Configuring True Wind Direction (TWD) and True Wind Speed (TWS) on a B&G Zeus 3 Chart Plotter

(Updated Dec 5 2019)

B&G make some of the best instrument systems used on sailing boats – but the lack of documentation on how to configure and use them – particularly some of the more advanced features is very frustrating.

I have a new (April 2019) Seawind 1260 catamaran and have been cruising in the San Juan Islands in WA (Pacific North West). I was noticing that when I tack the TWD would often change 30-40 deg after I’d tacked. Tides and currents are quite strong here in the San Juan Islands – often 4 to 5 kts. I suspected that both TWD & TWS were being calculated using boat speed (speed through water) and heading – not speed over ground (SOG) and course over ground (COG). In other words – the local current was not being factored into the calculation of True Wind Direction and Speed.

(The wind instruments on the top of your mast measure Apparent Wind Angle (AWA) and Apparent Wind Speed (AWS) – True Wind needs to be deduced factoring in the speed and direction of the boat – and any currents. )

After a lot of digging I found two settings under Settings/System/Advanced/Instruments :

“Use COG as Heading”	Controls whether COG is used as Heading when calculating true wind
Use SOG as Boat Speed	Controls whether SOG is used as Boat Speed when calculating true wind

In my case both of these were off. When I tried changing them the first thing I noticed was that my boat speed display on all my various dashboards was now identical to the SOG number. This was annoying as previously the boat speed display had shown my speed through the water – and given the strengths of the currents in the area – its really helpful to see both speed through the water and speed over ground. Fortunately I had spent some time configuring dashboards – and had noticed that under the Speed/Depth values there is both a “Boat Speed” and a “Speed through Water” inputs. My dashboards were all displaying “Boat Speed” to display “Speed Through Water” – but obviously changing the “Use SOG as Boat Speed” to calculate true wind changed this. I switched all my dashboards and instrument displays to use “Speed through Water” which correctly displays the “Speed through Water”.

Enabling the “Use COG as Heading” and “Use SOG as Boat Speed” immediately stabilized my TWD & TWS values. Now I can tack and the wind direction remains reasonably consistent.

Update 1 – 2019: After trying this for a while I noticed that the True Wind Angle (TWA) became inaccurate. TWA needs to be relative to the boat speed and direction through water – not speed & heading over ground. In the end I decided TWA was more important – so disabled “Use COG as Heading” and “Use SOG as Boat Speed”.

I’m still blown away that none of this is documented by B&G and important settings like this a buried in a completely random and difficult to find location. For sailors & cruisers this seems to be quite important setting – and seems to be turned off by default. Please join me in lobbying B&G to put more effort into improving their documentation. You would think that they could at least add a searchable community knowledge base so material like this could be shared.

Update 2 – Aug 2023: I have done lots more research into this as it is a very confusing topic. What I have realized is there are actually 3 different types of wind angle you need to consider. These are:

• Apparent Wind:
• (Boat) True Wind:
• Ground Wind:

The difference between Boat True Wind and Ground Wind is driven by the strength and direction of the current you are sailing in. In the San Juan Islands where I was sailing in 2019 – this is a really big factor. Obviously – if you are sailing in an area with little or no current this discussion is irrelevant.

I put this diagram together to illustrate all these different ways of viewing the wind.

If you switch your B&G Instruments over to use both SOG and COG to calculate True wind – you are changing the system to really calculate the Ground Wind direction and it will give you an accurate TWD – but as I discovered the TWA will probably be incorrect due to the fact that your heading and COG are different – so your boat is twisted relative to COG and in fact TWA is a much more important value for sailing – particularly when racing.

Ockram instruments have some excellent articles that explain this is greater detail that are worth reading:

http://ockam.com/category/wind-angle-and-direction/

In summary using Speed Through Water and Heading gives you a better understanding of the wind relative to the boat & your sails – so pretty important if you are racing. If knowing reasonably accurately TWD – and how its changing then using SOG & COG will provide an accurate picture – but your TWA information – which is used in polars will not be accurate.