At the outset, I just want to reiterate that I am no expert authority on hydrodynamics. I have no computational 3D fluid analysis of my own. I am good at reading and researching, but because of a lack of formal training, I am guilty of making extrapolations and assumptions that demonstrate 2+2=5. That said, here are my thoughts on the design process.
In three words: it's Kato's fault. ( as a separate issue, this lengthy ramble is all beer swilling Tardy's fault
)
Yes, Kato is freaky good, and yes, LG is freaky good, but to manage 38 knots on the FF28(mod) was still so much faster than I could comprehend was possible. To my mind it was the equivalent of breaking the sound barrier in an A380. So time for a big rethink and revisit my assumptions. A chance comment by Pacey about NACA 16 series foils got me started. The NACA 16 series are not as efficient lift versus drag foils as the ubiquitous Eppler foils, but they have very low drag at low angles attack. They have been used extensively in propellor design and there is a stack of info about pressure distributions etc available.
The leading edge shape: Maritime propellors have very rounded leading rake. This is very effective at countering leading edge cavitation. They are not the most efficient leading edge design for lift versus drag, but they do present the possibility of better handling at high speed at low angles of attack because of their resistance to cavitation bubbles forming on the leading edge.
Angle of attack is an area where my previous assumptions were wrong. I looked at old videos of mine taken from the mast head when on a speed run. I measured angles of 2-3 degrees. An old research paper suggested the same. I checked with Martin Love and he confirmed a much lower AoA was indeed correct. (As point of reference, a high rake delta wing doesn't start forming attached vortex lift until angles approaching 15-20 degrees. At lower angles, it is the foil shape and traditional flows that influence the amount of lift on a delta wing.)
Most traditional triangular 'Delta' fins on the market have low amounts of foil shape. They have relatively large amounts of surface area compared to raked elliptical wing designs. Despite this apparent surface area drag handicap, they have proven to be fast but not particularly forgiving in turbulent flows. The delta shape projects a small frontal area. Given frontal area on a streamlined body increases exponentially as the velocity increases, this adds to the effectiveness of this design.
So, this plus Kato's effort on a 500cm^2 plus fin had me thinking maybe I should consider a slippery foil shape that presents the smallest frontal area and not be so concerned about the surface area. Similarly, perhaps I should not be so dictated by the low pressure lift side of the equation, rather focus on the high pressure displacement lift side of the foil for the lateral resistance I need.
The FF18V4 has its wide point a long way back, so if you angle the fin at 2-3 degrees the low pressure side is almost parallel to the flow for the first half of the foil and the the water only has to recover a relatively small degree of pressure differential to satisfy the Kutta condition at the trailing edge. The down-side is that it produces low levels of lift.
The high pressure side presents a long rounded foil shape that displaces flow forward and laterally, in turn providing lift. Because this is a displacement type lift, the larger the surface area exposed to the flow, the larger the amount of lift force generated. Hence the FF18V4 has a large surface area for a fin of its depth, but projects a small frontal area at low angles of attack, and should be a nice and slippery solution on a downhill run. At higher angles of attack however, it is nowhere near as efficient as the traditional Eppler like solutions.
Thin foils always suffer in unsteady flows and turbulence as the flow readily detaches across the entire low pressure side foil surface. Often this occurs when a leading edge separation is joined by a low pressure separation migrating up the foil surface from downstream. The combination of the fillet, rounded leading edge, rounded rake, and a foil that has reduced low pressure peaks, should combine to keep the flow attached in a broader range of conditions than would be expected from a thin foil delta. I do not expect the fin to be able to provide the same level of tenacity of grip as the standard Fangyfin foil section.
With that in mind, I expect the fin to be found wanting in normal reaching and uphill conditions. I expect it to hang on and not spin out unless adversely loaded, but its pointing ability and speed will be less than a standard Fangyfin. The pay-off is hopefully its performance on the downhill, where it should be fast, stable and capable of dealing with chop at the end of a run with relative assuredness.
Finally, the structural stresses in the fin are borne by the the 'walls'. The loading on the centre core of the fin does very little, hence it can be hollow. The only reasons for making it hollow are to making casting easier and reduce weight. However, in thin foils, a hollow makes the casting process harder and there is a greater failure rate. This in turn pushes up production costs. By virtue of its size, a hollow in a thin foil does not offer much weight reduction. Thus whilst a solid FF28 is unusably heavy versus its hollow counterpart, a solid versus hollow FF18 is 100 grams at worst and probably a lot less.
So that is my ramble through the mess of my brain when it comes to fin designs for Budgewoi. My next step is to test the theory and see what I have got right and wrong this time. I hope to get a couple of castings done and I will send one over to Budgewoi for some crash testing as soon as I can.
