Know your air draft
If your lottery numbers come up you could hit it rich. If that’s not your thing, you can still play the numbers every day — the critical numbers of length, beam, draft, and air draft.
If you get some of these critical dimensions wrong, you can also hit it big. But it won’t be the jackpot you were hoping for.
A vessel’s specifications are not just random Quik Pick lottery numbers. Boat specs are carefully calculated and then measured when a boat is designed and built. I’ve seen many boats with its critical specs on signs that are mounted or painted on the wheelhouse bulkheads. These signs are merely a convenient reference to help vessel captains decide what to do before passing through constrained structures or restricted waterways.
Believe it or not, erroneous data is sometimes enshrined on the wheelhouse wall. It seems ridiculous, but it occurs more frequently than you’d imagine. I’ve seen different numbers posted in the lower and upper wheelhouses — that’s if the upper wheelhouse even has signs.
One thing is constant about towing vessel specs: They are subject to change. Length and beam don’t change much unless you’re hanging extensions over the side like a new fender system. Draft and trim changes are due to a number of factors such as fuel and water onboard, equipment moved or replaced, and even water temperature and salinity.
The boat was built, usually years ago, with a then measured air draft. Since then, the mast may have changed and new antennas may have been added. The VHF whip that is now fitted at the mast truck is great for extended radio ranges, but the whip increases your air draft by about six to eight feet. And how about that new radar you had been pushing the company to buy that now sits on the wheelhouse roof? And then there are alterations. I once saw a deck chair erected as a tanning crow’s nest, but it sure looked comfortable.
The aforementioned is for a light boat. Knowledge of the full air draft of the entire tow is an important safety tip. Cranes and stacked containers could increase your towing clearance height. In one bizarre case, the barge crew astern jacked up the crane without telling the tug. It was several feet higher than the bridge’s clearance. The watch in the wheelhouse sure got a surprise.
Another example of air draft exceeding clearance height is when a boat hit high voltage power lines running across the river, a shocking and fiery experience.
So, make sure that the plates showing the boat’s critical numbers are correct. Do not assume anything. When was the last time you or the port engineer ran a tape measure to the highest point on the boat and checked the draft? Just because you can see the waterline doesn’t mean the draft hasn’t changed. Boats have notoriously painted new waterlines higher on the hull when loading changed — sinking and capsizing are now closer to the gunwale.
Now back to those plates inscribed with the boats specs. Make sure they are correct and have the latest data. Make sure that the lower and upper wheelhouses have the same numbers posted, and know these numbers like the back of your hand.
If you use the correct numbers when planning your trip through tight spots, safe passage will not be like winning the lottery.
The views and opinions expressed in this blog are those of the author and do not reflect the official policy or views of the U.S. Coast Guard or its commandant.