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Calibrating your Instruments for the Round the Island Race

Getting your instruments correctly calibrated for any race is an important step – without correct calibration you can’t rely on the data to tell you what you need to know.

Here’s a quick guide to ensure you get the best out of a basic instrument system. More sophisticated instrument systems and processors have advanced calibration routines and polar performance inputs to ensure optimum accuracy: check out ‘Calibrating an H5000 system’ for an insight into high-end instruments and the accuracy that they can offer.

1: Depth calibration


This is one that it is crucial to get right to avoid an unexpected grounding as you try and tack in out of the tide. Make sure the depth offset is set so that the depth reading is measuring what you want it to – whether this is depth under the keel or from the waterline, ensure the offset is converting the depth below the transducer to a more useful figure. On a B&G system, you can find this under Settings > Network > Calibration > Depth.

2: Boat Speed calibration


IF your boat speed is over- or under-reading, your wind and tide calculations will also be off, so it’s worth getting this one right if you can.


First, make sure your paddlewheel transducer is clean and not clogged with weed.


Next head out on a calm day and compare your speed through the water with your Speed Over Ground. Basic systems let you adjust the % calibration factor – while more sophisticated systems use measured distances to get the calibration correct. Try to do this at slack water to remove the effect of tidal flow. If there is tide running, do a run up and down tide and compare the SOG and Boat Speed figures, increasing or decreasing the calibration % as you go until you have a speed through the water that seems about right when compared to the tidal flow in each direction. When it is calibrated correctly, your tide data (visible on the sailsteer page of your chartplotter) will match that in the real world.

3: Compass calibration


B&G compasses are calibrated with a slow turn through about 390 degrees. When the compass is calibrated, you can check the heading against a known bearing and adjust the offset until it matches up.

4: Wind angle calibration


You need to set your Masthead Unit (MHU) offset to iron out any installation issues and ensure that your wind angles appear the same on each tack. You can adjust this by sailing close hauled to the telltales on each tack and monitoring your Apparent Wind Angle (AWA), and adjusting the MHU offset value until the numbers read the same on each tack.

5: Target tables


B&G chartplotters display laylines on the chart to help with upwind sailing. To ensure these are accurate, it’s worth filling in your Target Tables. These provide realistic targets for your particular boat, and let you set simple upwind, downwind speeds wind angles to make sure the laylines and sailing timers are as accurate as possible.