When I was sailing tankers on the West Coast
, we routinely had to discharge ballast to clear Pinole Shoal in San Pablo Bay (at least 3' UKC, IIRC), then immediately take on ballast to clear the Carquinez Bridge. This took coordination between the bridge team and C/M in the CCR to accomplish. Even then, we had to lower the signal masts and still only cleared the bridge by a few feet. There was only a certain tidal stage this would work so that also added a variable. The pilot was also very careful to make sure the house passed under the highest point of the bridge, but off to the side just enough so as to not take out the lights and/or RACON. On some bridges, these hang quite a few feet below the bridge structure.
Also, I'm pretty sure the QM2 needs to transit The Narrows in NY Harbor at only certain stages of tide so as to clear the Verrazano Bridge. Exciting? Yes. Fairly Routine, though? Yup.
And yes, squat is most definitely taken into account for these types of transits. However, the amount of squat is determined not only by the speed of the vessel, but also greatly by the UKC. Therefore, small vessels are unlikely to see any benefit of taking advantage of squat to reduce their effective mast
Originally Posted by Arch Stanton
The "20 inches" figure makes for a good headline but it's misleading: 20 inches isn't the clearance, it's the known margin of error in the draft
That's a pretty big margin of error. Like David M said, an inch or two is typical. If a calculated draft
and actual, observed draft differ by more than three of four inches, something is not right. Twenty inches is enough to exceed bending limits and cause some very bad damage if you're off in the wrong direction. BTW, the same ship I was referring to above would routinely have about 17" of sag after loading in Valdez.
Passenger vessels typically have much more static loads and whatever minor changes in trim or list do occur, they can be corrected with small ballast tanks
transferring, if necessary.