Four things we learned on day four of the America's Cup - from the Sailing Professor

Emirates Team New Zealand to battle with Artemis. Photo / Ricardo Pinto

The Sailing Professor Mark Orams provides four things we learned on day four of the America's Cup.

1. Team NZ will be be exploring how to keep the speed gain they found today while improving stability. It's always a balancing act in foiling boats.

In the previous race between the two, Artemis showed better boat speed up-wind so

Team NZ simply did what they could to keep chipping away and maintain pressure. Yes, a controversial umpire's call then handed Team NZ the race, but without fighting to stay close they would not have had this opportunity.

Team NZ showed the same fighting character today, keeping the race close when they were behind, closing up then putting the pressure on to engineer a pass.

They had stayed out after racing on day three to test new settings, a good sign. The refinements made the boat quicker in a straight line upwind against Artemis, but it was much less stable with more crashes off the foils than in any other race.

There is always a trade-off in foiling yachts between straight line speed and stability, but it is worth trying this sort of thing at this stage of the regatta. This is about evolution, not revolution. Nothing radical or high risk from this point, no magic bullet, but rather an incremental process of finding little gains.

2. Team Oracle USA were impressive again, and gave France no chance of causing another upset a la their wins over Artemis and Team BAR. Oracle are smooth and clinical, and are showing good composure. They had a problem with some control component in their wing, but were far enough ahead to keep it together to complete a solid win.

Communication on the boat was clear, measured and calm. This team continues to look the goods. They face Team NZ on Sunday, their final races in this qualifying stage. Will either team show their hand by using their best gear and mode? I doubt it.

If Team New Zealand are unable to overtake Oracle on the points table, there may be little up-side in using their best set-up. Team NZ will be focused on the end game. Nobody will care who won the qualifying series in three weeks time. It's all about who wins the big show, the America's Cup itself.

3. Artemis had a tough day. They will have been desperate for redemption after the injustice they felt over the contentious umpires' call the previous day.

Skipper Nathan Outteridge wanted to take the race out of the umpires' hands today - but Artemis did the opposite via a very close cross of Team NZ while on the give-way port tack.

Team NZ protested and the umpires penalised Artemis again! This was a much clearer call, with no controversy. Artemis was in the wrong and was rightly penalised.

4. Team BAR looked like they did on day one, with good speed and stability. They sailed a very strong race against Artemis.

Interestingly, the conditions were similar to day one and most yachts looked like they had their light wind foils on (Team NZ and Oracle looked to be unchanged).

Ben Ainslie is a great sailor and has always had a "refuse to lose" mentality. No surprise then to see him lead his team to a comeback after four straight losses. This showed great mental strength - to come out upbeat and deliver a start-to-finish win is a reflection of his pedigree and the quality of this team. They have struggled, but the longer they stay in this regatta, the more of a threat they will be.

Bottom line: This regatta remains wide open. Great sailing, minimising mistakes and continuing to develop and improve is key. There is an advantage for the winning challenger at the end of the qualifying series in that they get to pick their opponent for the semifinals. Team NZ will want this option to avoid facing a team on the upswing.

Mark Orams - The AUT Sailing Professor