Knowing the boat’s position in relation to the sea-floor is important. But how about what is above you? How will you know that your mast will clear the bridge coming up? Throw in the dynamic mediums of water and air through which the boat travels and the answer to the equation may not be so easy to solve for. The video below shows how technology made things easier when the newest ship in the Royal Caribbean cruise line just barely cleared the Storebaelt Bridge in Denmark.
With tight tolerances in navigation datums become very significant. The prudent mariner always knows what datum the material they are referencing is on. Typically, as is the case with most NOAA charts, objects that are above the water are referenced to the Mean High Water line (MHW) and objects below are referenced to the Mean Lower Low Water line (MLLW). Waterline height (Tides) is another variable to be considered by the prudent mariner. We will look for a topic related to tides that we can post about if there is much interest in this post.

“The great weight of the ship may indeed prevent her from acquiring her greatest velocity; but when she has attained it, she will advance by her own intrinsic motion, without gaining any new degree of velocity, or lessening what she has acquired.” –William Falconer