Video here of a very large freighter passing by a beach. The freighter is a couple hundred yards offshore, but as it passes, the water level at the beach drops by several feet. The wake from the ship, rather than being a simple oscillation/wave like I’m used to seeing from a small powerboat, is more like a tidal bore; it suddenly and violently restores the water level at the beach to its original level.
Similar phenomena appear to be at work in this video, too.
So what’s going on? Are the ship’s screws moving so much water out from under the boat that they lower the local water level - and consequently raise the water level in the ship’s wake? Or is this some kind of oddball hull dynamics that would happen even if the screws weren’t turning?
Interesting video. The location appears to be on the St. Mary’s River, between Lake Superior & Lake Huron. The rivers around Neebish Island there are dredged to maintain adequate depth for the ore boats (you can see it on Google Maps); I wonder whether the different in water depth between the shipping channel and the water near the shore amplifies these effects.
I think this could be the same phenomenon as you can see with ordinary waves at the seashore, just happening at a longer wavelength.
If you watch a wave coming in and breaking on the shore, it is always preceded by water ‘drawing back’ - and this isn’t just the previous wave running away, it’s a trough preceding the wave - waves aren’t just a lump of water - they are a wave phenomenon, composed of alternating peaks and troughs.
I think what you’re seeing is a phenomenon of shallow water and a large vessel, and doesn’t have much to do with the screws. (Though their rotation probably causes localized seabed scour too.) From articles like this one, (B. Allenström et al. 2003. Amplification Of Ship Generated Wake Wash Due To Coastal Effects. Transactions.) these types of waves are called Bernoulli waves, and are distinguishable from the Kelvin waves we’re used to seeing in a ship’s wake. From the link:
In addition, per this article on ship-induced waves within the Venice lagoon, (K.E. Parnell et al. 2015. Ship-Induced Solitary Riemann Waves Of Depression In Venice Lagoon. Physics Letters A, 379(6):555-559.) there may be non-linear sizeable Riemann waves that arise in shallow, wide bodies of water, which may lead to higher than expected water movement.
I can’t describe the phenomenon as well as the linked authors do, so I’d recommend giving those papers a look or two.
*The *standout signature of an approaching tsunami is the ocean receding, leaving the beach or bay high and dry. That’s your cue to run for the hills. You might already be doomed, but at least you’ll die getting some exercise. Jim Fixx would be proud of you.
Yeah, as I understand it, this sometimes results in people reacting in the complete opposite way - running down to the exposed shore to look at what’s going on.
Depends on which side of the tsunami you’re on … some come in “peak” first then trough … and tsunami events come in wave sets typically … if you escape the first wave … keep running … there’s a bigger one next …
I use to fish in the Ohio River near Cincinnati. I noticed that the river level fell as a barge would be coming up the river. Seemed strange until I saw the amount of water that the engines were pushing behind it. It was certainly pushing water downstream faster. Once the barges passed, the river level would rise more than before.
Bernoulli’s principle?
There was a much less pronounced drop as barges came down the river. If nothing else the engines didn’t need to work as hard going downstream.
The guy in the second video gives an explanation: since the ship propels itself by pushing water backwards, it leaves a wave behind it and a trough where it is.
It’s not directly prop related. The props just don’t shift that much water. Gray Ghost is onto it.
That vessel is a fully loaded bulker probably drawing the maximum it can draw in that channel. I don’t have much experience of laker vessels but if it’s anything like where I’m used to it might have 1.5m (5ft) or less under keel. It is also in a one way channel which means the channel might be as little as 1.5x the beam, which for a laker means about 6m (about 18 feet) either side. Could be more but I’m talking minimums. The end result is that the vessel is a bit like un undersized cork in a tube. As it pushes up the channel, water rushes past underneath it and to either side to fill in the “hole” behind it.
This creates “interesting” navigational effects because as we all know (don’t we, kids!) fast moving fluids in a restricted space create low pressure. The entire vessel will experience “squat”: a fast moving vessel in shallow water can actually increase draft by as much as a metre (3 ft) or so due to this effect. There is also “bank effect” where a vessel can be sucked towards either side of the channel if it starts to get closer to one bank than the other.
What is happening in the video is that as the vessel passes, water is getting sucked out of the little cove those boats are in by the low pressure fast moving water racing back along the side of the ship. The Bernoulli effect doesn’t occur in an unrestricted space; but note the shallow water to either side of the cove.
The end result is that the ship/channel/cove system is like you blowing across the top of a straw extending down into a jar with water in it: the water gets sucked up the straw. In the same way, the ship is causing water to move very fast across the top of the cove, which causes water to get sucked out of it.
The same effect would occur without the cove, but the cove amplifies the effect because of the restriction on water rushing in from the sides to replace the water sucked out by the fast moving water caused by the ship.
Yesterday I did more reading and found out about the squat effect. The Wikipedia page cites a case where a ship grounded in a location where it should have had enough clearance based on the static water depth and ship draft - but because of its speed, the ship was riding lower than it would have if it were stopped.
Years ago I read an extremely interesting article about the the Port Revel Shiphandling Training Centre. Built on a lake in France, they teach ship captains how to deal with all of these peculiarities of handling big ships. To reduce the hazards and speed up the training, they use 1:25 scale models. Through engineering analysis they are able to ballast these models and fit them with appropriately sized engines, such that they handle exactly like full-size ships - except everything happens on shorter time and distance scales, e.g. if the real ship needs a 1-mile turning radius and a half-hour to complete a turn, one of these models only needs a turning radius of a couple hundred feet and six minutes. They position the trainee so that his eyes are exactly where they would be on the bridge of the real ship: everything looks approximately as it would if he really were at sea. To properly understand the limited maneuverability these big ships have, my recollection is that these models were ballasted to weigh many thousands of pounds, and fitted with an engine that was on the order of a few horsepower; they don’t start or stop moving very quickly.
Anyway, as you’ve noted, the flow of water around these big ships creates some surprising navigational challenges. Some YouTube videos from the folks at Port Revel here:
This all reminds me of the piston effect, which presents some interesting challenges for high-speed trains passing through tunnels.
This. A tsunami happens when there’s vertical slippage of a subduction fault, i.e. one plate jumps up and the other plate drops. If you’re on the side of the fault where the plate has jumped up, then there will be no drawback preceding the arrival of the tsunami.
Good description. I heard this when I was a 1st Class Midshipman. In going through the Panama Canal. Another 1st Class Deck midshipman explained to me why the pilots in the Canal are in command of the ships. When passing another ship in the Canal this effect will try and pull the two ships into each other so the pilots have to steer the ships away from the center of the canal, but at the same time they have to be carful that if the steer to the starboard too much they can get sucked into the edge of the canal. And yes the Deck Midshipman was enjoying explaining something to an unknowing Engineering Midshipman.
When a ship is underway you see a hill of water behind it. Placing a ship in a tank would raise the water level. That big tanker is already in the water. Displacing it at all times. But the power being expended to drive it through the water does displace more water. Behind it. So as it passes, it must take water from around and under it and put it behind it. The bow makes waves on the surface as it pushes through. But there is much more of the ship below. That water is being sucked out by the screws. Pushed back. Thus the hill behind the ship. At slow speeds it is not so evident. Water is not compressible. It has to go somewhere. The screws push against the water. Piling it up to the back. Water all around the ship is drawn in.
