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Sailing Instrument Functions

Sailing Instrument Functions are their reason for being

Sailing Instrument functionsSailing Instrument functions are the system outputs like boatspeed and wind direction. These are the things you buy an instrument system for. The hardware is only the means for obtaining them. Of course, the better the hardware, the better the functions.

Some sailing instrument functions are the same for all watercraft such as position, speed and heading. Others are unique to sailing, and mostly have to do with the wind. This is because sailboats are driven by the wind, and perform differently depending on wind speed and the boat’s angle to it. For Sailboat racing, knowing what the wind is doing and how your boat should perform improves performance.

True Wind is the sailboat’s fuel supply. Of all the functions that a sailing instrument system produces, true wind is the most important for performance, tactics and strategy during racing (see Sailing instruments win races).

Wind shifts can produce orders of magnitude more gain than tweaking your speed (see Lateral Separation). But you need to measure wind direction to know all about them.

Targets. Polar curves predict how fast the boat will go in a specified set of true wind conditions. Given a true wind angle and speed, they predict the boatspeed to be expected. (Learn more about Polars)

Time to laylines. Overstanding is bad,but not as bad as having to short tack amongst your enemies. Laylines are the final courses to the mark at the current wind speed and direction. They are calculated from the position of the mark, the true wind speed and direction, and your target angle and speed.

Shift & Puff. Shift is the amount the wind direction differs from average; i.e. +10 means the wind is 10° right. Puff is the amount the true wind speed differs from average; i.e. -2.6 means the wind is in a lull. The amount of average applied to the wind for these sailboat instrument functions determines how long it takes for a ‘shift’ to become the new ‘average’. The default is 5 minutes, but they can be adjusted with an Averages command.

The Wally. Taking advantage of small or short changes in wind direction can produce significant improvement in speed to weather. Read more about it on the Ockam U page and the Polar white paper. Wally is output by BIF #4a of OckamSoft.

Opposite tack. Knowing the heading on the opposite tack helps with decisions about when to tack; for the mark or whether you can clear an enemy vessel on starboard. It also gives a heading target on the new tack or jibe. Opposite track includes the effect of current. See Approaching the Mark for an example of its use.

Performance functions

Boatspeed is the most fundamental input to the Ockam instrument system, and is the first thing the crew looks at when trying to make the boat go faster. It is also used by many many other functions.

Current set & drift. The Ockam system computes current by comparing COG and SOG with Boatspeed, Heading, and Leeway. Current is used in laylines and opposite tack.

Vmg is the component of boatspeed in the direction of the true wind. When beating (or running) this is the number to maximize (or minimize) for best performance. Because of the dynamics of ship’s handling, Vmg is not a usable helming function (see Targets for an explanation). A more practical way to sail optimum Vmg is to use target speed.

Vmc is like Vmg except in the direction of the current rhumbline. Sailing maximum Vmc gets you toward the mark faster than sailing directly there, but at the cost of going off rhumbline. On a long course, the wind will shift and allow you to tack and consolidate back to the rhumbline. On short courses, this may not be possible. However, you can still sail Vmc on an upwind leg by doing the wally.

Trim & steer

True wind angle is useful for helming, orienting the boat relative to the true wind. It is especially valuable downwind where the human sensor system has less input.

Apparent wind angle and speed is sometimes used for sail trim because the sails feel apparent wind. It is the equivalent of the windex.

Trim and sea state are derived from pitch information provided by a 3D compass or AHRS. Trim is the average fore-and-aft angle of the hull, used for crew positioning. Sea state is the average amount the angle changes per wave.

Stopwatch controls from Matryx and 005 displays and Lynx controller allow start/stop and reset/sync.

Heel is an indication of wind pressure on the sails and thereby the force driving the boat. Most crews believe their inner ears rather than reading the heading display, but heel is used by the Ockam system for true wind and to correct apparent wind, calculate leeway and switch and calibrate boatspeed.

Loadcells monitor proper rig tension.

Displacement keeps tab on movable components e.g. rudders, tabs, centerboards, mast ram or cunningham.

Rudder & tab help in helming and for analysis of log files.

Mast angle Catamarans and other high performance craft sometimes have rotating spars or wing sails. This causes a problem getting wind data, because the masthead sensor rotates with the mast. The Mast Rotation Interface takes care of this problem by measuring the mast angle and adding it to the measured masthead angle to get apparent wind angle.

Navigation functions

Heading tells you where the boat is pointed but not necessarily where it’s going. Vital to navigation and helmsmnship, it is used by the system for dead reckoning, true wind and current.

  • Heading can be switched to Course Over Ground when needed (see Option 16).
  • There are 3 software settings (Options 17-19) to correct for lubber offset and semicircular errors (here’s a tutorial on compass adjustment Bowditch APN chapter 6).
  • The compass can also be the source for heel, pitch and turn rate.

Lat/Lon is the position of the boat. Sounds like a function only a navigator could love, but in racing, marks are usually maintained by on-board software because they tend to be ‘floaters’, defined by where the true wind is blowing from. This software needs to have the current position in order to calculate their own waypoint range and bearing.

Waypoint range & bearing is the GPS output describing the next waypoint in its waypoint list. However, on-board software sometimes has a different value for this function. Ockam provides a way for on-board software to override the GPS waypoint. When the software stops updating its waypoint, the system returns to the GPS output.

