All adjustments require me to vist to your vessel where we go out "for a boat ride." I will try my best to accommodate your schedule.
Compass adjusting is as much art as science. To adjust the compass on your boat, we must take the boat out on the river to swing around in circles. I will give you a heading to put the vessel on. Then I'll shoot a relative bearing on a distant object, using my polaris, and then compare the result to what the bearing should be. The difference is compass deviation. We will repeat this process for all four cardinal points of the compass. From that point I will begin to add equal and opposite magnetism around your compass, continuing to take bearings, until I am able to remove as much of the error as possible. Large ships require the use of two tug boats and a River Pilot to do the swinging around in circles.
Sometimes I need to mount small magnets to the deck near the compass but in other compasses there are built in magnets that are adjustable to "compensate" for the offending magnetism on your vessel. After the cardinal points are as "clean" as possible, we will then read the inter-cardinal error and record it. Only on a steel vessel with inter-cardinal balls is this error adjustable. So for most vessels I will only record this error, which should be minor, if the cardinal error is mostly eliminated.
I will provide you with a deviation card laminated in plastic for you to keep at your navigation station to use to correct your course headings to allow for the remaining compass error.
This process normally takes from 1-2 hours per compass. This is best done early in the morning when there are few other vessels on the river and never on weekends if possible because of river traffic being much increased on weekends. Of course, fairly good weather and good visibility is a must to get a good compensation of your compass.
I get a nice boat ride and maybe a cup of your coffee, and you get a professionally compensated compass!