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LogScanner Tacks

What LogScanner tells you about your tacks

There are several things LogScanner can tell you about when you look at tacks.

  • How much distance you lost to weather – the cost of tacking
  • How much wind shear you are experiencing
  • How good your wind solution is (your upwind calibration)
  • Several measurements of how you tacked

TackProfileWhat does a tack look like?

The performance cost comes from the reduction in your Vmg because the boat slows down. Learn more…

  1. When you first turn up, boatspeed hasn’t slowed down yet, so the Vmg increases.
  2. As you turn away the boat slows down because the sails aren’t driving the boat yet.
  3. When you get to the new bearing, sails begin to restore boatspeed.
  4. When you achieve your pre-tack boatspeed and angle, your Vmg returns to its pre-tack value (ideally).

LogScannerIdealTackWhen looking at a stripchart of a tack, you don’t use a Vmg plot because averaging hides all the detail, and the two components of Vmg (boatspeed and true wind angle), are what the crew controls, and therefore the things you need to look at.

  • True wind angle is heading – primarily the helmsman’s job.
  • Boatspeed is the result of sail trim (mostly)

Of course, these two items interact considerably. Getting them right takes practice. There is also wind’s 3rd dimension – shear and gradient. Learn more…

In pure terms;

  • Shear makes the wind angle larger on one tack and smaller on the other. Since this changes the sail drive, boatspeed will be faster than target on the tack with the larger angle, and slower than target on the other.
  • Gradient raises or lowers the boatspeed equally on both tacks.

Bad sail trim can also change performance in ways that look like shear and gradient. But bad trim won’t be exactly the same on every tack, so by looking at several tacks, you will probably be able to suss out whether God is messing with you or your crew is screwing up.

LogScannerTack1A real tack

Here’s a real tack plot. It is the first tack in our Block Island log (see LogScanner Race Analysis). Ok, real world has noise. But this tack does look pretty much like the idealized tack.

LogScanner has done the analysis and has listed out the various parameters:

11:32 1.2min DLvmg:-56ft Vt:8.4kt
TkAng:84.9° Shoot:0ft @0.0s
HTW:5s VsLow:4.65kt 9.8s Recov:70s
BtLow:-39° 45.0s Trans:15.5s Wig:-8.8° Shear:7.0°

  1. Time; 11:32am. Length of tack; 1.2 minutes. Distance lost; 56 ft. True wind speed; 8.4 knots
  2. Tacking angle; 85°. Point of maximum Vmg shoot; none.
  3. Head to wind; 5 seconds. Lowest boatspeed; 4.65 knots 9.8 seconds after head to wind. Boatspeed recovery; 70 seconds.
  4. Widest angle (on new tack) 39° at 45 seconds. Transiton (time from pre to post tack wind angle) 15.5 seconds. Wiggle (change in wind direction, i.e. miscalibration) 9°. Shear (difference in true wind angle) 7°.

There are a few things that should be pointed out about this tack;

  • Boatspeed was less than target pre-tack, even though the true wind angle was greater than target. Post tack looks OK. Could be sail trim issue.
  • Boat began to slow before the tack started. This is not good.
  • The boat didn’t go slightly wide of target angle on the new tack (in order to accelerate more quickly). In fact, it stalled out for a bit before finally getting to target angle. This reduces the acceleration and therefore extends the time before the boat gets back to speed.

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