I think the bottom configuration at the mouth of the Orinoco is very different from the case we have in Houston.

The point I was trying to make is that vessels do not squat in very shallow water so long as the approaches are uniform and gradual. This is an easily verifiable fact that Dr Barass and the hydrodynamics experts choose to ignore. The math that predicts squat in these circumstances is flawed.

Theory will predict squatting for the specific conditions modeled in developing the theory that may or may not reflect the conditions at specific locations. Hence, there may be difference in what Dr Barras predicts and the Houston Pilots experience.

Dr. Barras talks about grounding but does not mention what happens next. When the ship “grounds” due to squatting the flow around the hull will change and conditions will refloat the vessel and provide passage for water under the vessel again but subsequently the vessel will “ground” again. In other words it will bump / scrape and lift at a reduced speed until it speeds up again and causes another squat / bump.
At the mouth of the Orinoco crossing the sand bar we experienced several bumps till be passed the sand bar.

De. Barras concern with safety due to gounding may be becuase he assumes that gounding will damage the hull but I believe, in most dredged ship channels these gounding / bumps area against mud or sand with little no no paint damage.

Shape of the Houston Ship Channel may not be condusive to squatting. It is not clear if the Houston Pilots experience bumps or scrapes against the channel with no other disturbances in the channel.

Nevertheless, Theory is as good as the assumpsion made and should be vidicated with real life experience. The analysis of the measurements taken on the vessels that Pilot Louis West mentioned should be completed, if for nothing, then just for acedemic purposes.

Thank you for this article which expresses exactly my own thoughts and experiences in 25 years of piloting. I have experienced the quarter wave breaking effect when navigating a 5m least depth channel only to discover that a 4.1m shoal had now extended nearly halfway into the channel. We scraped the bottom paint off the ship and I was mortified to later be told by 2 other pilots that they had “smelled the bottom” at this point some time earlier – but had neglected to report the matter. Pilots!

I am now going to look for your navigable mud article because Wyndham, WA, Australia has mud aplenty – a tidal range of 8m and mud/sandbanks which require annual surveying so we can plot next year’s route.

Thanks again for your excellent article.

Regards

Brian Tod
Harbourmaster/Pilot
Wyndham Port

Interestinly, Dr. Barass; after trying to discredit me in the magazine with his second article, has never contacted me about my offer to observe squat (or lack thereof) in Houston. Nor has any other hydrodynamacist.

Regarding the reliability of models. Another of my disagreements with the professorial experts has been the validity of observations/tests/predictions based on data obtained from test tank models or mathematical models. Validation of these models is important because ship designs, rudder designs, etc are increasingly made and tested on computers. In some cases portions of sea trials are allowed to be simulated or performed in a test tank.

In one of the numerous attempts to verify squat calculations that you described above, the USCOE funded an effort here in Houston. The goal was to install highly accurate GPS units (+/- 2cm)on the bow, stern and beam ends of vessels making Houston transits. The recorded data was to be made available online for investigators to use. Last time I checked no one had done any work on the data.

More importantly, there were several ships calling in Houston at the time for which mathmatical models were available; the same models used for generating the graphics of simulators for example. The researchers made the extra effort to record data from these vessels and in at least one instance two vessels met in the channel, both of which were equiped for data recording and both of which had known mathmatical models. When the ships met bridge cameras were used to record the exact timing and sequence of pilot orders during the meeting. (For those who aren’t familiar, a meeting in Houston involves two ships meeting head on until about .5 mile and then swinging right to pass within 60-70 meters of each other in a 160m channel.)

After the study, two of the researchers tried to recreate the maneuver on a simulator using the ship models, the recorded sequence of commands, and the recreated channel outlines. The results did not conform to the actual meeting situation. The simulator ships could not recreate the maneuver. The reported the results were significantly different on the simulator from real life. I tried to convince them that a negative result was important and worth an article in one of the leading scientific journels, but they didn’t seem to think so.

So. How accurate is simulator training or simulator ship design? It’s not just about squat.

Louis Vest.