Thanks Hawaiis, heres a bit of theory of mine.
Any shape can be used for a foil and the shape carries its performance characteristics...wider foils are more stable side to side and longer foils are more stable nose to tail, but how do you work out the best dimensions ?
Over the past decade this is what Ive tried to find out...I started with twin lateral foils much like the commercial foils you can buy but I had both foils the same size and shape. This meant that I wasnt riding off the front or back foot but had a longer sweet spot to stand over.
The twin foils were 23 or 24 inches wide, NACA profiled, with an 8in chord so there was ~400 sq in of planing area in total and those foils would plane at just a few knots. Sounds great and I thought Id cracked a great secret but as the speed increased the foils would go wild and start leaping out of the water.
The problem was too much lift, to maintain a submerged
position, foils dont need constant lift but just enough for the speed and weight of the rider.
Soon enough I found a basic formula that said that Lift is multiplied by a factor of 4 when the speed is Doubled. so a foil that lifts 80 kgs at 5 knots will lift 320 kgs at 10 knots..... the problem with foils isnt lift, its control.
The 400 sq in foil setup shrank down to 100 sq in and they were well behaved at most speeds.
But the other problem with foils is that lateral foils lift, at any instant, for the distance of their chord, so no matter how wide a foil was, the measure of its chord was linked to its stability.
Now I could see why commercial foils had a larger lifting foil at the front and then a smaller regulating foil aft, much like an airplane where the bulk of the lift was in the bigger front wings.
The problem there was that there was part of the submerged setup lifting and another part not lifting so the aft section was definately playing a part but it wasnt focussed on pure lift... so why not build a foil setup that was inherently stable and totally designed for stable lift.
The foils had come down from a total of 400 sq in to a total of 100 sq in, no NACA profiling- just flat plate.
Then I started altering the distance between fore and aft foils and although the setup was more stable as the distance increased I found that the overall setup felt too wide. So as i brought the width of the foils down I increased the chord to maintain the overall area, the more the width decreased, the looser the foils were but they started to be a much smoother ride, a better feel overall, it seemed that the longer the chord, the better the foil would run through chop, the better it would handle minor drops in speed, longer chord and less width were a step forward up to the point where I was riding just one long foil that was 13 inches wide. I went down to a single foil 8 inches wide but that was too unstable.
Previously I had used twin lateral foils with an overall length of 30 inches and now I was riding a single foil 13 in wide X 23 inches long. 3mm flat plate Aluminium.
Hvave I bored everyone to death yet ??