DESIGNED primarily for sailboats 16 ft. (4.88m) to 36 ft (10.68m) in length VERTICAL Mounting Angle Through 40 Degrees. Calibrated for Zone 1. CLINOMETER, protective Front & Back Covers are Included. Blue LED Night Lighting MOUNTING HOLE is 4 5/8" (118 mm) to 4 13/16" (122 mm) and 3 3/4" CombiDial CERTIFIED this compass is Wheelmark Certified Dials in magnetic compasses should be balanced to compensate for dip caused by the earth's magnetic field. All Ritchie Compasses are standard balanced for Zone 1 (essentially all of the northern hemisphere) All Magnetic Compasses are vulnerable to magnetic interference, which will produce errors, called deviation. It is the Owner/Operator and/or Helmsman's responsibility to make sure the compass is properly installed and compensated. It is important to understand that magnetic compasses point toward Magnetic North. There is a difference between Magnetic North and True North, and that difference is called variation. Variation differs depending on your geographical location and can be determined by referring to a local chart. The Venture SR-2 is designed primarily for sailboats 16 ft. (4.88m) to 36 ft (10.68m) in length. NOTE Compensation is the act of correcting for deviation. Magnets (speakers, microphones etc.), ferrous metals (steel, iron, etc.) and current carrying devices are common causes of deviation.