Talador. This is my belated response to your post on 3rd May.
Talador: "Yes, in your configuration it is extremely hard, especially with a kick up rudder at the extremities. Close to impossible, unless weight and money isn't a consideration. What I envision is a cassette in a rotating drum, that does not kick up and is not at the extremities. Like I said, trivial."
Not necessarily trivial if you hit something hard. You'll require pivot adjustment to get zero lift and lower drag in light airs. The foils will require regular cleaning as was discussed in another thread.
I realise the difficulty of my proposal but I'm working on a simple design / engineering solution that overcomes the problems you imagine and I confronted. No detail will be provided till I find time to run an analysis and that's not high on my priorities. I am confident, but not convinced that my idea will work - I treat the proa I'm building as test platform for these ideas. It has been finite element engineered to tolerate the loads.
Different approaches are valid but I needn't accept that stable foiling is unachievable. It's already been done Williwaw and C-Fly for example; and L'Hydroptere uses surface piercing foils.
Talador: "No I didn't bother reading the links you provided, because I have already exchanged a few emails discussing the C-Fly and foiling with Tom Speers and I have been trying to figure out the cavitation and ventilation issues they have. I was kind of waiting for you to bring them up first. You wouldn't happen to be Doug Lord would you? And yes you are correct the C-Fly's asymmetric configuration is not very suitable for shunting, where simply rotating the GOE611 foil is."
David: Certainly not Doug Lord - half a planet away.
Talador: I think you have a fundamental misunderstanding of how foils work. A foil loses lift at the boundary layer and when ventilating or cavitating. C-Fly's trick seems to be using controlled cavitation (stalling) of the front canard to keep the canard submerged and maintain altitude control. Kind of an automatic wand if you will. It is a nice elegant solution without needing a mechanical fix.
"Fundamental misunderstanding"? I've discussed why C-fly is using super ventilated foils before along with a reference to the origin of that configuration - Bras D'or - again refer to UpTip thread. The shape of the ventilation cavity determines the lift characteristics of a super ventilated foil. Cavitating foils can be pretty ugly at low speeds but it seems that C-Fly have a reasonably efficient foil. Rather than a base ventilated wedge it resembles a conventional foil, but with a segmented ventilated lifting surface: Very Fast Sailing - Boat Speed to 40 knots
(1 second of video - pause for a good look). Unlike a T foil, the angled canard foils will be required to work at a wide range of attack angles due to the steering. Their solution to negative steering induced attack angle on one side of the canard or the other is a high angle of attack to begin with - making cavitation inevitable. So why use the V canard? Passive pitch and heave stability. Auto wand? Just call it passive (as opposed to active) response.
As for boundary effects, I shan’t conjecture concerning how significant (or insignificant) they are with this super ventilated foil. Instead I suggest that boundary losses will decrease as a fraction of total lift with increased immersion (a greater proportion of foil operating as intended). This is important when the canard is in the concave section of a swell as the bow must accelerate upward relative to the water plane at the foil at that instant. This upward acceleration won’t match that required until sufficient foil is buried. Then as the concavity diminishes approaching the next wave crest, the foil will rise seeking a reduced lift equilibrium. Transiting the convex crest, foil immersion can diminish further, as the bow trajectory is now downward (relative to the inertial frame which is again the instantaneous water plane at the foil. Dynamic wave profiling. This analysis is compounded by the wave induced oscillation in water velocity at the foil and becomes even more significant when sailing fast (wide true angles) but into a seaway. Water velocity at the foil will cycle: highest at the crest and lowest in the trough, due to the circular progression of the wave. So lift for a given angle of attack and immersion depth will decrease in the trough. This will further amplify the change in foil immersion. The into a seaway trajectory presents wave fronts at a greater rate than with a seaway, but this other option presents its own problems. If the craft is fast enough to surf over these crests, the following occurs: surfing down the wave with reduced lift due to direction of flow, the foils will be more deeply immersed. Fortunately, relative surface water speed increases markedly in the trough, as flow direction reverses, generating increased lift and raising the craft in the approach to the next crest where lift again reduces. Watch this C-fly video Open Sea Hydrofoil Sailing - Portland, UK
The craft pitches in the way I describe (even if all the conditions I’ve described can’t be observed). Dynamic pitch and heave stability. This I interpret as being counter to what you suggested. Mind you, I don’t know for sure what inertial frame you are referring to when you speak of maintaining altitude control. Given you refer to the requirement for 3m deep foils to keep the boat level in 2+ m seas, I believe my critique serves to demonstrate that your altitude maintenance claim is not sound.
It isn't possible to rule out all craft pitchpoling if sufficient speed (can be and) is realised in certain sea states, so the claim that the canard will lead to a pitchpole doesn't do much. The feature in favour of foils: increasing lift with increased speed reduces the potential for pitchpole. Plenty of empirical evidence on the web for this from anecdotal experiences of foiling. Hull volume forward is constant with speed (obviously), so reserve lift doesn't increase. The disadvantage of foils: the risk of loss of lift.
Wave profiling: once the wave length is significantly greater than the foil spacing, the craft can more comfortably profile the wave section. Ocean wavelengths far exceed foil separation, so hull clearance does not need to exceed wave height. Offshore foiling is dynamic. Pitch and heave stability become dynamic with respect to the sea state. The sea state becomes the inertial frame in which the dynamics of pitch and heave stability operate. Chop is a different matter, and fetch in storm conditions. Clearance is required for both dynamic wave profiling and chop. The confused sea states found in storm conditions are another matter, but I’d only choose to foil in these conditions if the craft was more seaworthy by doing so (unlikely).
I'm focused upon building at present so it is highly unlikely that I will allocate time to be drawn further.
David - Drawn Onward
Posted by: cruisingfoiler@yahoo.com.au
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