With it, they could tell just how fast Emirates Team New Zealand are going and what they have to do to beat them.
Until yesterday - although comparatively light airs may have thwarted the watchers from picking up much. Both Oracle yachts seemed quick but the relatively light winds meant foiling was not easy and, without that, comparisons may be muted.
Some preliminary observations were possible: Boat 1 (skippered by Sir Ben Ainslie) may be slower downwind but might be a touch faster upwind. However, it still has non-compliant rudders, suggesting that the change to compliant rudders may affect its speed.
Boat 2 (skippered by Jimmy Spithill) won the start and looked to have a faster package overall though it is still difficult to tell without foiling. It has compliant rudders and is expected to be the yacht Oracle field in the Cup match.
"It was a difficult day," said Ainslie. "It was a bit puffy and that sort of wind produces the hardest conditions for foiling. These boats are actually easier to sail if they are going quicker."
Spithill said he felt the New Zealand team were still ahead when it came to foiling.
"I think they are still ahead in that department," he said. "You know, we are not hiding from our weaknesses - we still have work to do in that area."