The Routing Process

In Real Life [IRL] I route a handful of times a year. In between those times I use sailonline.org [SOL] to practice my routing skills in a virtual sailing venue. In both worlds the process begins with new forecasts.  These WXs arrive every six hours.
Here are the steps to process them:

• Download the new forecasts:
  IRL, I will be downloading GRIBs with current and wave data in addition to wind data.
  Also, the GFS data will include "surface" pressure and rainfall forecasts.
  I load all the GRIBs into my charting & routing program of choice -- Expedition.
  [I am looking at BlueWater Racing as a shareware alternative to Expedition.]
  
  In addition, I will download the old-fashioned hand-drawn forecast maps, as well as the narrative that goes with them.  I have the ability to overlay the maps into Expedition, but I can also show them in Google Earth.
  
• Update the boat's position:
  IRL, I get the position from email messages the boat sends automatically via Spot-Tracker.
  Last trip I hand-entered selected positions into a spreadsheet that calculates COG, SOG, and distance traveled since the prior position.  I'm looking at ways to streamline that process for the passage coming up in June.
  
  In SOL, I alt-click a spot on the projection header that is roughly calculated to be 5 minutes into the future.  That captures the lat/lon onto the clipboard.  I then copy that position into a new line in the 'notes' tab of SOL and type in the [5 min in the future] time.  Then, in Expedition, I edit a course 'mark' called "javakeda" to place it at the lat/lon I have captured.  The 'javakeda' mark is set as 'active', and will thus be the starting point of any optimization calculation.
  
• Get the router[s] involved:
  With Expedition and SOL, I sync the 'optimize' start time with that '5 min in the future' time.  Usually, I do that by waiting a few seconds for that time to come up, and then setting the optimizer start time to "NOW".  [And that should explain why I choose a position about 5 minutes into the future ... it just works with the natural timing of the process.]
  
  After that, I just click on "optimize" and, depending on the settings, Expedition will paint an optimized course on the screen in somewhere between 1 and 1000 seconds.  In addition, clicking on "results" yields a cell-by-cell table of suggested courses and times, together with other columns of related data,   You can save the table to a CSV file and do with it as you wish.
  
  The process is almost the same IRL.  The only difference is that I am sync-ing the start time to Spot-Tracker time and position data.
  
• Validate the data:
  The most common reason I hear for not using routing software is, "I don't want to be a slave to the computer."  My response to that is that I couldn't agree more.  Once I have a "results' table from the routing software, the next job is to determine whether the results make sense.  The full process of validating results is a subject unto itself, and more than I will go into in this blog entry.  That said, here are some questions I ask myself:
  
• Are the results more or less along the lines I expected?
  If not, why not?
  Usually, the model will see something I missed with SOTP analysis.
  Occasionally, it works the other way around
  [e.g., there are storm generated waves not yet in the GRIB data but that are described in the storm warnings.]
  Either way, I learn something.
  
• Am I sure that I had all the settings properly adjusted?
  Settings to check include
   -- GRIB file selection,
   -- start time and position
   -- optimizer granularity
   -- boat polar
  Contemporary routing software has all kinds of knobs, dials, bells, and whistles to play with.  The more choices, the more opportunity for making mistakes.  Hey, I am human and make mistakes on a continuing basis.  Best to double check the settings.
  
• If the new optimized route is different from the prior optimized route, do I understand why?
  [That sounds like a good subject for a subsequent blog post.]
  
• Do the results want me to do something now in anticipation of a future event?
  If so, what probability would I assign to that event taking place?
  What are the consequences if the event does not take place as forecast?
  [An example of 'risky' routing results are those that depend on the winds inside a HP ridge 72-hrs from 'now'. This is how skippers get caught in 'blue goo'.]

As a practical matter, I spend more time validating the routing results from day-time WXs then I do those that arrive at 0330.  IRL, I try to do a daily comprehensive routing update based on 1200utc WX data.

