What’s your draft? Do you think you know what it is by reading the draft marks or – if load lined – your Plimsoll marks? How about the beam and air draft? The key dimensions of your boat are numbers that you better have burned into your memory – and maybe even on a plate fixed to the wheelhouse bulkhead.
No matter how well you know the numbers, the aforementioned dimensions are valid when the boat is tied up to the pier. When you get underway, they will change slightly or greatly depending upon a number of factors. This translates to the difference between running aground, striking an overhead obstruction, or bouncing off the bank of another vessel you are passing in the channel.
The laws of physics and nature come into play when a vessel gets underway. Water starts flowing along the bottom of the boat and winds start exerting force on the exposed hull and superstructure.
First, let’s consider under keel clearance (UKC) – how much water is between you and the bottom. Without sufficient UKC, you’ll run aground, so guessing at this number is a bad bet. Your Safety Management System should reference it, and so should your voyage planning. A senior pilot told me that maintaining 3’ UKC is a good general rule.
UKC is typically determined by how you load your barges and trim your boat, factors that change constantly as you burn fuel and use water onboard. Combine this with how fast you are moving through the water (add the current if you are stemming the tide) and it can add up to a huge difference in UKC. Though you may be going 8 knots, the Bernoulli principle dictates that you will sink deeper in the water, losing buoyancy.
The most affected place is aft under the propellers as prop wash sucks the stern down – a phenomenon called “squat.” In the case of a towing vessel going slowly it might only be a few inches, but if you cut UKC close, that could end up grounding your wheels. A big ship traveling at speed in a channel can squat feet and not inches. When a huge cruise ship was getting underway for the first time from its builders it had to clear a bridge that was a little too low for the ship’s masts. They used squat by cranking up the speed and getting the ship to hunker down and slide under the bridge.
The other dimension subject to underway conditions is your beam. As you crab down the channel in a cross wind, just a few degrees off the track line can make your effective beam much wider – “sweeping” the channel. For example, a large ship could sweep the channel with an extra 300’ beyond its own beam, making a meeting situation in a narrow and constrained channel interesting.
So take into account the effect of squat on your drafts and cross winds on your swept channel beam, and, as always, sail safe!
