MPX / SRT Test Model
So, for the ama foils I'm probably going to go with a 63412 section with the "flatish" part outboard. Each board will be made with 20 pieces of 2X2 twill, .007" ,5.9 oz carbon fabric and 12 pieces of .012" uni carbon. Final thickness will be about .284" for a T/C(thichness/chord) ratio of 9.5% for a 3" chord. This foil has the heaviest loading of any foil on this boat at max. speed.
The daggerboard and mainfoil, are being built one off with .125" plywood core(main and rudder foil=2 .065" pieces wth flap material bonded between them) with 5 pieces of 5.9oz carbon each side and 3 pieces of.012" uni each side.
The rudder and rudder foil will use the same core with 4 pieces of 5.9oz carbon cloth each side and 2 pieces of .012" uni carbon each side.
All will have T/C ratio's of about 9%. All(except the curved foil) will have symmetric sections.
The main and rudder lifting foils will use short flaps similar to the ones shown below from the F3, with sealed mylar flap hinges built into the foil. The main reason for this is the "thinness" of the foil which increases the propensity of the flap to bind under load. And, considering everything, the best thing these foils have going for them in this model application is that their design is proven to work on the F3 /MicroMoth.
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Most of the above post is still correct for the building of the foils, except the part about curved foils is omitted because the new shape, inspired by TNZ's AC 72 main foils, is sort of a combination of an "L" foil and a curved foil. It has some of the advantages of a surface piercing foil in that its area will automatically reduce as speed increases. All foils will have their angle of incidence adjustable on the beach and the main foil uses a flap that is adjusted automatically with a wand(surface sensor). The main foil will be set at +2.5 degrees, the ama foils at about +4 degrees, and the rudder foil at 0 degress -all relative to the design waterline.
There is a relatively big difference between the model and the full size boat in terms of how it is sailed. When the full size boat is launched both ama foils will be retracted with the lee foil being lowered as speed builds. But on the model, both ama foils will be down to start with. This gives extra foil area to get the main hull up but as soon as it flies it will heel a bit, begining to load the lee ama foil and main foil only-the windward foil clears the water at this point and that drops 33% of the foil wetted surface right off the bat. Next, the main foil begins to unload as the speed picks up, reducing induced drag until, at about 10mph, the ama foil has most of the weight on it. The boat will be, essentially, sailing on one foil with far less drag than a Rave or Trifoiler configuration. This foil system will help enormously in light to moderate wind getting the mainhull flying early-much earlier than possible with any oversquare tri and earlier than possible with most trimarans period.
That is, if the system works like I think it will.
Pictures,L to R,1-4-The foil loading info from the F3 in pictures 1 & 2 served as the basis for the design of the model MPX foils. The main foil will have a partial span flap as shown in pictures 2 and 4 as well as an almost identical planform. The rudder foil will appear similar but w/o the flap since the angle of incidence is adjusted on the beach(not while sailing) by moving the whole rudder +foil. Should there be a need to adjust the rudder foil underway it will require adding a servo but it will be very simple.
The ama foil in picture 3 can be set at almost any depth-the depth shown is what appears to be ideal for full flying. With the foil retracted it loses some vertical lift and can be sailed with some ama contact with the water. Normally, the ama is designed for intermittent contact. That brings up another difference between the model and the full size boat: there is no movable ballast on the model as there is on the full size boat so the model will likely heel over a bit as it gets started getting the ama wet.
