A marine compass is a dome-shaped bulb with a dial that rotates on a pivot point. It is usually installed near the steering wheel. The north/south line of a marine compass runs through the pivot point and through an indent mark called a lubber's line that aligns with the centre of the vessel. A marine compass usually lasts about five years and is prone to leaking if the area experiences significant temperature changes. Serious issues like a cracked dome or a worn pivot should be repaired by a technician, but fixing a leak can be done at home.
Bubbles in the compass dome are the most common signs of a leak. In cold weather, water contracts and pulls air in. In hot weather, water expands and sometimes pushes liquid out. These events cause the bubbles at the top of the dome and are unavoidable.
Turn the compass on its side and locate the screw that attaches to the backing. Remove the screw. Fill the syringe with compass oil and inject the new oil through the hole into the dome. Replace the screw.
Turn the compass upright and sway the dome back and forth gently to help the excess air get out. Repeat the filling process as needed until no more bubbles are present.
Bubbles will continue to appear as water contracts and expands. Most compasses are built to withstand this, but if the problem persists, consult a technician.