NavList:

We do use plumb bombs from the main deck for draft surveys.  They are very difficult to get accurate.  I can't imagine trying it from the bridge deck.  Between wind blowing the string/sound tape around, and the waves affecting the height as well, it's a crap shoot.  We actually tend to put people over the side in Jacobs ladders and other devices to read the draft marks as they are far more accurate to determine.

Next week I'll be on my ship and I'll try the laser range finder in my binoculars and see how that compares to my calculated values.  Sometimes the laser doesn't read too well off the water surface.

My Dip exercise with cadets is a didactic.  I am attempting to teach them how to think critically and pay attention to details.  Dip calculation is an easy and harmless way to convey this lesson.  The REAL use of this methodology is calculating air draft.  There are things at sea when being sloppy can lead to accidents; like hitting bridge spans because you "guestimated" too many factors. See this story.  I have had to bring two different ships under that very same bridge with about 3 ft of under-keel clearance, and less than 4 feet under the bridge span.  You can bet my math as as painstakingly detailed as anything I run a cadet through.

Jeremy