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

How to look ahead on the racecourse with your Vulcan or Zeus chartplotter. 

What If?

We are going to look at one of the most useful tools on the Zeus and Vulcan chart plotters – the What If? app. This 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.

What’s what on What If?

The top line below the Home and Menu buttons shows the course for which you are calculating the data (Boat to 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. 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), TWA (True Wind Angle) and then AWA (Apparent Wind Angle), Apparent Wind Speed (AWS) 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.

 There will be some variability between different chart plotters – for instance to calculate the AWA and AWS the app will need a polar table installed (see Race panel – Starting)

 Ok, so how do we make good use of all this data? 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. The What If? app can tell you how far you are from the layline at the extremes of the wind shift pattern. For instance, in the What If? screen shown to the left, the boat is on port tack on a big lift with a True Wind Direction (TWD) of 080 degrees. The What If? says that there is 5mins and 25secs to go on port before the starboard tack layline.

The next shift

All good – you might think – and relax... but how far away is the starboard tack layline if the 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 (by unticking the Live box) and the sailing data will recalculate based on that number. In this case, the boat would now be only 1min 16sec away from the starboard tack layline if the wind suddenly went 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 120degrees 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.

 A quick look at the What If? tells you that it’s a TWA of 130 – in the current TWD of 120degrees. If this is what you tell the tactician, they might well be tempted to go for a spinnaker.

What about a wind shift?

What about when or if the wind goes back to the left-hand shift of 080 degrees? If you enter 80degrees into the What If? then you get back the answer of a TWA of 91... and suddenly that spinnaker is looking pretty extreme – a reaching sail would be the more conservative call.

Tackling the Tide

The What If? App 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 avoid a powerful current out in the channel. The software is calculating a current of half a knot flowing towards 346 degrees, and it says that the final starboard tack layline to the mark is 10mins and 50secs 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 a two-knot 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 value of the tide in the channel (again, just untick the live dialog box and then enter the correct values of 345 and 2 knots) and in this example it tells you that it’s now 16mins and 50secs until you reach the starboard tack layline.

 These are just a couple of examples; the What If display can be useful all around the race track. To make best 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 a great tool to help you with this too, the Wind Plot and we will look at that in the next article.

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