Aug 19, 2013

New Zealand 40knots Pitch: Saved by Design



Only this weekend I went searching for aerial views of the Tractors and Oracle AC72s. I had the side views for a while now and the overall volume somehow established. But for a more accurate comparison I needed those plan views.
Of course these are all estimates as the only data we have are overall lenght and hundreds of pics for trying to guess the AC72 actual hull shapes and measures.
From the aerial views the data I got is that both hulls have a similar width range. Prismatic Coefficient is higher for TNZ though.

Sketches are in Parallel camera view for a better cross section comp, so diffs can be exagerated. Just finished them minutes ago and will refine later.
Based in these lines, the hull volume diffs are mainly found freeboard height / Profile view (both boats are showed here with the complete freeboard side view including the cockpits height)

The F18 comparison always comes in to play... The fastest F18 in any conditions from 3 to 20knots is the Hobie Wildcat that can be placed as an analogy for Oracle as the two platforms excels in speed but they have less error margin when racing in 20knots.

TNZ can be the Cirrus R on its volume and negative rocker sections forward & aft. The R as the SI Phantom has some penalty while racing in 4knots.
Needles to say they excel in the mid range and above, and provide a smooth ride in 20knots, specially the R.

Alex Udin gave a pretty good description on Prismatic Coefficient while describing the design work done by SI & Fischer on the Phantom, that features more volume than its Wildcat predecessor (Fischer's design):  "....One the the characteristics we look a lot was the prismatic coefficient, that is the difference between the biggest section of the boat, probably in the middle and the slender part of it.A high prismatic coefficient is the shape of a tree

- So your boat has a pretty high PC, which is the benefit of this feature?
You reduce the pitching increasing the fore and aft stability and you reduce the drag at certain speeds but as a compromise you increase the drag at low speeds in calm, specially in the transom.

- I compared the Phantom transom at LA, and it is huge, not as big as the Cirrus but still quite large..
The transom is 3-4 cm wider than the Wildcat , the same for one meter behind the bow , 4-5cm wider there. So the width is exactly the same but we add some volume on the back and front of the boat.
This combined with a pretty straight rocker..."


Oracle & TNZ/LR are showing the same characteristics and compromises of their F18 counterparts, I've been saying this for a long time too, but now it is even more evident with Oracle showing excellent floating speed upwind in AC72 'San Francisco low wind range" of 15knots on their low drag hulls. (a la Wildcat and compared to Cirrus R/Phantom in the F18 5knot wind low range)


TNZ pioneered the off the wind Flight mode, and they might compensate their higher drag hull penalty in upwind floating mode  foiling to windward as they did in  Sat LV Finals Race 2 for some moments. But they will surely need those +18-20 knots? for that 'trick'.


This will be the battle of Design Compromises, and as both teams have first class and well trained  crews, wind conditions may define the winner at the end.
As if a great percentage of the Finals are raced in 15knots Oracle seems to have the advantage if they refine their downwind handling in that low 15knot tws range. In the upper wind range TNZ should prevail.

I hope these Pros & Cons of each platform will merge in the Cup Finals for an ultra competitive Match Racing Show.

One little detail to remark again is that Oracle digs the bow too much even on the pre start dial up at low speed and although they are super fast and a serious thread to the seemingly almighty TNZ , there is now way that platform can stand a similar 40knots flying Pitch situation.

Aero considerations are as important, but from what I'm seeing till now platforms and foils are having a major role over the AC72 performance.


Pitch Sequence can be found at catsailingnews.com/2013/08/ac34-lv-finals-etnz-super-pitch.html
Video at catsailingnews.com/2013/08/ac34-lv-finals-new-zealand-reviews.html

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