A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2017 Jan 24, 20:00 -0800
And if you don't have any string, but you do have your digital camera (with a lanyard!), then drop a rock over the side. Drop it from your height of eye, and record the event from drop to splash with the video mode of your camera. Or if you have an audio recorder, you can time it by saying "drop... splash". It's also possible to time this with a common stop watch if you can read it to tenths of a second or with a stopwatch app on a smart device. In any case, take the drop time in seconds to the nearest tenth of a second and multiply by 3.9. This directly yields the dip in minutes of arc. The object you drop needs to be something small and dense... in other words, a rock or maybe a large bolt (don't use the one that holds the rudder on}. Otherwise, air resistance becomes significant. Note that throwing things over the side may get you tarred and feathered, so get permission or be discreet.
Yet another approach. Take a photo of the vessel from the pier before you depart. You can measure some convenient object at your leisure on board and that gives a scale for the photo. This is easy and effective.
A fail-safe technique: take a bunch of sights (that's the plan anyway), and treat dip as a systematic error. This is especially easy if you have the exact position of the vessel (GPS coordinates) during the run of sights, but it's not essential. There will be some dip value that minimizes the systematic error. Done.