Getting Under Bridges

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Recap on taking your boat under objects such as bridges.

  • Know what your vessels AIR DRAFT is
    • Approx 36 – 42 ft Sailing Yacht is 40-50 ft – 13 – 16 Meters
  • Northam Bridge has a clearance of 4.2  (METERS)
    • This figure is measured from HAT (Highest Astronomical Tide) and represents the smallest clearance you’ll likely ever have.
    • The range of tide locally makes a large difference.  (if HAT is 7.5m then when the height of tide is 1.0m we have 4.2 +6.5 = 11.7 meters of clearance)
    • This is the greatest high water height possible and is rare in most areas and greater than MHWS. (Mean High Water Springs)
  • For a Motor vessel with an AIR DRAFT as high as 5 or 6 Meters, this is a realistic bridge to pass under where as a yacht this would be unrealistic without dropping the mast.
    • A 12m tall yacht would require the HEIGHT OF TIDE to FALL by 8m and would only allow 0.2m room for error + the HAT bonus.

 

Extreme measure can be taken to reduce you vessels AIR DRAFT as seen in figure 2.  We don’t recommend this approach!

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Figure 1

 

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Figure 2. Extreme measures

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