I am planning the installation of a B&G Broadband 3G radar,
using a Zeus Touch 7 (T7) for display. I have now received the display that
will also act as a chartplotter. So it was natural for me to check if I could
use the T7 Sailsteer feature, by using my own data.
It happens that the T7 has an input for NMEA0183 data. My
system produces a 10 Hz stream of binary data that I have to convert to NMEA
sentences supported by the display. The conversion is done by a SAMD21 32-bit
microprocessor, connected to the display through a MAX3232 converter. Note that
the T7 has its own internal GPS, so that I don’t need to convert and send the
GPS data from my system.
Here is the schematics and wiring used. Each NMEA sentence
is sent once per second, through the NMEA 0183 Talker port (orange and green
wires), using a baud rate of 38400.
To test the behaviour of this setup, I used an example presented
in an earlier post. I reproduce here the data from this example for
convenience.
As a first test (System A of the example), I send continuously
the 4 following inputs:
Heading : 0.0 deg
Boat Speed : 6.0 knots
Apparent Wind Angle (AWA) : -26.71 deg (-26.71 + 360 = 333.29)
Apparent Wind Speed (AWS) : 16.43 knots
$HCHDG,0.0,,E,,W*50
$IIVHW,,T,,M,6.0,N,,K*7D
$WIMWV,333.29,R,16.43,N,A*1B
This corresponds to the basic inputs that the display would receive
if it was directly connected to sensors (masthead unit, speed log and
electronic compass). So the display is left calculating the following outputs:
TWA, TWS, VMG and tide current. Here is a screen capture of the display for
these conditions.
The calculated outputs of the display correspond exactly to
those of the basic System A example. It is interesting to note that the display
has correctly calculated a front current of 6 knots, as its GPS gives him a 0.0
knot SOG (the test is made at home, and the home does not move!).
But we know from the example that these results suffer from
2 problems: the AWA is not corrected for heeling, and the leeway is not
considered. In the second test (System B), I send instead the AWA corrected for heeling (available from my
system).
Heading : 0.0 deg
Boat Speed : 6.0 knots
Apparent Wind Angle (AWA) : -28.17 deg (-28.17 + 360 = 331.83)
Apparent Wind Speed (AWS) : 16.43 knots
$HCHDG,0.0,,E,,W*50
$IIVHW,,T,,M,6.0,N,,K*7D
$WIMWV,331.83,R,16.43,N,A*19
Here again, the values calculated by the display agree with the
results of the example for System B.
In the third test (System C, corrected for leeway), I send
these additional inputs correctly calculated by my system (instead of letting
the T7 make these calculations):
True Wind Angle: -45.0
deg (-45.0 + 360 = 315.0)
True Wind speed: 12.0
knots
Wind Direction: 315.0 deg
$WIMWV,315.0,T,12.0,N,A*11
$WIMWD,,T,315.0,M,,N,,M*73
I cannot send the result of my VMG calculation because the
VPW NMEA sentence is not supported by the T7, so that the VMG calculation is
left to the display.
Here we see that the VMG value is not calculated correctly
by the display. This is not surprising as the display has no information on the
leeway. To do so and to reproduce all the correct data, this system would have
to be completed by the B&G H5000 computer.
But despite this problem that may somewhat affect the
resulting calculated laylines, I expect that the Sailsteer feature will still
be very useful in practice, as it takes in account the tide current values in
its calculations. This is a much more important correction in my sailing area
characterized by strong tidal streams.
For now, the erroneous calculated VMG value will not be displayed on the T7, to avoid confusion with the correct value displayed on my tablet.
