Thursday, 27 August 2015

Rodrigues Inbound Day 10

Hi everyone,
We are currently in position 17 47S 75 12E, sailing at 6.5 knots on a course of 290M.  The wind is 20-25 knots from the SSE with a 2m sea and roughly 2m swells from the SE and SSW.  The swell from the SSW has very long period.  We are under triple-furled yankee and single-reefed main.  Interestingly our day's run was 151nm yet again.  We have 680nm to go - just a Sydney-Hobart!

Yesterday we had a fantastic sail in 15 knot winds under full sail.  It was the first time we had the full main hoisted on this passage.  The seas gradually settled and we had a very pleasant day.  In the late afternoon a line of small thunderstorms came over the southern horizon and brought drizzle with 10 knots more wind.  We dropped a reef in the main and a couple of furls in the yankee to keep our speed down to around hull speed (~7.2knots).

The night went by with frequently changing weather as lines of thunderstorms passed overhead.  We think they are associated with a high ridging in down south.  As each came overhead the wind increased and veered from ESE to  SSE.  The SSE wind allowed us to bear away and edge northwest towards the rhumb line to Rodriguez.  We're expecting the wind to return to ESE so the chance to make some northing was welcome.

We are obtaining weather info from two main sources.  One is PredictWind which provides GRIBs based on 4 weather models - GFS, CMC, PWG and PWC.  We generally download the PWG and PWC GRIBs.  We also download MetArea text forecasts from PredictWind which are useful.  The second is zyGrib which provides GFS-based GRIBs but with more parameters than PredictWind.  This is the GRIB I use most of the time.

The program we use to present GRIB wind and pressure data is qtVlm.  This is open-source software which also provides routing functions.  Below is a screenshot from the early hours of this morning.  It shows our position surrounded by polar diagram, relative wind angle circle and compass circle.  There is a velocity vector showing where we'll be in 12 hours.  The orange line is the recommended route to Rodriguez.  Background colour shading shows wind strength and arrows show wind direction.

qtVlm screenshot

We don't use these tools to "tell us what to do".  Rather we use them to check our own analysis of the GRIBs.  On this passage the GRIB wind data has been pretty accurate, perhaps a little on the light side compared to our own estimation of wind strength.  Also the timing of weather feature movements is often a little off.  For example I believe the High SW of our position above is actually further east than shown so we're in the "yellow" wind.

We find GRIB files very useful, allowing us to position the boat for likely future wind changes (to some degree).  For example that's why we bore away beyond the course to Rodriguez when the wind went SSE.  We expect the change to last only a day or so before reverting to ESE, so took the opportunity to get that northing.

qtVlm receives GPS data from our instrumentation system, which is how it shows our SOG and COG at left.  I only had to wait a moment to capture the 8.6 knots!  ;)

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