A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Bill Ritchie
Date: 2022 Dec 1, 09:49 +0000
Since it was brought up on another thread, I will let the list know how we calculate height of eye onboard a modern merchant ship. This is one of the first lessons my cadets must go through with me when they start their celnav project.
Contained within the ship's draws is a schematic that gives the height about the baseline (keel) of all the accommodation decks. Since we shoot our bodies from the bridge deck, we find the distance from the baseline to the bridge deck. This number is usually found posted somewhere on the bridge. This however does not consider the draft of the vessel or the height of the observer.
Drafts are either observed or calculated. Most ships are trimmed to some degree, so the drafts at the forward and after perpendicular, which are the ones recorded, are not equal to the draft under the bridge in many cases. I ignore hog and sag and use straight plane trig to determine the angle of trim (some stability programs also give this number) based on the difference of the forward and after drafts and then use the distance between the bridge wing and the nearest perpendicular to find the draft under the bridge. As a side note, this number can change dramatically after a port call and even slightly as fuel is consumed during the voyage, so it's best to keep note of it.
Once the draft is known, it is subtracted from the height of the bridge above the baseline. Finally, the height of eye of the particular observer (height minus a few inches is what I use) is added to the equation to get height of eye. So BLheight-Draft+ Obs Height = HoE.
The dynamic variables that are far less controllable are the components of the ship's motion, particularly heave, roll, and pitch. Each of these can change HoE by several feet. To minimize these effects, I try to shoot from as close to the centerline as possible, and time my sights to where the ship is as centered as possible. I also resign myself that rough weather sights aren't going to be as accurate.