So how do you know what fast actually looks like ?
We all know the feeling, we hit the top of a wave the boat surfs, accelerates then goes quiet, everyone watches the numbers on the mast instruments and at that point you set the highest speed score for the day. You put a score on the board for everyone else who helms the boat that day to beat….. and when they don’t you have bragging rights for a little while.
The question though is are you actually going as fast as you should be and if you are do you actually know ?
Most sailing boats these days have extensive ranges of instruments that can tell us everything from how fast the wind is blowing to angle the boat is sailing at in 3 dimensions. You have a whole range of sensors that if correctly calibrated will tell you a lot about what’s going on with your boat. But its actually really difficult to understand the big picture and work out how the numbers interact.
When we created Rockit (version 0.1 is quite a long time ago now) we set out to start to understand those interactions. By recording the information we were able to record against time and place of the boat.
So what do we use to benchmark our performance ?
We picked on through the water speed (STW) as the benchmark number.
So for every minute we knew where the boat was and the average STW. We picked average STW as this seemed the best guide to how effectively the boat was harnessing the energy of the wind and turning it into forward motion. Speed over ground (SOG) didn’t (and still doesn’t) really tell us anything useful in this context. There are too many other factors at play; leeway, tide etc for it to be a good benchmark.
So we track STW as a measure of how much wind energy is translated into forward motion. Once we have a measure of forward motion we can start to relate that to other variables.
True Wind Speed (TWS) is an artificial construct as is the true wind direction. We don’t sail in true wind we sail in apparent wind; the more wind the faster we go so the more wind we feel. Think about riding in an open top car at 50 miles an hour on a windless day your true wind might be one or two miles an hour but the apparent wind is going to be 50 miles per our or more; that’s what messes your hair up.
But as a way of measuring the input energy True Wind Speed and True Wind Angle are more useful because they are effectively standardised numbers; and we can calculate it using good old high school trigonometry. We can take SOG, Apparent Wind Speed (AWS) and Apparent Wind Angle (AWA); do some maths and come up with a value for TWS and the True Wind Angle (TWA). Notice that’s SOG not STW. SOG is a measure of how fast we are moving relative to the earth and the wind. It removes factors such as tide which we expend wind energy fighting against to move towards our goal.
Think of it this way.
If our open top car is on a giant conveyor belt going in the opposite direction at 10miles per hour (some day I will think of a better example …) then if the car moves at 50miles per hour along the conveyor (the number you get on the speedo) you are only going to be moving through the air at 40 miles per hour so the apparent wind will be less and you hair will be slightly less mess up; but the True Wind won’t have changed from the first car related scenario above.
Whats the point of polar data ?
At this point we can build a table that looks something like;
This is whats called a polar table. The values in the table are the Polars for the boat. BTW is the bearing to the True Wind of the True Wind Angle (TWA). The figures along the top are the TWS in knots. The numbers in the individual cells are boat speeds. So we can determine that the boat should do 6.16knots STW in 10knots of wind when sailing at a TWA of 60 degrees.
So where do these numbers come from ?
These numbers come from a Velocity Prediction Programme. A piece of software that when fed a lot of numbers about the size and shape of a boat, hull, sails, weight, rig etc will do a lot of complicated maths and produce a table like the one above. In the case of the numbers above they are for our test boat and were generated using a piece of software called Sailfish Yacht Analyzer.
Coming back to our real world sailing boat. Everything is apparent (well it is on my boat other skippers may have other opinions) so we have to do a nifty translation to get some numbers that are a useful guide to us when sailing so we end up with a table which looks like this full of important targets for our crew to aim for…
So you have a table of boat speeds to aim for in a given set of sailing conditions. This table again is from the Sailfish VPP.
Whats this got to do with bragging rights ?
Ok so you might ask what does this have to do with bragging rights.
Valid question.
With Rockit we have replicated the Polar table above but based on the actual performance of the boat. We take the data from the instrument sensors and produce a table that records the boat speed against the True Wind Angle and True Wind Speed. So we can see for a given sailing day how the boat is performing. We also allow you to record other custom pieces of information against the time series data. So you can record who is helming at a given point in time and validate their claim to the fastest boat speed.
Better than that though we can aggregate together all the sailing sessions that relate to a boat to produce an overall real world profile. Whilst we know from the VPP how fast the boat should be going in a given set of conditions we can tell from Rockit how fast the boat actually is.
We can also tell the magnitude of achievement of the person doing the bragging. They might be the fastest today but compared to the rest of the season they might be distinctly below average.
So the Rockit Session Polar Display shows both the best value for a session in terms of performance and in addition the best ever performance (in brackets). So to get true bragging rights you need to get into the pink. We colour code based on percentages of the boat best performance pink is hot; blue decidedly not.
How can we apply this further ?
With recorded information we can look at how you perform on the race course. For each sample period we can assess the relative performance of the boat. The result is a multi coloured snail trail that shows you which bits you need to work on. Dodgy mark rounding’s and spinnaker drops stand out like a saw thumb. We overlay the snail trail on an outline map to see where you are and make it possible to include Waypoints that you set for your course.
As with the polar table pink is good blue is a sign that someone should try harder.
Can we see the overall performance of the boat ?
Bringing it full circle we can see how the boats performance overall relates to the original VPP prediction. the screen shot below shows the Polar table for
The information is made up of 3 components. From bottom top;
The VPP data. We use this as the initial seed data.
The Session Polar Data.
A graphic representation of the polar data.
Rockit actually combines the VPP and session derived data to arrive at the second table. This example sadly doesn’t include the VPP data I need to get around to importing the values from Sailfish. So if the crew is faster than the VPP that will be the number that is used in the Boat Polar. If they never reach the VPP target the VPP will be the max speed. To get a Pink segment you have to be at least as fast as the VPP and all the other session data for a boat for a given True Wind Angle/Speed combination.
The Rockit Polar Data is more granular than that produced by the VPP. Our aim was to capture data that could be loaded into a program such as Adrena. It could then be used as part of a routing calculation; a piece of software that uses boat performance, current and weather data to calculate the most efficient route between two locations. To get an accurate route you need to understand the performance of the boat sailing at different angles to the wind.
So to answer the original question. Fast is pink.
Notes;
Before anyone else points it out Apparent Wind Speed and Angle is not consistent at height. So what you see at deck level will be different to the top of the mast. Not necessarily by much but its important to remember and its one of the reasons why we put twist in sails.
Wind Angle and Wind Direction are often ill defined. For the purposes of this article and Rockit generally an Angle is relative to the Bow of the boat 90 degrees is on the beam. A direction is relative to the real world so 90 degrees would be a wind from the East.
Adrena is here : https://www.adrena-software.com/en/
We very much appreciate the help that we got to produce the VVP data for our test boat from Kjell at Techsail. Sailfish Analyzer is here : https://techsail.com/sailfish-yacht-analyzer/
Consistently calibrating your instruments is vital. If the sensors don’t provide consistent values they will lead to misleading aggregated data results over time.
and finally;
Fast doesn’t have to be Pink; You can reconfigure the colours in Rockit to have fastest any colour you like. We took quite a long time to arrive a pick so that it worked with the other colours; changing one colour tends to lead to a need to change another… its a rabbit hole we wouldn’t recommend you go down unless you have a lot of time on your hands…..
Bill/February 2022