What do you mean by powerful? Most people find a pound of C4 to be surprisingly unimpressive. Maybe you can use one point of impact but you would have to either improvise one very large charge or combine hundreds of individual charges into one. Then unless you tamp or drill into the structure to place the charge most of the energy of the blast will dissipate into the air and not the structure.
What would happen if a suicide bomber filled a large truck with TNT and detonated it in the middle of, say, the Geo. Washington bridge?
He’d succeed in…killing himself, whoever was close by, and seriously damaging the roadbed. The blast isn’t going to damage the suspension cables, probably even if the TNT was right up against them.
The bridge would be closed until it is inspected.
Remember the I70 bridge over the Mississippi? Had a crack through an entire beam? It stayed standing until it could be repaired.
I haven’t seen this in any discussion, but we need a consideration of the total energy level imparted to the bridge pier by the ship, and calculate the explosive equivalent. That would be a back-of-the-envelope assessment of how much explosives a putative terrorist sapper would need.
I remember the accident of El Al flight 1862 in Amsterdam in october 1992 and thinking: terrorists will take note. Nine years later we had 9/11. Granted, it took a lot of luck for the terrorists and some incompetence/distraction on the side of the people who should have prevented this, but the prospects are worrying. If somebody can get hold of a big ship, even worse if it carries some explosive or flammable cargo (LPG, oil and fertilizer have been mentioned) they could be unstoppable. And now they know, so somebody may try. Crazy fanatics exist. I hope they have other priorities, but if mayhem is the goal, that way they may get what they want. 
I think the failure of that logic is entirely in thinking that no terrorist ever considered the idea of crashing a ship as a weapon.
YouTube is full of videos of ships crashing and wrecking a surprising amount of shit in slow motion. Only the dullest of young men fascinated by destruction will not have seen them.
Just as in business, getting the big idea is not the hard part of a successful project. That part is trivial. The hard part is doing all the detail work to bring it to fruition.
I think part of it is that these giant ships are a lot harder to steer than one might think.
We saw what happened on 9/11 but I bet it is harder to steer a container ship into a particular place than it is a plane (and that is not easy).
And, of course, there is the whole slow motion aspect to it. It evokes the scene from the Austin Powers movie where he is being threatened by a slow, chugging steam roller coming at him. That leaves a lot of time for the police or military to respond.
Agreed, hijacking passenger planes would be minor league stuff compared to gaining access to the control of a large container ship. First, the harbormaster would need to be taken out of the equation. Harbor security goes back to the age of pirates and harbors are more controlled than one may think. In addition, the large ships are controlled by tugs while in harbor, one that isn’t or a tug that is taken over would elicit an immediate response. Then there is the Navy, Coast Guard and the Air Force. Any large ship that was hijacked in US water would immediately see a response that could escalate to taking out the ship very quickly since there are no passengers to be concerned about.
Slight quibble that was the I40 (Hernando de Soto Bridge) over the Mississippi.
What do you mean by “taking it out”? We don’t have disintegrator rays. There are lots of things that the military can do to a ship to “wreck” it, but none of those will change its mass, and most of them won’t stop it. And of the few things that will stop it, most would do as much or more damage to the surrounding harbor as the collision would have.
I should have checked.
If a ship were to be hijacked in a harbor, it is very likely that the ship would put out a Mayday call and alert everyone. It would not be as simple as overpowering a few flight attendants. The engine crew would be alerted and could delay a total takeover for a significant amount of time.
Many US harbors are also very close to US Navel stations that have things like destroyers that are armed with cruise missiles and torpedoes. In addition, the Air Force has these things called fighter-bombers. Taking out a ship before it hits a bridge would not necessarily shut the harbor down; a cargo ship in harbor would be an easy target, especially one that is fighting off an attack by whatever means possible, ever seen the movie Captain Phillips?
Pearl Harbor was still an active harbor after the Japanese attack because the Japanese did not sink a ship in the mouth of the harbor.
By my math, the Dali moving at 8 knots had about as much energy as 375 pounds of TNT:
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Wolfram Alpha gives the kinetic energy of the ship as 786.3 megajoules.
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786.3/4184 = 0.188 tons, or 375 pounds.
The catch is that the Dali was able to dump all of its kinetic energy into one support pier (along with the deformation of enough trusses to assure that the rest of the bridge would bring itself down). I think a 375-pound charge on the side of that same pier might not bring it down, because most of the energy from the explosion wouldn’t be coupled into the pylon.
Rather than attacking the massively thick piers, sappers might have better luck going after the truss elements or the road deck support cables using linear shaped charges:
OK, right, so they take it out. I agree that that’s easy. And then after they take it out, it still hits the bridge anyway.
Well it really depends on when they recieve the Mayday and respond. I’m not talking about preventing the accident that actually happened and trying to stop it seconds before it hit, I’m talking about a terrorist take over of a vessle in a harbor. There isn’t much that can be done in the situtaion that happened since we don’t know the root cause. Once the root casue is determined perhaps then we can talk about the how to prevent the actual accident.
Every map I have seen of a navel battle has two icons for a ship that has been sunk. One for where it was attacked and one for where it sunk. They are often quite far apart on the map.
Before tomahawk cruise missles and smart bombs. Just look how fast some of the Russian ships went down in the black sea. Poke a huge hole in a ship and watch all of that mass sink straight down.
But really, wouldnt the first reaction of a ship in harbor being hijacked be for the harbor master to order all tugboats to pin it in or veer it off course, possibly run it aground? And for the Coast Guard to organize a response.
I recall after 9/11 going to to Boston Harbor and Coast Guard was very visible. A huge tanker full of what I believe was propane was coming in and armed ships surrounded it.
Are plane dropped torpedoes still a thing?
Because I think modern torpedoes swim under the ship and detonate which breaks the back of the ship. It snaps the ship in half and sinks on the spot (or near enough).
But yeah…a Harpoon into the bridge or side won’t stop the ship in its tracks.
The thing that’s missing here in @Si_Amigo’s fever dream is that USN & USAF are not on hair trigger alert to be able to attack a ship in a port on less than a day’s notice. And probably more like 2 days’.
USCG has 30’ gunboats (overgrown center console sport fishing boats) with 0.50 cal machine guns in every commercial port. Which gunboats are often patrolling around and so immediately available to respond. Which mighty weaponry they can use to rake the sides of the hijacked ship’s bridge with gunfire and that’s about all the harm it’ll do to a freighter.
In some few harbors, USCG has a couple of more heavily armed ships that could do real damage. Unless those ships are out to sea and 4 days’ sail from their home harbor or are out of commission. Of course the smarter bad guys would be aware of those cutters’ movements and plan their attack accordingly.
The idea of cruise-missiling or torpedoing a freighter in a US harbor because some bad guys have seized it and are threatening to run it into a bridge is too stupidly over the top for even Tom Clancy or Dan Brown to consider.
Trust me, there are, starting with the fact that the Dali is of Indian licensure.
If it WAS a terrorist attack, IMNSHO they’d have done this in broad daylight, to produce the highest casualty count.