Placing the Boat in the Right Place
Posted on 14/07/2019 by John Martin
PredictWind, our go to weather and routing source!
Placing the Boat in the Right Place
We all know that weather can have a profound effect on having a good or bad passage and we're all looking for a good weather picture (window) for a safe and comfortable passage. Why is it then that two boats leaving from the same place at the same time and therefore into the same weather window can have a wholly different story to tell at the end of the passage.
Sitting here in New Zealand this weekend in one of our favourite anchorages in the Bay of Islands saw a not untypical weather event for this time of year. A low pressure cell passed over the South Island with its frontal tail stretching to just below 30S, (tap on the video link on the left for a video showing this weather event both live on the boat and on the PredictWind App).
This is a classic case of being in the right place for whatever weather is coming. In many cases regardless of what Planning we did prior to the passage and the anticipated route we will take, as we leave port the chaos theory takes over. You've no doubt heard the tale of planning a military event and that regardless of how well you plan the offensive it all goes out the window after the first shot is fired, well weather can on some occasions be a bit like a military offensive and that's where Passage Management comes into play. This means always having the latest information so you can make informed decisions;- the PredictWind Offshore App coupled with Iridium Go is our choice, regular downloads to ensure you've always got the most up to date information, make sure you're on the unlimited airtime package from PredictWind so that little bit of Scottish in you doesn't say "Och nay, once a week is plenty at that cost lad!" and a willingness to deviate from the rhumb line. The shortest distance between two points isn't always the least number of miles.
Money in the Bank.
It's an old racing expression, if you're hard on the wind heading for the mark and you get a lift, take it! With a little bit to windward when the inevitable knock comes you've still got some windward "In the Bank" to bear away during the knock without pinching or even worse tacking. So how does this relate to Passage Making? In the SW Pacific once your south of the intertropical low, thats where the trades are, you'll enter the Horse Latitudes where the migrating weather systems move from west to east with low pressure, in the southern hemisphere, rotating in a clockwise direction and High Pressure systems rotating anticlockwise which influences the direction the wind blows. With these migrating systems you've really got to think in three dimensions, not only are the systems moving they're also rotating but it's not just the system that needs to be looked at, you're moving too, so you're relative position to the system also changes. The speed of migration, cased by any number of influences may also vary during your passage. Good Passage Management takes all this into consideration and knowing what the prevailing conditions in an area are and where you'll get the best sailing angle to your destination is, will allow you Place the Boat in the Right Place for a great passage.
Heading South to New Zealand.
Particularly when coming south from Tonga and Fiji, Vanuatu and New Cal need slightly different planning see passage-planning-from-the-sw-pacific-to-nz , a curved route often serves best. Depending on your boat speed will dictate when you depart the tropics but unless under ANY conditions you can definitely reach New Zealand in the migration of a single high pressure system, 6-7 days is the norm in Sept/Oct., then both fast and slow boats are likely to want to aim for a spot in the ocean colloquially known as 'John's Corner' 30S 173E. But that means I'm out there longer I hear you say! Yes it does, but this is the prime reason why the two stories of the same passage are so divergent. Imagine this, a passage from Fiji to New Zealand expecting the weather patterns we saw here today but one week from departure; boat A takes the rhumb line from Fiji, firstly as they leave Fiji as the new high pressure system presents the winds will be south east and strong, the easterly swells will be topped with the SE wind waves, hard on the wind into a lumpy and variable seaway. If they've not been quick enough or the system has gone through faster than first thought they may be still some distance from landfall when the next front passes. Having taken a more southerly route they are likely further south, looking at the PredictWind image we can see the frontal band has significantly more punch the further south you take it. The next days until landfall will see squally disturbed S-SW headwinds, this time though they won't be warm! Boat B on the other hand leaves Fiji 36 hours later once the winds have backed to the East, they took a track with some westing in it and at departure they it approximately 5.5 to 6 days to when this weather is expected and 780nm to John's Corner. They reach that point today and see a broken line of shower cloud, as they pass through the NW winds change to W or SW 18 - 22 kts on the beam their next waypoint should be off North Cape for land fall there two days from now. They arrive 12 hours later after a comfortable and pleasant passage on the Q dock at Opua's Bay of Islands Marina and guess what? Boat A is also there with a sad tale to tell. Good Passage Management in this scenario has seen Boat B trade time for comfort and arrived safely. Welcome to New Zealand.
For more resources on this subject read passage-planning-from-the-sw-pacific-to-nz or attend one of John Martins Passage Planning and Pacific Weather seminars in Fiji during Fiji Regatta week at Musket Cove and at both Port Denarau Marina 1:00pm Thursday 19th September and Vuda Marina 4:00pm Thursday 19th September. Follow Sail South Pacific on Facebook for updates and information.
Why is the GUST forecast from PredictWind important?
The bigger the differential between average wind and GUST will tell you how stable the system is. In this scenario there is a significant difference which would indicate the likelihood of severe squally conditions.
Note the increased severity of the frontal band the further south you are.