for the rear struts. Cutting through the glass fiber skin and the foam was no problem but some, at the time very stylish looking, reinforcements of glass fiber covered triangular stock were hell to get out of the way. Eventually we managed to remove them. An other exciting moment followed as we put the almost finished wing into the mill to make the cutouts for the struts and cut the trailing edge straight. Next the struts were milled from 14.7mm and the supports from 17.7mm plywood. After accordingly preparing all surfaces of the joint and some measuring, the rear struts and rear foil were joined permanently together. A lot of micro-balloon rich resin was injected into the wing to reinforce it locally at the struts.
As we waited for the joint to cure we started design of the front struts. We decide to use 9mm plywood and a truss like geometry. After several hours the hull was turned and the joint was reinforced with resin and 4mm carbon fiber snippets. We had finished the 9mm plywood so nothing was more logical than to go buy some more. This proved more difficult than expected: at two large do-it yourself stores it was out of stock, fortunately Obi had it! Obi saved the day and the project. After a lot of playing around on CAD with constraints and measures, we milled the parts and started the most complicated alignment task we had ever attempted.
Thanks to the CNC milled parts the whole assembly went together reasonably well without any kind of jig and everything was jammed into place. We now noticed we suddenly had an angle of about 28° for the front foils, we had wanted 25°. After a brief moment of panic we checked our CAD sketches and the wrong angle was entirely our fault: we had forgotten an angle constraint.
All joints were first glued with epoxy resin, then a few hours later reinforced with carbon fiber snippets and put into the oven. Because of the high ambient temperature of about 35°C the whole structure could be handled very soon without fearing any loss of alignment. We were on a very thigh schedule as the boat had to be in the water the next day.
In the middle of the night the hull and the Yamaha 15A were already moved to the yacht club. To do this we had to start the other car with jump leads since the battery had died. Even boring tasks can be exciting!
After a few hours of sleep we fetched the foils and everything we could possibly need and headed for the yacht club once again. We assembled the boat directly on a bogie, this way we could then comfortably let it roll into the lake. The front structure was aligned and screwed to the two front bulkheads with ten self tapping wood screws. Next the rear foil and the outboard were mounted. Last came our new plastic fuel tank (much smaller and lighter than the one used in Easter). The foils also got a yellow stripe at their tips, the primary function of this measure is to close them in some way. A nice side effect is that it makes the wingtip clearly visible at all times.
The angle of the rear foil was set a bit more than zero and the front foils received a bit more than five degrees of incidence. These angles were just wild guesses, they would soon be changed anyway.
The big moment came and we set the bogie in motion. First tests were carried out with a crew of one. With the very approximate settings the boat took off at about four to five knots. After takeoff the unloading of the motor, and consecutive acceleration could clearly be felt.
Encouraged by this we continued tests with full load (crew of two). Takeoff speed became much higher and because of the higher loading of the foils we had severe ventilation issues on the front V. The cause of this was the lack of a leading edge of the struts. Flow separated at the beginning of the strut, creating a large air bubble which went till the foil. Ventilation had several unpleasant effects such as high spray generation and sudden loss of roll stability if one V ventilated before the other.
We carried out several tests with different incidences but ventilation was always present. Only one clean run, in full flight, of about 100m at 10 knots was accomplished that day. Most of the time only the bow was sustained by the foils and we constantly kept crashing.
The next day we cut some leading edges made of EPP (NACA 0024 cut at maximum thickness), some carbon fiber fences (0.75mm prepreg) and hit the lake again. The difference was simply astonishing!
The struts generated only a fraction of the spray. Flow remained attached and therefore ventilation was less prone to develop, if it did develop the three fences were ready to stop it. Roll and pitch stability was much improved and in general the behavior of the boat became more predictable. We managed a few stable runs at 12 knots. The rear struts however had no leading edges yet and this could be felt: the motor had less thrust at speed than without foils. The struts pulled air under that then spread on the foil and was pulled into the prop, killing thrust. During the tests we hit some driftwood, one fence was ripped from the foil and the leading edge of one strut was also damaged.
Day three started with the installation of a leading and trailing edge on the rear struts. The difference again was quite spectacular, we had to decrement the incidences of the rear foil a lot, else we would continue to nosedive. The motor also pushed as hard as during the tests at Easter. After a bit of tweaking 12 knots were no problem any longer. Finally the boat behaved how we had hoped. At this point however police spotted us and was of the opinion that we were having a bit more fun than was legal. Indeed our boat is not legal in Switzerland because of several aspects of its construction. Even though they would not tolerate us on the lake any longer, they did have a sense of humor: we were allowed to floor it on the way back to the yacht club.
Sadly this was the end of the tests. We are convinced that with a few more hours of tweaking it should not have been a problem to beat the 17 knots we reached at Easter. We never managed a full throttle run without nosediving or crashing in some way.
Our GPS told us that the maximum speed was 14.6 knots and that during the three days we covered 7.54 nautical miles. We burned about 6 liters of fuel.
We hope to be in the water again soon in Italy to find out what the maximum speed of our boat really is.
Here is some video footage of the project