easy calculations and adjustments 59
- Find the adjustment screws. Now
locate the compensator-magnet
adjustment screws. You’ll normally
fi nd these beneath small, round
caps near the base of the compass.
One will be on the side and the
other on the front or back of
the base. Remove the caps to fi nd
the adjustment screws. - Set up board, book, and compass.
Use the felt-tip pen to draw a
straight line in the center of the
board, parallel to one edge. We’ll use this mark to align our compass. Place the
compass on the board so that the lubber line aligns with the line you’ve drawn.
Alignment is easier if you look down at the board from above. Screw or tape
the compass fi rmly in place all around its base. Make sure the compass doesn’t
move even if you move the board from side to side. - Test for north-south error. Th e compensator magnets controlled by the
side screw are used to remove northerly or southerly compass error. Turn
the board until the compass lubber line points to zero degrees. Place the
hardcover book along one edge of the board and hold the book fi rmly in
place. Now move the board away from the book, rotate it 180 degrees, and
place it fi rmly against the book’s edge again. Does the compass read
180 degrees? If not, remove exactly one-half of the error. For example, if the
compass reads 176 degrees, turn the compensator screw on the side of the
compass until it reads 178 degrees. If it reads 182 degrees, turn the screw until
it reads 181 degrees.
Now we’ll start again from a new point. Move your board, aligning
the lubber line to 180 degrees. Re-align the book edge along the board.
Again turn your board around, placing it fi rmly against the book’s edge.
Th e compass should read zero degrees. If it doesn’t, remove one-half of the
error. Th en turn the board to line up the lubber line to zero degrees, reset
your book along the edge, and spin the board. Continue in this incremental
fashion until you remove all error. - Test for east-west error. Th e compensator magnets controlled by the
front (forward) or back (aft ) screw are used to remove easterly or westerly
compass error. Point the lubber line to 90 degrees. Reset your book edge
to the board, then rotate the board 180 degrees and place its opposite edge
against the book. Th e new lubber line reading should be 270 degrees. If
not, remove exactly one-half of the error, this time using the forward or aft
compensator screw.
Tools and Supplies
Needed for Zeroing
a Steering Compass
X 1 small nonmagnetic fl athead
screwdriver
X 12-inch-square piece of
plywood or pressboard
X duct tape
X felt-tip pen
X 1 large hardcover book