America's Cup - Pessimistic forecast not shared by Teams - Day 5

by Richard Gladwell, Sail-world.com on 31 May 2017 Although wind forecast models used by America's Cup Race Management are only for 3-5kts for race start time of 2.00pm on Day 5, Race Director Iain Murray has just to the daily media briefing that the teams' own projections are less pessimistic.

The official prediction is for 3-5kts at 2.00pm; 2-5kts at 3.00pm and 2-4kts at 4.00pm.

The predictions from Predictwind.com, developed by top sailor and former Alinghi weather forecaster, Jon Bilger is far more optimisc than the ACRM models - with all four weather feed developed by Predictwind from independent feeds at 8-9kts average with the wind gusts only 1kt more.

The key point is that there is a strong co-relation between the four feeds and usually that means the forecast can be taken as quite reliable and accurate,

The minimum wind limit for racing to start is 6kts.

Murray explained that while the AC50's will foil in the current conditions or 5kts of wind, they will only be able to reach across the wind or sail to windward.

Downwind is the difficult point of sailing, and Murray said the AC50's would not be able to reach the bottom mark.'Sailing an AC50 downwind in these conditions is like using a blunt saw to cut a piece of wood,' he quipped.

The AC50's sail on apparent wind, and in light to moderate conditions will sail at over three times the true windspeed.

However apparent wind is a complicated combination of the actual wind and the wind that the boats generate themselves as they gain momentum. Apparent wind is in effect the wind you feel putting your hand out of a moving car window.

To generate apparent wind from the true wind the boats either sail into the wind and then alter their course away from the true wind as their new apparent wind build. However, in very light conditions, they loose the true wind benefit the more they sail in the opposite direction.

Apparent wind sailing is a very complicated, theory riddled exercise made even more complex with the use of foiling daggerboards and wingsails. That is why there are so many Ph.D's in the design teams - and even they struggle.

Harking back to the last America's Cup in San Francisco - a similar model is used in Bermuda - but with a sanity check.

While the breeze is still measured over a rolling period, this is just 30 secs in Bermuda, not five minutes in San Francisco.

The average wind strength measured 5.5 metres above water on the race committee vessel and a 30-second average taken - if the average windspeed is less than 6kts or more than 24kts then the start is abandoned and a new attempt made.

In Practice racing Murray says they did try to get started in low winds and had 25 attempts at starting before they gave it away.

Organisers are allowed to start any race that will finish before 5.00pm, however for late starting races - after 3.30pm there is not a commitment for live coverage, or for the race to be even recorded for later replay.

Currently, on the Great Sound, the Red Bull Youth America's Cup teams are out training and foiling. From current observation stations monitored by Predictwind.com, the breeze is currently 6.9kts and gusting to 11 kts.

Only Land Rover BAR (GBR) is on the water currently of the six teams based at the Royal Dockyard. She has just done one run down the harbour foiling easily and it would seem likely that, at this stage racing will proceed as scheduled.

The wind is from a new direction, the east which means a new course configuration as the wind blows down the Great Sound in the direction of Hamilton.

Racing Schedule – second Round Robin for Louis Vuitton America’s Cup Qualifiers

RR 2 R4: SoftBank Team Japan v. Groupama Team France 2:08 pm
RR 2 R5: Land Rover BAR v. Emirates Team New Zealand 2:37 pm
RR 2 R6: ORACLE TEAM USA v. SoftBank Team Japan 3:06 pm
RR 2 R7: Groupama Team France v. Land Rover BAR 3:35 pm

The race management team will go on the water at 12.30pm to assess the wind situation.