What causes this phenomenon?

One of the posts suggests the venturi effect but the comment is so abbreviated, I am not sure I understand it.

At first I would thing that the water would rise due to the displacement of the massive hull.

But obviously the water level decreases. Why? Does it have something to do with the bow compressing the water in front of the ship?

 

Being that it only happens in that spot.......I would guess the ship's displacement is a fair proportion of the channel volume in that area.

The 4 propellers on the USS Enterprise....could pump lake erie dry in 24 hours. That's displacement.

 

The boat briefly acts like a giant plug and the propellors are pushing the water out of the bay faster than it can return. Once the "plug" is removed, water again seeks its own level and rushes back in. Obviously, this is a bay, with no water being fed in at the head (or at least not enough to overcome the pumping action of the props).

 

I can go with SLK001, the boat is a cork and the props are a pump, because the replacement water arrives from behind the ship.

 

I can go with SLK001, the boat is a cork and the props are a pump, because the replacement water arrives from behind the ship.

After thinking about it. I am leaning that way myself. Those props must move an enormous amount of water.

But it still brings up the question, what happens to the water displaced by the ship? Is that tall also pushed out?

and speaking of which, do ocean levels rise slightly every time a ship is placed into the ocean? For that matter any object?

 

I don't know what happens to the bow wake, but yes, every time you displace water with an object, the sea level rises. That would be an interesting math problem. How much does the ocean level rise when you drop a thousand tons of ship in it? Probably less than a micron.

The opposing argument would be that the land rises equally when you remove the weight from it, but I don't think so.

 

...and speaking of which, do ocean levels rise slightly every time a ship is placed into the ocean? For that matter any object?

Yes, they do - but it's not a "drop in a bucket" thing, but a "drop in the ocean". How much does it rise? Take the displacement of the ship, in pounds and divide that by 62.8 lbs/cu ft (approximate weight of sea water) to find the cubic feet of displacement. Next, find the surface area of the oceans on earth by taking the radius of the earth, 4100 miles and applying 0.7 × 4 × π × r^2, to get 4×10^15 square feet. Divide the first by the second to find the level rise for the ship (the 0.7 is the 70% of the earth's surface covered in water).

EXAMPLE: A 50,000 ton ship displaces ≈ 1.6 million cubic feet of water.
1.6 × 10^6 ft^3 / 4×10^15 ft^2 = 400 × 10 ^-12, or 400 picofeet of sea level rise. This reduces down to a more manageable 33 picoinches of rise.

So any eyeball should be able to detect this!

 

Yes, they do - but it's not a "drop in a bucket" thing, but a "drop in the ocean". How much does it rise? Take the displacement of the ship, in pounds and divide that by 62.8 lbs/cu ft (approximate weight of sea water) to find the cubic feet of displacement. Next, find the surface area of the oceans on earth by taking the radius of the earth, 4100 miles and applying 0.7 × 4 × π × r^2, to get 4×10^15 square feet. Divide the first by the second to find the level rise for the ship (the 0.7 is the 70% of the earth's surface covered in water).

EXAMPLE: A 50,000 ton ship displaces ≈ 1.6 million cubic feet of water.
1.6 × 10^6 ft^3 / 4×10^15 ft^2 = 400 × 10 ^-12, or 400 picofeet of sea level rise. This reduces down to a more manageable 33 picoinches of rise.

So any eyeball should be able to detect this!

That is just one ship. There are probably 10s of thousands of ships in the ocean. Not to mention smaller vessels, structures and even people.

 

That is just one ship. There are probably 10s of thousands of ships in the ocean. Not to mention smaller vessels, structures and even people.

Yes, that's correct. Just think of the 33 millionths of an inch that one million 50,000 ton ships would raise the ocean levels! Why, it would flood out New York City (but who would care)!