I've heard from both Marstrom and Wouter that the curved foils should have a damping effect, but I fail to understand the mechanism behind it? See Wouters text below.
/hakan
/********** From a post from Wouter ***************/
I classify the curved daggerboards much more as a refinement then a breakthrough. This because its effects are far more in the field of damping the boats movements then lowering the overall drag. I also feel the latter is already partly adressed by inward canted straight boards. (whatever happened to outward canted boards guys, the hot and exiting development of a few years back ?) And there are some conflicting design problems that are not obvious at first glance. One of those is that simply curving inwards the boards doesn't do that much to lift the boat when the foils are symmetric. In the way of damping movement they will be effective that way. In order to create meaningful lift (in this case, enough of it to matter) assymmetric profiles are needed or a permanent angle of attack of say 4 degrees. At low speeds or when having two boards down these characteristics are actually a disadvantage. That is why patient lady C-class design, raised one baords and lowered the other after each tack. Patient lady is one of the very few examples of a sufficiently succesful implementation of curved daggerboards as a performance enhancing item.
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Hakan,
I believe Wouter's quote is exact except for one detail: in my opinion pure pitch damping can be obtained from symmetric foils at zero angle of incidence, like the pitch damping provided to a plane by a symmetric section stabilizer at zero angle of incidence.
It is intuitive that a flat horizontal board damps pitch. It is also intuitive that the flat board (as the hydrofoil) will increase the boat's total drag. I guess the only requirement for a pitch damping board to work is that it is placed fore or aft of the CG.
The mechanics of the pitch damping effect from hydrofoils can be described this way:
When the boat pitches down, the foil moves downwards and its angle of attack increases, also increasing the existing upwards lift force, which counteracts the downwards movement. Drag usually increases with the higher then normal angles of attack that occur in this case. This is relevant only to understand the difference between a hydrofoil and a flat board.
When the boat pitches up, the foil moves upwards, its angle of attack decreases and lift is reduced or even inverted, counteracting the upwards movement with the reduction of upwards lift or increase of downwards lift.Added drag is less of a problem in this case, for the angle of attack is being reduced at first. It is only when it becomes negative and relatively large that drag will surpass the levels associated to horizontal motion. This explains the difference between a hydrofoil and a flat board at pitch damping:
The horizontal board forces the boat to decelerate its horizontal motion during pitch up or down damping, regardless the pitch direction. Interestingly, the hydrofoil will allow the boat to accelerate due to less drag while damping the pitch up movement, and force it to decelerate due to increased drag while damping the pitch down movement. There's no magic in it, though: the hydrofoil was sailing with higher drag than the board from the begining, due to its initial angle of incidence or asymetric section.
Hydrofoils work almost exactly like wings on a plane. There is an excelent (and free) online book that teaches a lot about lift, drag, stability and damping from wings (read "foils"). Just follow
this link. and enjoy.
