Learn from sailing data: good and bad tacks
Charles Swanson contributes to ChartedSails and is a winning competitive sailor. Charles is also the Sales & Support Manager at Velocitek. Prior to Velocitek, Charles worked on America's Cup winning electronics at Ockam Instruments.
Losing a quarter of a boatlength less than your opponent in each tack means one length gained in a four-tack beat.
Ed Baird - Staying on Target
Looking at the shape of the turn and the speed through a tack can tell us a lot about the maneuver. This is very easy to do using ChartedSails and the data from your Velocitek ProStart or other GPS logger. ChartedSails makes the quantitative and qualitative analysis of our maneuvers very simple.
You can look at each tack in three parts: entry, turn and exit:
For the entry you want to see that you are on a close hauled angle and the boat was at target speed going into the turn.
During the turn you want to see that the momentum is carried into the wind and the rate of turn is consistent. The rate of turn will vary with the conditions (wind and sea state). But generally speaking, you want a consistent rate of turn in each maneuver.
For the exit you want to see that the turn stops at a good acceleration or speed build angle, the boat is able to accelerate straight away and you are close to a close hauled angle.
Let’s take a look at a couple tacks recorded on a Melges IC37!
First, a good tack
In this tack we have a nice smooth turn and the speed bottoms and increases on approximately the same angle out of the tack. This means the momentum is carried through the turn. We can see the low speed point and the speed carried through the turn using ChartedSails’ speed-colored tail.
Entry
Using ChartedSails, we average 10s of data before the tack (notice the blue background on the boat card when averaging) to see see our boatspeed and VMG going into the maneuver.
Looking at the boat polars for our Melges IC37, we can see that we are a little low and fast wth 7.2 knots of boat speed but we are not far off (we had about 10 knots of wind that day). We’re moving up the racecourse well with 5.3 knots of VMG.
Turn
Looking at the track we can see a nice smooth carve up into the wind as evidenced by the maintained momentum. The boat card shows that we’re still traveling 6.8 knots at 100% VMG! We only drop 0.4 knots of boatspeed and our VMG has jumped 1.5 knots!
Exit
Looking at the track we see a nice steady course out of the tack. There is no searching for the exit angle. Using the heat mapped track and the speed stripchart you see the speed bottoms and turns as the boat accelerates.
Tack 2 - A wild one!
Looking at the track, we can see right away that this tack does not have the same nice smooth shape of tack 1. The shape is more complex with a tightening radius as the turn progresses.
Entry
Looking at the track and the boat card, the entry is pretty standard. The course is relatively straight and the boatspeed is good.
Turn
The turn into the wind is good and the momentum is carried up the course. However the speed is not carried as well into the wind. Using the boat card, we can see that we lose 2.4 knots of boatspeed.
Exit
Here’s where things get exciting! Looking at the track you can see the radius of the turn tightening in the later stages of the tack. The boat continues turning, well past a good exit angle. We measure with ChartedSails a TWA as high as 68° for a moment. Using the boat card we can easily compare the exit angle with the entry angle. The entry is 43° and the exit peaks at 68° or ~25° of over tack.
Why is this bad? Won’t the boat accelerate back quickly at 68° TWA?
Yes, the boat will accelerate quickly at 68° TWA, but it is accelerating across the course and we lose gauge up the course (see image below). Additionally this means the sails need to get eased to a reaching angle and then retrimmed to an upwind angle as the boat course corrects.
Quantitatively, using ChartedSails boat card, we can see that the VMG at this exit angle is 1.8 knots with a boatspeed of 5.0 knots. Compare that with just a few seconds before a VMG of 3.8 knots with 5.3 knots of boatspeed.
Conclusion
Using ChartedSails makes it easy for sailors and coaches to debrief a day of sailing. The more you can learn from a day on the water, the better you can be the next day.
Adding training partners’ and friends’ tracks to ChartedSails gives you even more to talk about and even more learning opportunities as a group.
