BG - Tuning Guide - X
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1. Brookes amp Gatehouse Ltd Premier Way Abbey Park Romsey Hampshire N D E 5 G N England Tel 44 0 1794 518448 Fax 44 0 1794 518077 www BandG com X 35 Performance Guide March 2008 Prepared by Nat Ives INTRODUCTION B amp G has been involved with the X 35 Class from the very beginning when we consulted on the specification of the electronics Having now worked with the first boat in the UK during the first season we have now released our guide to the use of instruments onboard such that you can truly maximise your performance and have accurate data to help assess changes to sails trim and set up INSTRUMENT CALIBRATION Accurate calibration of your chosen instrument system is essential for ensuring that you are getting meaningful information from it that you can then use to measure the performance of the yacht The following sections describe how to do this on a typical B amp G system and where appropriate the headings detail the relevant B amp G menu structure Where possible typical values for use on the X 35 have be given DEPTH DEPTH DEPTH CALBRATE DATUM For obvious safety reasons depth should be calibrated before any other function A DATUM offset value is set such that the depth display refers either to the waterline or bottom of the keel Enter a positive number for waterline or a negative number for keel Typical value 2 1m for depth below keel HEADING Heading calibra2. you tack to tack then TRUE W A is reading too wide half the error must be subtracted from the correction table e If TWD is heading you tack to tack then TRUE W A is reading too narrow add half the error to the TRUE W A table 2 If according to the compass you are tacking through an angle different than the sum of the TRUE W A s on each tack Port TRUE W A Starboard TRUE W A then the following rule applies e If the tack angle lt the sum of the TRUE W A s the TRUE W A is reading too wide half the error must be subtracted from the correction table e If the tack angle gt the sum of the TRUE W A s the TRUE W A is reading too narrow add half the error to the TRUE W A table A Worked Example Your TRUE DIR is telling you that you are being lifted by 10 degrees from tack to tack or the sum of your TRUE W A is 10 degrees more than the angle you are tacking through and you know this is wrong This is in 5 kts of TWS When you go into the TRUE W A correction table for 5 Kt upwind you see that a value of 1 5 degrees is already entered To correct the error you are seeing a further 5 degrees needs to be subtracted so a total correction of 6 5 degrees is applied Calibrating and refining TRUE W A is an ongoing process Differences in TRUE W A can been seen from morning to afternoon races as sea breezes develop and changes in gradient conditions TRUE W A should be checked at the beginning of every race any necessary changes made a
3. he log 3 42 1 3 39 If you are using the latest B amp G Hercules system with a heel sensor you can also adjust the speed from tack to tack and apply more correction at higher speeds in the case of the paddle wheel See the Owner s Manual for more information on this Once boatspeed has been verified as accurate it should be monitored against other functions and only adjusted if the wrong speed cannot be attributed to different sail trim helmsman rig settings wind shear crew weight and hull fouling APPARENT WIND CALIBRATION If wind is not calibrated accurately then performance is next to impossible to assess Proper calibration of the wind function is as vital as selecting the correct head sail for the next upwind leg 1 APPARENT WIND ANGLE WIND gt APP W A or MEAS W A CALBRATE gt MHU ANGL Installation offsets and other asymmetric factors can affect the symmetry of measured wind angles An offset is all that is required to correct for tack to tack errors in APP W A The perfect scenario for calibrating APP W A is in non tidal flat water with stable conditions no building sea breeze and with a correctly calibrated boatspeed Using identical trim and crew weight distribution not necessarily a full racing crew complete half a dozen tacks with the helm concentrating purely on boatspeed A consistent speed not necessarily from the targets table on each tack should be achieved without changing trim and without u
4. ical corrections for TRUE W A and TRUE W S for the X 35 TRUE W S UPWIND TRUE W A CORRECTION 7 0 3 0 0 5 4 5 6 5 8 0 eer eraers eras CORRECTION DOWNWIND TRUE W A CORRECTION 02 0 TRUE W S CORRECTION ANGLE Hercules Only Polars Polars are used to profile a yacht s performance and B amp G s Deckman for Windows software can be used to store these and then compare actual performance against a set of target polars The image below shows a basic set of polars for the X35 stored in Deckman and their associated curves plotted alongside They are a good starting point but as you sail the boat you will most likely develop these into a set of your own specific to your sails and set up 6 0 5 40 13S 110 470 143 387 180 8 0 640 6 40 737 70 7 38 io 530 150 513 180 0 0 670 7 88 70 793 110 600 156 110 6 00 186 5 57 557 180 120 680 6 80 s17 7 8 40 110 660 159 641 bar 180 4 0 690 8 32 70 8 78 110 740 167 728 180 16 0 700 7 00 8 40 70 6 88 110 7 80 169 7 68 180 roe 710 850 70 9 21 110 840 170 820 180 250 720 3 seo 70 355 110 3910 172 860 180 Conclusion The information above should help you get up to speed as quickly as possible and then move forward with your own polars based upon an accurately calibrated instrument system Setting up your instrume
