Race boat instruments â understanding it all - L-36 Fleet

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Correct for mast rotation and sensoroffsetMeasured Wind Angle / Speed Correct for boat speed and headingCalculated True Wind Angle / Speed Correct for wind sheer and twistTrue Wind Angle / SpeedTrue Wind Direction Factor in boat speed to back-calculateApparent Wind Speed / AngleGrand Prix instrument systemMast head unit samples data at upto 100Hz Correct for mast rotation and sensor offsetMeasured Wind Angle / Speed Gyro correction for mast movementCorrected Measured Wind Angle / Speed Correct for Heel, Leeway, Boat speed,HeadingGround or Original True Wind Angle/ Speed / Direction Correct for Wind Sheer / Gradient and TwistTrue Wind Angle / Speed / Direction Factor in boat speed to back-calculateApparent Wind Speed / AngleAs you can tell, the big difference between the various options is the speed at which the data is sampled andthe number of corrections (shown in italics) introduced into the calculation, hence the need for a powerfulcentral processing unit.Staying with the Grand Prix solution, we will now step through the process:The reason for accurate wind data is simple.Although we sail and trim our boat to the apparent wind speed and angle, as this is what the boat is actuallyseeing, the boat’s performance is measured at a given true wind speed and angle. This is because the truewind data stays constant irrespective of where the boat is pointing (Not effected by apparent wind).Effectively it’s a constant reference point.Mast Head SensorAs stated earlier, this data is sampled at 100Hz but typically is averaged to 10Hz. To achieve good winddata, the wind sensor needs to be secure and aligned with the centreline of the boat. Vertically mountedwind sensors are used to get the wind sensor up and away from the effects of the sail plan on the wind. Toget your wind sensor completely clear of these effects is not practical due to the length of the vertical wandrequired, however as a rule of thumb, 1 inch of wand for each foot of boat length.So a 40’ boat would have a 40” vertical wand.Correct for misalignment due to poor installation or mast rotation (if wind sensor is fitted to arotating mast with the required sensor connected.)The motion of the mast can be substantial 100’ above the hull, so gyro sensors are used to compensate forthis motion.Corrections are then made for boat speed, current, leeway and heel.Boat speed is simple enough as is current thanks to GPS.
Heel is another important part of the equation.Measured Wind Angles can change as much as 4’ when heel changes from 10’ to 24’. This anglewill continue to narrow as heel angle increases until the Measured Wind Angle equals 0 at 90’heel.Corrected MeasWA = atan (sin (MeasWA) * cos (heel)) / cos (MeasWA)Now we correct for Wind Sheer and Wind Gradient. These two effects are often just called “windsheer” although they are quiet different but generally appear together.Leeway is the thorn in your side, as there is no really accurate way to measure leeway. Multiple GPSsensors have been used with varying degrees of success. The best source of expected leeway is often theboat’s designer.The accepted formula for leeway is:Leeway = (K * Heel) / VS²orK = (Leeway * VS²) / HeelSo with an expected leeway angle from your designer, K can simply be calculated.Typical K values are around 1 for an AC boat and 10-15 for a typical modern race boat.Wind Sheer is the change in wind direction with altitude. This is easily detected by your instrumentsreading different wind angles from tack to tack.Wind Gradient is the change in wind speed with altitude, primarily due to the effect of friction with thewater. The wind speed will increase with altitude and is most noticeable in lighter winds.Finally we now have True Wind Speed / Angle and True Wind Direction. This information is what is seenon the instrument displays. Apparent Wind Speed / Angle are then back-calculated and available on the instrument displays.AWS = √ (b² + c²)AWA =a tan (b/c)Wherea = TWS * (cos TWA)b = TWS * (sin TWA)c =ab + VSBoat Speed Sensor: Paddle wheel or Sonic Sensor?Sonic Sensors are very popular on superyachts due to very low maintenance.Paddles wheels are considerably lighter and some believe more accurate and hence are normally found onrace boats.Like the wind sensor, proper installation is paramount, and boat speed sensors should be installed on thecentre line of the hull, in front of the keel about 2/3 of the way back from the bow’s waterline.In this position they should not come out of the water at high speed or in big seas and will not be affectedby water flow around the keel.If your boat speed sensor can’t be installed on the centre line due to hull shape, then consider 2 sensors,positioned so they are vertical when the boat is at normal heel (Which is about 15-20’).Multi-hull’s often use a sensor positioned in each hull. Typically the wind angle is used to calculate whichsensor’s data to use.
Page 1: Race boat instrumentsPart 1 - Under
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Page 7 and 8: sails for speed and as you approach

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Race boat instruments â understanding it all - L-36 Fleet