America's Cup: Why Team New Zealand's VMG is the key

"We were young and strong, we were runnin' against the wind": Team New Zealand living Bob Seger's lyrics in Bermuda.

The key to Team New Zealand's success in the 35th America's Cup may rest in three little letters - VMG.

Along with the aggressive design features in the New Zealand boat, the way the Kiwi crew are sailing the boat has also impressed onlookers. What has been particularly noticeable over the latter stages of the regatta is the advantage Team NZ have upwind, which is all down to their superior VMG, or velocity made good.

In simple terms, VMG measures the progress a boat is making towards the mark. As yachts cannot sail directly upwind, they sail across the wind.

The rule of thumb is the lower the angle, the faster the boat will go, but it will be sailing further away from the mark. The higher the angle, the slower the boat will travel, but it will be sailing a more direct route to the mark. The crew therefore need to balance this trade-off between true boat speed and VMG.

What is VMG?

VMG stands for 'Velocity Made Good'. In simple terms, it measures the progress made towards the destination. The concept is useful because a sailboat often cannot, or should not, sail directly to a mark to reach it as quickly as possible. Yachts cannot sail directly upwind. It is usually less than optimal and can be dangerous to sail directly downwind. Instead of sailing toward the mark, the helmsman chooses a point of sail that optimises VMG.

Where Team NZ have a significant advantage over Oracle over the opening four races of the first-to-seven series is they are not only sailing faster upwind, but also at a higher angle, meaning it is travelling a shorter distance on each beat to the top gate.

At one point during Monday's fourth race, Oracle tactician Tom Slingsby was picked up on the on-board audio saying "they're way higher here".

While we don't get full access to the numbers coming off the boat, the television coverage has shown in terms of actual boat speed, Team NZ are around one to two knots quicker than Oracle upwind, but their VMG has at times been three to five knots quicker.

Team NZ designer Dan Bernasconi has been coy on how the team are able to achieve this, but said it is a combination of both design features, and the way the crew are sailing the boat.

"I think our boat speed has improved massively over the last few weeks as the guys get more and more familiar with how to get the best out of the boat," he said.

"Being able to sail against the other teams in the Louis Vuitton really helped us develop, because when you're sailing on your own it's really hard to work out what's fast and what isn't. But when you're alongside someone and you see how they're sailing and what they're doing differently, definitely you learn a lot quicker."