IACC winged keels design question

[tim mcd]

americas cup boat winged keels have obviously evolved significantly since australia II. as i understand it the success of aus II's keel was partly getting ballast lower, part end plate effect and partly increased lateral resistance (with rule restricted draft) when the boat heeled (& part psychological!).

what is dictating wing shape on the latest generation IACC boats? it doesnt seem to be end plate effect as the wings are generally located quite a way aft on the bulb from the keel itself. is it something to do with modifying the flow off the bulb itself? or just increasing area of lateral resistance when heeled? -this must have quite an adverse(?) effect on the lead/balance when the boat is heeled?......fashion?

......any interesting related links?

thanks
tim mcd

[Stephen Ditmore]

It's an open question right now whether the wings should be in the middle of the ballast bulb near the main foil, or if they should be at the aft end of the ballast bulb. Wings at the aft end interrupt the vortex after it's going 'round the bulb, recapturing the energy. In the 1995 AC finals NZ had wings aft and Young America had wings in the middle, and NZ won decisively. But just when we were all thinking that settled it NZ goes to wings in the middle in 2000, and wins decisively again!

What does seem clear is that the wings have become much higher aspect - in the same neighborhood as glider wings - and less raked.

[Stephen Ditmore]

What's driving it? What's fast, as informed by CFD and wind tunnel testing. A high lift/drag ratio for the overall geometry is what they're looking for. I think early on people were contemplating putting volume in the wings, but they've come to realize that high aspect ratios are as important for wings as for any other foil, and that excessive whetted surface doesn't pay. Better to put the volume in the bulb and make the wings as clean as possible.

The person to watch in this area continues to be Dave Egan of SGI who designed for New Zealand in 1995 and Italy in 2000 (does anyone know if he's working for Italy again this time?)

[Stephen Ditmore]

As for the adverse effect on balance, an A.C. helmsman would be in a better position to comment than I, but remember that most boats tend to round up as they heel, so a feature that moves the CLR aft as the boat heels may be compensatory, and result in a more neutral helm.

Interestingly, some A.C. boats have had a problem getting the ballast bulb to turn with the boat. The high aspect keels are not all that stiff torsionally, and the bulbs tend to want to go in a straight line, especially the ones with winglets aft and canted downward.

[tim mcd]

stephen - further reading agrees with the balance argument ( ian howlett in sailing yacht design practice by claughton, wellicome and shenoi) a further advantage being that the boats didnt need to ease the main so much through the tack as they were balanced and thus powered up earlier.

there is an interesting thesis (especially given the resurgence in interest in aerofoils prompted by the international 14 and the moth foilers) by a dr bussard (ayrs catalyst vol1 no. 2) that the wings contribute a vertical lift that reduces wetted surface area with a positive effect when lift drag ratio of the wing is larger than the displacemet drag ratio of the boat which seems counter intuitive for a 75foot keelboat but is possibly a factor. this would then be an argument for shifting the wings back to the centre of the bulb or as closeto being under the LCG as posible. nick holroyd (co designer of nzl 60's keel) is giving a lecture in southampton this week so maybe some answers.

[Stephen Ditmore]

See also ErikG's post of 02-02-02 at
http://boatdesign.net/forums/showthr...p?threadid=319