• Set headings for the next 6 hours:
  And this is trickier than it looks.
  A basic problem is that a tightly-granular results table generates far more heading instructions that I am willing to convert to DCs in SOL.
  IRL, the problem is even worse.  Just try convincing the watch-captain on the mid-watch to adjust the auto-pilot by a degree or so every fifteen minutes [and that assumes auto-pilot is working.]
  Don't even mention headings of 175.12*T.  I mean, that's a joke ... right?
  So, after all this, we are back to SOTP decisions for headings, but with guidelines developed by the router.
  
  My current approach is to set an optimization granularity equal to about 1/2-hour of travel.  So, if I see the boat is likely to average 15kts over the next 6 hours, I'll set a granularity for 7.5nm.  That gives me about 12 heading instructions in the next 6-hours of the results table.  I will try to combine those and bring them down to 3 or 4 if I can.  If I can't simplify it, then perhaps I need to sail that section hands-on [or the real-life crew does].
  
  IRL, the 'boat' and I agree on intermediate WPs for the boat, and I use the router to help develop COG and TWA guidelines for getting to those WPs.  Usually, I talk about being above or below the Rhumbline, and why that makes sense.  The boat, in turn, sends me 'actual' data that lets me know if my GRIB forecasts are on target, or just pipe-dreams.  Mutual trust is essential in making on-shore routing work with a boat at sea.  Without that trust, routing efforts are far less effective.
  
• Repeat!
  It is, indeed, an on-going process.

Routing Software

If you want to know the 'optimal' route from where you are to where you want to sail, the answer is simple ... use routing software.

Does routing software actually give you the 'optimal' route?
Well, that depends....

If you are talking about an on-line sailing venue, where the winds are guaranteed to be as forecast and the boats always perform exactly to their polars, well, then, yes ... routing software can potentially calculate an optima route.

BUT ...

As soon as you introduce 'future' winds, uncertain positions, or different update granularities, the guarantee of an 'optimal' route is gone.

You can still get close, but now there is uncertainty.

The routing chart on the right was produced by Expedition routing software for 'javakeda' -- my SuperMaxi100 -- in the race around Cape Horn from New York City to San Francisco.  The race is produced by sailonline.org -- which has a very realistic 'virtual' venue.  The routing chart covers about 5-days in a race that will take a couple of months [for me, at least].

This 'optimal' route has four kinks in it -- each [subtly] highlighted by a red arrow.  I'd like to know what is going on with the weather at each 'kink'.

But before going there, I want to describe the process that produces the routing chart.
And I will do so in the next blog entry.

Après Ski, Let's Dance

In an email to me, Vendée Globe skipper "YR" compares weather routing in the virtual race to playing chess -- it is always important to thnk a few moves ahead.

I agree, as far as that analogy goes.
Here a two more ways to look at it:

• The race is like skiing down a mountain.
  You are starting at the top of the lift.
  You are headed for the lodge.
  There are many trails that will get you to the lodge.
  It is up to you to pick the best one.
  This is your synoptic weather-routing.
  
  But on your way down you encounter details you could not anticipate from the top of the mountain.
  A traverse across a smooth slope might be the equivalent of a beam-reach in a boat.
  A field of moguls requires careful routing [at the wind-cell level].
  Where you exit the mogul field is determined, in large part, by your entry point.
  Selecting that entry point is a crucial decision.
  -- and so is selecting the wind-cell you want to be in at a weather update.
  
  What I like about this analogy is that it reflects the dual-level planning needed for weather-routing.
  Moreover, this dual-level of planning applies in both virtual and real-life ocean passages.
  
• Back at the lodge, a jazz band is playing and it is time to get on the dance floor.
  In this analogy, the wind is the music, and your routing is the choreography you use to dance to the music. 
  
  [A personal note: My Dad was a musician and my Mom was a dancer.  I have played in bands and orchestras.  I have choreographed dances.  It is from this perspective that I offer....]
  
