Real-time Display and Tracking of Actual Current (Set & Drift)

One of the nifty capabilities of SailFast is the ability to determine the actual surface current; both current magnitude and direction. Knowing what the current is can be helpful in determining the best course to sail. Current affects the course made good on each tack and the laylines, and therefore is a factor in your racing tactics.

Often you will be able to adjust your course to sail in more or less current. Sometimes you can even choose whether to sail in favorable or unfavorable current, as sometimes happens with Gulf Stream eddies. While sailing in unfavorable current (current that is slowing you down) by choice is counterintuitive, it can be the best and fastest course if that course is shorter or the wind is better. SailFast's optimum routing algorithms takes this all into account and recommends the fastest course to the finish or destination, based on the available wind and current grib file forecasts.

Whether or not you are following an optimum route strategy, there are times when being able to determine and track the actual current over time and position can yield a big competitive advantage.  Let's take an example using the Gulf Stream.

Below is a screen shot from SailFast showing a segment of the Gulf Stream. The color gradient background is the surface water temperature with orange being the highest temp. The arrows represent the surface current. A black arrow is the highest current, followed by dark purple, purple and blue. The displayed sea temp and current are from NOAA RTOFS grib file forecasts. The box in the center with current and temp values shows the forecasted values at the cursor's position. The cursor is not shown here but is near the  box's top left corner.

     Figure 1.  Gulf Stream Segment Optimum Route

The green line shows an optimum route when starting from the waypoint near the bottom left and assuming you want to get to the upper right waypoint. For this example the wind is a constant 8 knots 0 degrees true and we are sailing a Beneteau First 40.7. While this optimum route is great guidance for the navigator it cannot be blindly followed. In the real world neither the current nor the wind forecast will exactly match actual conditions. By observation you will want to determine where the primary flow of the Gulf Stream is and how current increases or decreases as you move left or right. There will likely be tradeoffs between maximizing speed over ground and sailing the shortest distance. With higher winds and boat speed the current becomes less important and sailing the rhumb line becomes preferred.

Let's use the RTOFS grib model to look at a typical relationship between current speed and water temperature for a cross section of the Gulf Stream. Using SailFast's Range/Bearing tool and the cursor data we can easily gather the data shown in Figure 2. We've arbitrarily defined the edges of the stream at the current = 2 knots point, and find that the stream width is 38 nm. In this example current velocity rises quickly as we move into the stream from the North wall. Initially current increases before hardly any temperature increase is observed. The current peaks before temperature peaks by a few miles. Note that the temperature curve has a relatively wide flat top. It would be difficult to use temperature to find the peak current location. Once sailing on the temperature flat top it's even hard to determine the direction to more favorable current. We clearly want to be tracking both current and temperature data.

 
     Figure 2.  Gulf Stream Cross Section

 

SailFast is an effective tool for gathering accurate and consistent surface current data needed to make tactical course decisions. The simplest way to view the actual current in real-time is to include "Current" in the Instrument Data display panel. In this example (Figure 3) the current is flowing at 0.9 kt in the direction of 283 degrees magnetic. This is the actual current, not a forecast from a grib file.

Note that the boat Heading is 207.8 degrees while the COG (course over ground) is 219.1 degrees.  The difference is due to the current.  

     Figure 3.

Figure 4 shows the large number of parameters that can be displayed as Instrument Data. Some parameters are repeated directly from your GPS or boat instruments via a NMEA 0183 interface. Other parameters are calculated values, such as Current, Set and Drift. The parameters most useful in analyzing the effect of current have been highlighted. "Speed" is the vessel boat speed through the water; ie. your knot meter reading.

     Figure 4. Instrument Data Selection

SailFast can also record any of the available Instrument Data parameters along the vessel's track as illustrated in Figure 5. In addition, Current can be periodically displayed as arrows along the track. The arrow color and size indicates current magnitude just like grib file forecast data.

Active or saved tracks can be evaluated in detail by stepping along a recorded track and viewing the parameters in the Track Data panel. To learn more about setting up tracking, refer to the Viewing Active and Saved Tracks section in the SailFast user manual.

     Figure 5.  Tracking Current

What Is the Competitive Advantage?

Let's say your navigation decisions result in a 0.5 kt speed advantage over your competitors during a 6 hour period, while sailing an average of 6 kt. After sailing 36 nm you will be 3 nm and 30 minutes ahead.  A very nice gain!

How Is Current Calculated?

SailFast calculates the actual current in a manner similar to traditional navigation. If you refer to The American Practical Navigator (Bowditch), Chapter 7  , you will find a complete discussion. In traditional navigation a course is sailed and later a position fix (FIX) is made. A corresponding dead reckoning (DR) position is plotted. Any difference between the FIX and DR positions are attributed to current set and drift. Set is the current direction and Drift is the current speed. In reality the FIX and DR difference is not just due to current but is the combination of all navigation errors and vessel leeway.

Solving for Set and Drift is done by solving a vector triangle, such as the Figure 6 example.

 

     Figure 6. Bowditch - Set and Drift 

Traditional navigation requires sailing a significant distance and time to adequately resolve FIX and DR positions. SailFast does near instantaneous computations by relying on modern instrumentation.  The equivalent vector triangle uses Heading, Speed, COG and SOG to define the A-B and A-C vectors. The B-C vector is then solved for Current.

     Figure 7. SailFast - Set and Drift

SailFast requires the following NEMA 0183 inputs for current computations. Be sure that Heading and Speed instruments are well calibrated.  Input errors will directly translate to computed current error.

Heading	Vessel heading in degrees magnetic from the NMEA VHW or HDG sentence. This can normally be obtained from an auto-pilot. Alternatively a dedicated heading sensor can be employed.
Speed	Vessel speed through the water from NMEA VHW sentence.
COG	Course over ground from GPS using NMEA RMC sentence.
SOG	Speed over ground from GPS using NMEA RMC sentence.

 

Displayed NMEA data items are filtered to reduce chatter and give more stable readings. The default filter setting is 5 seconds. Current, Set and Drift data has an additional default filter setting of 1 minute. The user may modify these settings on the Options/NMEA tab page.

---