Mast Shadow Explained

 

 

 

 

 

 

 

 

 

 

 

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Look along the rows of boats at any marina and no doubt you will see a sail boat or two with a radar reflector bracket mounted to the mast the problem is, a 36 ft. sailboat fitted with an 8" spherical or fender shaped reflector mounted in front of its mast, will have a  mast shadow will be at least 33° wide. At a range of only 3.5 miles astern this shadow is 2.3 miles wide. Assuming a vessel is overtaking with a relative speed of 15 knots, it would only take 12 minutes to catch up with virtually no chance that the reflector ever returned any energy whatsoever. Even the most expensive reflector mounted in this fashion would be totally ineffective. The only reflector to address this issue is the Cyclops with its unobstructed masthead mount which ensures 360° coverage and a high RCS for long range detection no matter from what angle it is approached. 

Mast Size (Approx. length X width) Center to Center Spacing Azimuth Shadow Angle Boat Size
130mm X 93mm 192mm 27.2° 20' - 25'
155mm X 104mm 205mm 29.0° 25' - 30'
177mm X 124mm 216mm 33.0° 30' - 35'
206mm X 139mm 230mm 34.0° 35' - 40'
237mm X 162mm 245mm 37.0° 40' - 45

Notes:

  1. The mast sizes are typical and were supplied by a mast manufacturer.

  2. The center to center distance includes generous clearance to allow for the mounting bracket.

  3. The mast profile was considered as elliptical, but virtually every mast carries the full width further forward, which further increases the shadow size.

  4. No account is taken for Heel or Phase Interference effects between mast and reflector, reducing the effectiveness of a mast mounted unit even more.

The ISO 8729 specification in clause 8.2 requires the reflector to be installed in the optimum position to avoid shadowing sectors but does not give guidelines as to what degree of shadow angle is deemed to be significant.

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