5. nd the conditions logged for future reference Typically when graphed a TRUE W A table for upwind might have a curve like the following C a D T L Le 2 o If it has only been possible to achieve reliable values for the TRUE W S 5 10 and 20 range and the rest of the table left blank then the table would look as follows Correction It is clear to see that the 15 25 and 30 kt values can be accurately guessed at as opposed to being left with zero values You should repeat the process for reaching and running as well as beating 2 TRUE WIND SPEED CALIBRATION WIND gt TRUE W S CALBRATE gt CORRECTION TRUE W S errors are seen from sailing upwind to downwind This is most noticeable on Mast Head boats however all yachts are affected to some degree This is due to the acceleration of the airflow over the top of the mast and around the sails when sailing downwind The introduction of the Vertical Mast Head unit has gone some way to solving this however calibration may still be necessary As a rule of thumb it is safe to put a correction of 10 into the table before first Sailing and this is the value entered by B amp G as a factory default Monitoring the change in TRUE W S from close hauled to flat running will enable further refinement of this calibration value On a B amp G Hercules system it is also possible to define the TRUE W A at which this correction is applied typically your downwind target angle Typ
6. nts can be very rewarding for the whole team Should you have any questions or further suggestions please contact Racing BandG com Author Nat Ives is the B amp G Race Specialist where he focuses on ensuring B amp G meet the requirements of the best sailors and fastest vessels in the world His role includes Supporting regattas and teams all around the world including all of the current AC and Volvo teams In addition to this Nat sailed as part of the crew on Cool Runnings the first X 35 to sail in the UK winning the Warsash Spring Series 2006 As a professional navigator Nat brings with him a wealth of big race experience He has competed as navigator in a wide range of yachts and events including the TP52 Breitling Med Cup Swan Cup and Maxi Worlds
7. sing any reference to APP W A The navigator or tactician should monitor the APP W A from tack to tack and average each tack Any difference between Port and Starboard tack can be corrected in the MHU offset If Starboard APP W A is greater than Port then subtract half the difference from the MHU offset and vice versa Once a good APP W A is achieved do not change it unless necessary e g rig has been removed from boat new wind sensor put in place Any differences seen from now on may be attributable to effects such as wind shear and current 2 APPARENT WIND SPEED Do not alter the APP W S calibration values This is a wind tunnel based factory setting TRUE WIND CALIBRATION TIPS 1 TRUE WIND ANGLE CALIBRATION WIND TRUE W A CALBRATE gt CORRECTION A TRUE W A calibration matrix is utilised for corrections upwind reaching and downwind across the TRUE W S ranges The latest release of the B amp G Hercules system serial numbers greater than H374001 allows you to load an initial set of corrections for a fractionally rigged monohull that will be a good starting point to work from To do this change CAL VAL 1 on TRUE WIND DIRECTION from O0 to 1 Two methods of calibrating TRUE W A are available either monitoring TRUE DIR from tack to tack or gybe to gybe or using the compass to verify the angles the yacht is tacking or gybing through 1 If an error is seen in TRUE DIR then the following rule applies e If TWD is lifting
8. tion is carried out using B amp G s Autoswing functionality that calibrates the compass An offset is then entered to account for the alignment of the compass as installed on the yacht 1 Autoswing NAVIGATE gt COMP CAL CALBRATE START Prior to commencing the swing ensure that no magnetic or large metallic objects are placed near to the compass Choose a calm open stretch of water and then begin the calibration process as outlined in the User Manual ensuring you steer a steady circle not exceeding 2 3 degrees per second with a boat speed less than 5 knots 2 Offset NAVIGATE HEADING CALBRATE CAL VAL 1 Having completed the swing compare the compass heading with the bearing of a known transit and correct it by entering an offset value BOATSPEED All the work in the world on target speeds and angles or on polar tables goes to waste if boatspeed is inaccurate even by a few True wind calculations and performance functions depend on this input being accurate If boat speed is wrong the whole system s data quality is compromised e Itis important to keep an eye on boatspeed accuracy during the course of a season as paddlewheels do suffer from friction and hence changes in physical characteristics can occur over time e Regular cleaning of the paddlewheel is essential for accurate and repeatable speed readings 1 Auto Calibration SPEED gt BOAT SPD CALBRATE AUTO CAL The B amp G three run method eliminates an
9. y changes in current during the calibration procedure since the runs are carried out in alternate directions When carrying out each run maintain a constant compass course to ensure that the distance covered is the same as that measured on the chart If possible choose a measured distance with clearly visible transits marking each end Typical value 3 48 Paddlewheel It is prudent to manually calibrate boatspeed using the trip log function on another FFD whilst running the auto cal feature By recording the log at the beginning of each run and recording the log distance run for each leg we can deduce the error in the current speed calibration This can be useful to complete a calibration procedure if any of the runs have to be aborted due to a necessary change of course Example Current cal in Hz per Kt 3 42 Measured run distance 0 75nm Run 1 0 76nm Run 2 0 72nm Run 3 0 77nm It appears that the current is changing due to the difference in the first and third runs Log distance Correct this by averaging run 1 and 3 and then combine with run two to get an overall average log distance Runi Run3 2 0 765 Run2 0 765 2 0 7425 Comparing the log distance to the actual distance reveals that the log is under reading by 1 0 75 0 7425 1 Increasing cal Hz Kt in MANL CAL reduces boatspeed and vice versa Therefore by reducing the cal Hz Kt for the above calibration by 1 will address the under reading of t
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