Sailing from Bermuda to Chesapeake: The Weather Puzzle

Trying to figure out the weather puzzle to decide the best time to leave Bermuda (with the help of some new routing software).

We’re sitting in Bermuda, trying to figure out the weather puzzle. Everyone will tell you that the prevailing winds are supposed to be southwest this time of year. But since we don’t have our devil devil stick* aboard and younger daughter Sarah to operate it, the wind picture is, well, somewhat confused.

The highs are squashed, and the lows frequent. So the prevailing winds are NE to NW, on the nose as it were, with just a few hours of fair breeze from the SE to SW to tantalize us in between.

Beating, per se, if the breeze is not too strong, is not that bad. In winds of less than 20 knots Beowulf can go full speed, and be reasonably comfortable. Drop the true wind speed to 16, fill up the weather ballast tank, and we will get there quickly with the crew feeling chipper.

The distance we have to travel is 630 miles. That’s an easy two days if we are reaching. But beating is much harder to figure.

This is where the best tool we’ve seen since the devil devil stick comes in. It is the weather routing function built into the MaxSea navigation system. As we mentioned last week, this function combines a weather “GRIB” file – a numeric representation of the weather over the next three to five days – with the boat’s performance polars. The MaxSea program then chooses the fastest route – tacking (or jibing) as required to find the best course.

If the weather/course issues are simple, as with our passage from the West Indies to Bermuda, the routing function serves to help plan for the different wind speeds and angles which will be encountered (which makes planning in advance for what sails to carry simple), and provide a pretty good handle on ETA. However, it is with the complex patterns that we now face where the MaxSea routing algorithm really becomes valuable.

The main issue is when do we depart Bermuda to get the fastest, most comfortable passage for our combination of boat and crew needs/capabilities? To find out we’ve just looked at the passage leaving at noon today, and each of the next three days. MaxSea tells us the total elapsed time and miles sailed for each departure date. More important for us is the breakdown of wind speeds and angles which we’ll encounter. We can easily check the conditions every six hours on each of the projected voyages – with true wind speed and angle, magnetic wind direction,and boat speed called out.

Bottom line: It is going to take between 2.5 and 2.75 days, if the weather comes in as projected in this GRIB file. If we leave today we are seeing about half the passage in winds of 20 to 25 knots. If we leave in in three days, the winds drop considerably, making for a more comfortable passage.

Given the fact that in another day sea state will be much more relaxed than it is at present, we are going to be tourists today and take the bus to Hamilton (the big city hereabouts). We’ll check the weather tomorrow AM, and then look at the most recent GRIB file and perhaps we’ll unstick Beowulf’s anchor from the mud.

A couple of things to consider: One is that you can have different performance polars for your boat. For Beowulf, at present, we have four. One set of numbers is for high performance, sail only, pushing the boat in moderate seas – something we rarely achieve. Next is a sail only, backed off performance which is indicative of a relaxed state of crew awareness and sail trim. Then we’ve got two motorsailing polars – where the engine is used to augment progress on a laid back and then aggressive basis. For the purposes of the trip between here and the East Coast, we are using the laid back motorsailing polar.

The second issue is how do you get GRIB files? These will shortly be available on a special section of SetSail, for different cruising areas around the world. They can be downloaded to the boat using a variety of techniques – we are using the Globalstar phone to connect to the web and download – in two minutes per file.

More about how this routing software works later.

*A devil devil stick is an inanimate object used to cajole the wind gods to grant favorable (i.e. broad reaching) weather. Developed roughly 1000 years ago in the Outlier Islands at the bottom end of the Solomon Group, these wind workers have been used by those in the know for many centuries now. In double blind tests they have proven to be as effective – and often more effective – than modern weather forecasting techniques.

Posted by Steve Dashew  (May 10, 2001)

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