Before GPS became ubiquitous, swinging a compass (checking it for accuracy and adjusting) was a yearly ritual for most boat owners. The compass was the most important navigational tool onboard as even a small error could have a major effect on an Estimated Position calculation. These days, we are not as dependent on compasses, but the humble steering compass is still an important navigational tool.
We had the compass checked and the good news is that it is reading correctly; there is little deviation from what the compass shows and what is actually shows as you can see from the “report card”. However, we did discover that the air horn, which is used to signal other vessels, was causing a major deflection of the compass as it is a heavy metal container and was located close to the compass. Once we are out of San Diego Bay where we might need to give an audible signal, we will move the airhorn to a place far enough from the compass not to be a problem.
Voyage of Inspiration 〜全盲ヨットマン、夢と希望の航海〜 