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.
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 (10.04.2009)
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.
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.
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.
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.
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, maximum 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 way of saying that the person on the craft will have to move about.
With all the rigging taught 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.
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.
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.
Some video footage of the project:
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, speed was also not measured. Have a look at the following video:
Video of the dynamic stability test. Notice the exceptional stability provided by the four lateral tilted foils: