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RacePanel series with Mark Chisnell: Part 4: What If?

 In this article we look at the tools provided on your B&G Vulcan or Zeus series chartplotter to help you make smart tactical decisions.

What If?

This is the final in our short series of four blogs on the B&G Vulcan and Zeus series chartplotter software, showing you just what it can do. In this article we are going to look at one of the most useful tools in the navigator’s bag of tricks – the What If? screen.

The What If? screen allows the navigator to instantly see the changes that a wind shift will make to the current leg, or to calculate the tactical data required for smart decisions about the next leg. The display looks like this:

What to do with the display

The top line below the Home and Menu buttons shows the course for which you are calculating the data (Boat to RPT 001); along with the range and bearing of the mark (‘from the boat’ for the current leg, or ‘from the previous mark’ for the next leg).

The next two lines show the True Wind Direction and Speed, and the Tidal Direction and Speed. When the green box to the right says that these are ‘Live’, it means that they are showing the most up-to-date calculated values on the system. However, they can be changed to any value for tide or wind, speed or direction. The table below shows the sailing data for the leg in whatever tide and wind that you have decided to use.

In the far left-hand column, we have the data labels for the two right-hand columns; Heading, COG (Course Over the Ground), Boat Speed (target/polar), SOG (Speed Over the Ground), True and then Apparent Wind Angle, Distance and Time left on the leg. The second and third columns are then the calculated values of this data for port and starboard tack respectively.

If you want to look at a different leg, then you can toggle between them using the Prev, Current Leg and Next buttons on the bottom of the display.

Please be aware that not all systems will have all the data; if you have a H5000 CPU then you will see all of this displayed, but without the Polar Table in the CPU some of the data (like Apparent Wind Angle) cannot be calculated.

Ok, so much for the display… how do we make good use of it? In the case of the current leg it’s particularly useful when the wind is shifty. When you are going upwind or downwind, tacking or gybing it’s essential to know how much time you have left on each tack. This is vital tactical information as the general rule in a shifting breeze is to stay away from the corners of the race course.

Without the What If? screen, you might get close to a corner and not even realise it. It can tell you how far you are from the layline at the extremes of the wind shift pattern. For instance, let’s say that you are on port tack on a big lift, in fact, the wind is as far left as you have seen it with a True Wind Direction (TWD) of 080 degrees, and the What If? tells you that you have 5 mins to go before you hit the starboard tack layline.

How far to the layline?

All good – you might think – and relax… but how far away is the starboard tack layline if you had a maximum right-hand shift (starboard tack lift) is a TWD of 120 degrees?

It’s easy to find out; just enter 120 into the True Wind Direction box and the sailing data will recalculate based on that number. And now you might find that you are only a minute away from the starboard tack layline, should the wind suddenly go back to the maximum right-hand shift – great information for a tactician trying to play the fleet up a busy beat.

Next Leg options

The What If? can be just as useful when you are looking at options for the next leg. Let’s say that you tacked to starboard on the first sniff of a header about a minute after the scenario above; the wind then went to 120 degrees and you are now approaching the windward mark bang on the layline. There’s no time to revel in the admiring glances of the crew; the next leg is a starboard tack reach and the tactician wants to know the True Wind Angle (TWA) that you will be sailing.

Time for a reaching sail?

A quick look at the What If? tells you that it’s a TWA of 130 – in the current TWD of 120 degrees. If this is what you tell the tactician, they might well be tempted to go for a spinnaker. But what about when it goes back to the left-hand shift of 080 degrees? If you enter 80 degrees into the What If?then you get back the answer of a TWA of 90… and suddenly that spinnaker is looking pretty extreme – a reaching sail would be the more conservative call.

More or less tide?

The What If? doesn’t just help you make allowance for the wind shifts either, you can also use it to calculate the effect of more or less tide on the leg. Let’s say that you are beating up a shore, sticking tightly to the shallow water to duck a powerful current out in the channel. The software is calculating a current of maybe half a knot, and it says that the final starboard tack layline to the mark is ten minutes away.


Ashtray on a motorbike

Unfortunately, the mark is in the middle of the channel (isn’t it always?) and you know that there is nearly two knots of current out there. So a layline calculated using half a knot of current is about as much use as an ashtray on a motorbike. The What If? will allow you to enter the full value of the tide in the channel and it might then tell you that you have ten minutes until you reach that starboard tack layline.


These are just a couple of examples; the What If? display can be useful all around the race track. But to make really good use of it, and make smart tactical choices you will need to have a very good idea of the direction that the wind will be blowing on the leg in question. The system has great tools to help you with this too like Wind Phase Indicator, Wind Lift and Wind Plot.

Wind Plot works by calculating a mean True Wind Direction over a minute period of time (up to 30 mins) and then the current wind direction is displayed on a graph as a delta – or difference from the mean. So you can see the wind shifting in phases. It’s updated once per second, and also output over the network for display elsewhere on the system.

Lift or Header?

This gives you a lot more information than just a single number wind direction, and it can be further displayed to help even more. The system will also display the Phase (e.g. “right, 10 degrees”) or whether you are on a lift or a header. This is essentially the same data as Phase, but takes into account the tack that you are on. (H5000 CPU needed for this functionality)

When you see the wind plotted out like this, it’s remarkable how obvious things become, patterns emerge in graphically displayed data that remain hidden in raw numbers. An oscillating pattern might appear in a sea breeze, or a permanent, geographic shift might give itself away by popping up every time that you are close to a particular part of the race course. The wind phase display will show up all of these things, and lots more – all of this information can then be fed into the What If? to see the impact it will have on the race course.

And that’s it for our short series – hope it’s helped you to see some of the potential of this great system.