B&G X-35 Performance Manual

Brookes & Gatehouse Ltd.
Premier Way, Abbey Park
Romsey, Hampshire,
England
Tel: [phone]
Fax: [phone]
www.BandG.com

X-35 Performance Guide

March 2008

Prepared by:

Nat Ives

INTRODUCTION

B&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&G system and where appropriate the headings detail the relevant B&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 calibration is carried out using B&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 → 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.

• It is 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.

• Regular cleaning of the paddlewheel is essential for accurate and repeatable
speed readings.

1. Auto- Calibration (SPEED → BOAT SPD, CALBRATE → AUTO CAL)

The B&G three run method eliminates any 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:

(Run1 + 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 the log.

3.42 - 1% = 3.39

If you are using the latest B&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→ APP W/A or MEAS W/A, CALBRATE →
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