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SonarChart Live: Waterline and Keel Offset Settings

Waterline Offset and Keel Offset relationship with SonarChart Live
 
First, let’s define what each of these things are.  Both settings orient around the transducer or Zero offset point.
  • Depth Below Waterline a.k.a. Waterline Offset is the distance ABOVE the transducer to the water line.  Often expressed as a positive number.
  • Depth Below Keel a.k.a. Keel Offset is the distance BELOW the transducer to the bottom of the boat, or keel.  Often expressed as a negative number.
  • Depth Below Transducer is the distance below the transducer to the ocean floor.
The below Graphic is taken from a Raymarine MFD user manual.
 
User-added image
 
Example:  Raymarine Lighthouse 3 – Axiom can be found in the transducer settings.
Out of the box, a sonar installed with no settings altered will display the true depth below the transducer (shows as #2 above).

User-added image
 
  • If a Waterline Offset is set, the depth value displayed by the sonar will increase by that value indicating that the display now knows there is water above the transducer location.  On a smaller boat this may only be a foot or less while in a larger cruising boat or sail boat this may be several feet or more.  This value is not always set as many users prefer to know the depth of the water below their boat because it gives an indication of how shallow they can get before the boat hits bottom. 
  • If Keel Offset is set, the depth value displayed by the sonar will decrease by that value. 
Practically, this can be thought of as a safety buffer.  Meaning, the depth value shown on the sonar screen will be from the deepest point of the boat to the bottom.  As the on-screen depth value approaches Zero you are becoming dangerously close to hitting bottom or grounding.  Some boaters add an additional foot or more buffer to be certain that they always have enough water to float the boat  with a quick glance at the screen.
 
Offset relationships with SonarChart Live – Contour depth value is displayed considering the input variables for offset settings as well as any tide adjustment if needed.  Tide Adjustment is a governing variable controlling SonarChart Live.  Tide adjustment will allow the plotter to understand time of day relative to tide cycle and adjust the depth according to reflect low tide conditions.  In this way, each time you are boating regardless of tide stage the resulting SonarChart live reflects the same normalized depth values which also correspond to the charted depth values reflecting mean lower low tide.  It is strongly recommended to enable Tide Adjustment in your plotter settings when using SonarChart Live in a tidal environment for best results and make sure your time and date settings are correct.

Tide Adjustment Active (Marine Waters, 3 ft tide, High tide example)
  • No Offset Settings - SonarChart Live takes the absolute value of the depth below transducer when no offset settings are made.   With Tide Adjustment enabled and the sonar screen says the depth is 5 ft then SonarChart live will draw a contour of 2 ft.   Why 2 ft?  Remember that in this example we are at high tide in an area with a 3ft tide.  SonarChart Live will then subtract the extra depth caused by tide so that the contour drawn best reflects low tide conditions.
  • Waterline Offset applied – SonarChart Live will add in the waterline offset so that the contours drawn reflect the true water depth, not the depth below the transducer.  If the depth below transducer is 5 ft, the waterline offset is 2 ft, the tide is high at 3ft, the depth shown on the sonar screen will be 7 ft (5+2=7ft, 3ft tide is already part of the 5 ft value) while the contour drawn by SonarChart Live will be 4 ft (5+2-3=4ft).
  • Keel Offset applied - SonarChart Live will ignore the Keel Offset so that the contours drawn more closely reflect the true water depth which is the depth below the transducer plus any tide adjustment.  We do not know the waterline depth setting in this case.  If the depth below transducer is 5 ft and the Keel Offset is 2 ft, the depth displayed on the sonar screen will be 3ft (5-2=3ft) while the contour drawn by SonarChart Live will be 2 ft (5-3ft tide=2ft) which is the most true depth we can calculate without knowing waterline offset.  This is simply the depth below transducer adjusted for tide.
No Tide Adjustment Active (Inland Waters)
  • No Offset Settings - SonarChart Live takes the absolute value of the depth below transducer when no offset settings are made.   If on a lake with no tidal influence and the sonar says the depth is 5 ft below the transducer then SonarChart live will draw a contour of 5 ft.
  • Waterline Offset applied – SonarChart Live will add in the Waterline Offset so that the contours drawn reflect the true water depth, not the depth below the transducer.  If the depth below transducer is 5 ft and the waterline offset is 2 ft, the depth shown on the sonar screen will be 7ft and the contour drawn by SonarChart Live will be 7 ft.
  • Keel Offset applied - SonarChart Live will ignore the Keel Offset so that the contours drawn more closely reflect the true water depth which is the depth below the transducer.  We do not know the waterline depth setting in this case.  If the depth below transducer is 5 ft and the Keel Offset is 2 ft, the depth displayed on the sonar screen will be 3 ft (5-2=3ft) while the contour drawn by SonarChart Live will be 5 ft which is the most true depth we can calculate without knowing waterline offset.  This is simply the depth below transducer.
 
When depth adjusted by tide is < or = to 0 the boat would be hard aground at low tide sitting on a drying area.  You may be able to float freely over these areas at higher tides; however, if Tide Adjustment is active the SonarChart Live recording will be masked.  No depth snake is drawn under this condition.
 

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