centreboard sweep back angle

[hwindhagen]

I am racing a relatively large centreboard class (width 2.50m),
which is usually heeled by some degree (10-20°) to reduce
wetted area (yes, also upwind). This is different from most centreboard classes, which go best dead upright and dont´t have sweep in centreboards.
Following some older rules (Larsson) sweep back angulation
could help in heeled situations.

Can anybody comment on whether this is still the present
opinion in centreboard design? (Then why go additional centreboards
on keel yachts straight without sweep)

How do I shape the outline of a centreboard with the goal
of elliptic lift distribution and maximum area at the same time
when considering some sweep back?

Thanks
Henning

[tspeer]

You could use a vortex-lattice program like AVL

A big problem is accounting for the effect of the water's surface when you use an aeronautical code like AVL. At very low speeds, the water surface acts like a solid boundary, so you could use the "double-model" approach and use and airplane-like configuration with the water's surface as the centerplane. At very high speeds, the water's surface has the opposite effect, doubling the induced velocity instead of canceling it. And at the Froude numbers that dinghies sail, the surface effects are nonlinear - somewhere in between very low speed and very high speed, the surface effects may be minimal, or they can be much greater that at the two extremes.

So I'd consider both the double-model case and the asymmetric (no surface) case.

This lifting line spreadsheet can help design the optimum span loading (which won't necessarily be elliptical), but can't account for the effects of sweep when calculating the planform shape. However, even with sweep, the spanwise lift distribution you want to shoot for will be the same. So when you add the sweep in AVL, you will probably need to make the planform more tapered because sweep will load the tip more than the root.