  A jazz band usually plays the first chorus as written in their charts.
  After that, it is time to improvise.
  A big band will stay closer to the original tune than a combo.
  And this is exactly what happens with weather updates.
  The larger the weather system, the less likely that you will be surprised by the way the update has 'improvised' on the prior forecast.
  For light and unsettled winds, however ... well, who knows what those cats will be playing in the next chorus?

Neither analogy is perfect, and the chess analogy still applies.
Taken together, these perspectives help me to wrap my mind around the challenge of weather routing.

Passing by Storms, and Vice Versa

Whether sailing 'virtually' or in real life, storms that pass by can help or can hurt.

In real life, avoiding a potentially dangerous storm usually takes precedence over trying to take advantage of the storm. 

In virtual sailing, the dangers are virtual as well.  With 'real' dangers gone, it is easier to analyze how to use a storm to improve sailing speed.

As I write this, the Vendée Globe sailors [real and virtual] are moving across the Indian Ocean towards the waters to the south of Australia and New Zealand.  The storm pattern looks like this:

 It doesn't take a great deal of analysis to determine that:

• Storms rotate clockwise in the southern hemisphere.
  
• Sailors heading west will do best by bouncing across the top [that is across the northern part] of the storm's wind rotation.

Of course, a sailor's life is never quite that simple.

• Great Circle routing looks at the shortest distances between two points on the globe, and not on a flat map. The meridians [lines of longitude] are much closer together near the poles than they are near the equator.  For example: 10 degrees of longitude is:
   -- 459nm at 40*S latitude
   -- 383nm at 50*S
   -- 298nm at 60*S
  These are differences that a racer must take into account.
  
• The Vendée Globe race committee has established 'ice-gates' to keep the racers from sailing too far south -- and either hitting ice-bergs or freezing to death.

What is the best overall strategy?
That is the topic for the next blog entry.

Practice, Practice, Practice

Javakeda is sailing in the Vendée Globe because this race is one of the best opportunities for amateur weather-routers [of which I am one] to practice their skills.

Where else can you

• read a GRIB file
• make course decisions
• have those decisions confirmed or debunked by the elite skippers of a fleet almost 400,000 strong.

Only in the Vendée Globe.

But, in addition to weather-routing strategy, the race lets me practice the mechanics of routing.
This is the watch-to-watch:

• OK, here are the winds
• Looks like we should go this way for the next watch.
• Looks like we have a handle on the long-term route
  ... or NOT!

Last October, I was the weather-router to the Sea Witch as she made her seasonal passage from New Bedford, CN to the British Virgin Islands [BVI].
I was 'shore crew' -- not on-board, but in regular contact with Captain John and First Mate Dennis.

The first leg of the passage [New Bedford to Bermuda] went well.
Sea Witch set a personal best for that passage.
On the leg from Bermuda to the BVIs, however, the 'routing' became very serious.

We had TS-Patty passing to our west.
We had TS-Rafael, coming at us from the south-east.
Sea Witch was able to pass safely to the west of Rafael within 100nm of the center.
This was less than 24-hours before Rafael became a hurricane.
This was less than 12-hours after Rafael passed over the BVIs, our destination.

Definition:

Experience is what you get, just after you needed it.

Days before, we had made the right decisions regarding our route.
But during the critical time, I wished I had better routing tools and more experience.
In the days after the passage, I resolved to practice
 -- and here I am.







 

 

The Basic VMG Calculation

Understanding VMG (velocity made good) is essential if you want to sail your boat quickly from point to point.  This is true regardless of whether your boat is real or virtual.
VMG values are used in many 'advanced' racing tactics, but the calculation of these values remains the same.

From a 'trig' standpoint, it should be obvious to you that VMG = BtSpd * COS(40*).
Easy enough to let a spreadsheet or scientific calculator tell you the actual value of the Cosine.
You don't need to know the actual values of the trig function.
But the fundamental relationship between BtSpd and VMG should be clear.

If not, you can go to Google for help.
And, of course, you can comment on this blog.