Inquiry about adjusting compass

[Mark]

Hi all,I have an O'Day 23 which I'm refitting and getting ready for it's sailing days. When I bought the boat, it came with a bracket mounted compass. It reads "Chicago Airguide" on the center dial. I believe it's a direct-reading compass, but the North/South is all out of whack. Reason I know this, is the desk in my home office faces west-northwest, but the North/South on the compass is reversed. There are two adjusting knobs on the bottom of the compass, however I'm curious if anyone has ever adjusted these before. It's not mounted on my boat at all yet. (was in the box of extra "goodies" when I purchased the boat). Hope someone could help. I'm ready to go out and buy a new compass anyways...should I?Mark

 

[Rod Johnson,]

Are you sure?

Are you reading the compass correctly? Some compasses read with the lubber-line positioned on the forward side of the compass (flat-card type), others (front-reading) have the lubber-line on the aft-side of the compass. If the compass was off a few degrees, then I'd adjust it, but if it is off by 180 degrees......I suspect that you are reading it backwards.Anyway, do not make any adjustments until the compass is mounted on the boat, the magnetic influences on the boat will affect the compass and that is what you can usually adjust out. There may be something magnetic on your desk that is causing the compass to read wrong as well.Once mounted on the boat, you should run a few compass courses to check the compass. Plot a course on your chart between two landmarks, then run that course in both directions noting what your compass showed each way. Now plot and run a course roughly 90 deg. to the previous one. Again, note your compass readings. If you find that the compass is off by a few degrees, you can use the adjusting screws on the compass to compensate the error. Use a non-magnetic screwdriver (a Dime is often best) to adjust first the N-S screw while running a N-S course, adjust it to correct for 1/2 the error, then turn the boat 180 degrees and check again, adjust to 1/2 of error if needed. Now, do the same for the W-E screw. Finally, recheck the N-S adjustment. If further adjustment is needed, recheck the W-E error.