Back range and bearing refers to the last point the Reset Back R/B command was issued. The system maintains its own data on this point, independent of other hardware. This function is useful in prestart preparation.

COG/SOG is course and speed over ground.  By itself, this doesn’t seem to be useful, but, it is vital for computing current.

Depth surface & keel. The Ockam system produces both surface and keel depth. When depth is input to the T1 processor, commands K15 and K16 set these parameters.

Logs. There are two logs, one of which is resetable.

Cross track error is a report on how far off the rhumbline you are. It is a repeat of the GPS cross-track error (if provided)

Sea and air temperature (tags ‘g’ and ‘g”) display one or two general purpose temperature values. The “Sea temperature” has a resolution of 0.1°C or about 0.2°F. Usually used to detect the gulf stream, and possible wind gradient conditions.

Barometer & trend (tags ‘G’ and ‘G”) display barometric pressure and pressure trend (the change in barometric pressure over the last hour).

Status functions

Test configuration (tag ‘Z’) shows which interfaces are currently communicating with the processor, and therefore, which functions can be calculated and displayed (see Configuration).

Test errors (tag ‘z’) As the processor does its calculations, it occasionally finds out things that are wrong, e.g. if an interface suddenly stops communicating. There are currently about 150 error codes defined (see Error Codes).

Status1 (tag ‘ ‘) Many useful but slow-changing parameters are output on the Status 1 function. They include all cals, averages, options, date, disabled tags and many more.(see Status1)

Advisory (tag ‘S’) is a textural error and advisory function. Includes text versions of the Test Errors function plus interface status messages, e.g. compass in cal mode

Calibration functions

True wind correction. This is a calibration that adds or subtracts from the magnitude of the true wind angle. If the true wind angle is too narrow (too small in magnitude), wind direction will head when you tack. If it’s too wide (too large in magnitude), you will be lifted when you tack. QuikCal adds or subtracts an adjustable number of degrees from the magnitude of the true wind angle; for a positive 2.0 QuikCal, 21 degrees becomes 23, and –21 degrees becomes -23.

Cal Boatspeed Master & Offset. The overall calibration of Boatspeed is set by the interface signature switch and the CAL Boatspeed Master or the K1 command or AutoCal. If a heel is available, Boatspeed is modulated by heel and CAL Boatspeed Offset.

Cal leeway. This calibration is the factor used in calculation of leeway angle (See Leeway function). The leeway calibration is set to zero when shipped from the factory, so the leeway will always be zero until you are ready for it. Since leeway and upwash are compensating errors in true wind angle, you should go about adjusting both of these functions at the same time.

Cal windangle offset zeros the apparent wind angle. A value of 1.0 makes windangle apparent 1 degree narrower on starboard and 1 degree wider on port. A value of -1.0 does the opposite

Cal windspeed changes the apparent wind speed. K1=1.00 is 100% of the nominal calibration of the signature switch calibration. K5=0.99 lowers windspeed apparent 1%, while K5=1.01 raises it 1%

Cal Upwash & Upwash Slope The wind is bent by the sails as it approaches them. In fact, the drive created by the sails is caused by this bending. However, if the masthead is inside this disturbed flow, it is not reading the proper angle and speed. This effect is termed upwash and is a complicated concept to understand.
Upwash broadens (>0) or narrows(<0) the apparent wind angle upwind. Since apparent wind angle has a 3:1 effect on true wind angle (and therefore wind direction) upwind, Upwash is the calibration to use to set upwind wind direction.
Upwash slope is a calibration number that changes Upwash with True Windspeed. The complete Upwash function is CalUpwash+CalUpwashSlope*(Va-12).

Leeway (tag ‘l’) is the angle between the ship’s heading  and the boatspeed resulting from side pressure by the sails, resisted by the keel. It is used in true wind and dead-reckoning calculations, and is displayed as an aid to proper calibration. Leeway is calculated from heel angle, boatspeed and Leeway Cal.


True wind statistics have been added to the T1 processor beginning revision 20.05. These sailing instrument functions are designed to help in decisions involving laylines, Vmc sailing and sail selection. There are six parameters displayed in round-robin order.

Wind direction (e.g. Mt:224)  The average wind direction over the last hour

Trend (e.g. Tr:+5.1) The persistent shift of wind direction over the last hour (in degrees/hr). The example reads “wind direction has trended clockwise (+) 5.1º over the last hour”

Shift (e.g. Sh:5.5) The RMS wind shift (in degrees). The example reads “5º each way from mean”

Velocity true (e.g. Vt:12.6) The average true wind speed over the last hour.

Puff (e.g. Pf:4.5) The RMS variability in true wind speed. The example Vt and Pf together read “true wind has been 12.6 knots ±4.5 knots in the last hour”

Peak wind (e.g. Pk:17/15) The highest windspeed (over a 5 second average) and how long ago it occured (in minutes) during the last hour. The example reads “highest wind was 17 knots, 15 minutes ago”

 These Shift and Puff items are statistical; there are two other functions that give the current differences in wind direction and speed from the mean (see Shift and Puff).

Error codes associated with the Statistics output are:
181 Not enough data to calculate statistics. After 10 minutes of steady data, this error should go away.
182 No wind direction available or unstable conditions. Check your system for boatspeed, apparent wind and heading.
183 No tacks. Tacking is required in order to detect wind shear.

Ockam offers more Sailing Instrument Functions than any other Sailing Instrument Manufacturer.


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