During WW2, Germany owned ships along the eastern US coast with U-boats pretty much unopposed.
So... why didn't Germany head into US ports and destroy infrastructure? Why did they keep their actions out at sea (for the most part)?
If I were calling the shots, I'd be trying to be as distracting as possible by attacking high visibility targets all along the coast -- not modded shipping boats. What gives?
According to Gannon's book, Operation Drumbeat and a quote from U-123 commander Lieutenant Captain Reinhard Hardegen, from the article Sharkes in the Water, the issue that most deterred U-boats from entering American harbors was that they did not have detailed charts of the harbor and feared running aground. Hardegen said they nearly went aground near New York when they mistook a street light for a buoy light.
Attacking targets in ports is the least productive way of using your ships for at least two reasons: 1) The damage you do can be easily repaired and 2) the chances of your own ships getting "caught" or sunk are the highest.
The Japanese found this out at Pearl Harbor. All but one of the ships that they sunk were raised from the sea and recycled. (Only the Arizona was "blown up" and beyond repair.) And while several thousand U.S. sailors died in the attack, the death toll would have been much higher in the open ocean (from drownings as opposed to bomb explosions) with no rescue facilities.
Ports are like "roach motels." The enemy ships that enter them usually can't leave. There are exceptions to this rule of course, but they are the stuff of legend. And submarines are the most vulnerable in this regard, once they hit shallow water and can't "submerge."
The Germans sank hundreds of ships lying off the Atlantic coast in 1942 where neither crews nor cargo could be rescued, while losing a handful of submarines. There was no point of their launching "suicide attacks" in ports and inflicting damage that could be recovered or repaired.
Because it would have been suicidal and unproductive.
The bulk of the German U-boat fleet barely had enough range for operations in North America at all. They did things like filling water tanks with diesel just to get enough range to hunt convoys. The bigger, less maneuverable Type IX did have better range, but they were also clumsier and more detectable.
Even if they made it to US ports, the U-boats didn't have arms to attack ports properly. More to the point, they would not have been able to stand up to the conventional surface fleet. The U-boats suffered enough losses as it is doing convoy raiding.
Lastly, your whole strategy seems rather dubious. The U-boats wrecked havoc on merchant shipping. That is a concrete contribution to the German war effort. I'm not sure how helpful being "distracting" would be, compared to preventing vital supplies from reaching Britain.
The only ships the Germans could get to the US coast were U-Boats. These had small guns and no armor. A Coast Guard cutter could tear one to pieces. A 50 caliber machine gun hole could make the U-Boat unable to submerge again.
So they stayed just outside the port and sunk ships, until the US started convoying and escorting the merchant ships. The sunk some 300,000 tons of shipping until the US got its act together.
What the other answers are missing is a clear explanation of why it is dangerous for a sub to enter a harbor.
The reason why a sub cannot normally enter a harbor, like Boston Harbor or New York Harbor, is that they are shallow, confined areas. New York Harbor is about 50 feet deep. Boston Harbor is even more shallow, less than 20 feet in many places. It has to be constantly dredged, just to maintain 40 ft deep channels. A WW2 submarine is about 30-40 feet high including masts. So, a sub would be practically sticking out of the water in such a harbor and easily visible as a shadow from the air.
Also, if a sub entered a harbor and was spotted it would be trapped, because the ways in and out are so limited.
There is a very famous harbor attack: the sinking of the HMS Royal Oak in Scapa Flow on 14 October 1939. This was only possible because Scapa Flow is very large, deep harbor. Even so, it was very daring and dangerous for the U-boat to have entered.
(At least one U boat ventured up into Canadian waters, but there doesn't seem to have been much damage.)
The advantage of U-boats was their stealth, but when they were detected then countermeasures could be taken. When attacking a US port, their location would have been easy to narrow down, and there would be massive allied firepower nearby. Far easier, and safer, to attack merchant shipping in remote locations with the minimum of allied support.
Oldcat already answered for the naval aspects.
There were also plans to obtain a long-range strategic bomber that would be capable of striking the contiguous United States from Germany.
From Wikipedia:
The Amerika-Bomber project was an initiative of the German Reichsluftfahrtministerium, to obtain a long-range strategic bomber for the Luftwaffe that would be capable of striking the contiguous United States from Germany, a distance of about 5,800 km (3,600 mi). [...] Various proposals were put forward [...], but they were all eventually abandoned as too expensive, and potentially consuming far too much of Germany's steadily shrinking aviation production capacity after 1942.
My father was torpedoed off the US east coast by U-653 in May 1942. U-boats were more easily able to cause economic sabotage by sinking ships in US coastal waters than by attacking US ports.
U-boats are not equipped to duel with shore batteries. U-boats are very vulnerable because the slightest crack to their pressure hulls will prevent them diving. Their so called ballast tanks were mainly used to house fuel and even slight damage could cause them to trail oil behind them for hundreds of miles enabling patrol aircraft to follow such a trail and destroy them.
In September 1917 Scarborough was bombarded by a German U-boat and in Australia, Japanese submarines made bombardments of Newcastle and Sydney during 1942 but these were only pin prick attacks of no strategic significance. Such attacks were more dangerous to the submarines than they were to their intended targets.
The type VII U-Boat used at the time had a single, unprotected 88mm deck gun. The type IX had a larger 105mm gun. Their primary use was to sink unescorted merchant ships because torpedoes are expensive. While the range is listed as 12km, I have serious doubts they could hit even a large industrial target at half that range at night from the pitching deck of a U-Boat.
While 88mm (3.5") sounds big (it is for a WWII tank) by naval standards it is a pea-shooter and unlikely to do enough damage to be worth risking the boat. Your typical coastal defense gun will be 8" to 16", often guns stripped from old battleships.
And risk the boat they would! They would have to be on the surface, probably at night (the US left the lights on), in sight of the shore, with a large number of sailors on the deck, firing a cannon. They would be visible and vulnerable and slow to dive in shallow water. If spotted, any number of patrol boats and airplanes could arrive and harass them.
What would they shoot at that was worth the risk and they were able to hit and damage? Fuel storage tanks are the only thing that comes to mind. But a handful of U-Boats would not be able to make a dent in the US fuel reserves. That was the end result of many of Germany's plans to attack the US, too risky, too expensive, too little result.
Finally, there were very few U-Boats attacking the US. The first wave was just five Type IX's, which were designed to operate so far from home. The third wave in the Caribbean was only 11. I don't have data for the second, but the Germans never had enough U-Boats.
The Japanese did bombard the US West Coast with little result. They shot at fuel tanks, a fort and a lighthouse. The Japanese had the advantage of a much larger (140mm) gun, and they couldn't hit anything.
What about firing at civilian targets and causing panic? While we have the image of a crazed Hitler ordering downtown Manhattan bombarded, that sort of thing happened much later in the war when he'd fired or killed most of his good commanders and had nothing but party sycophants left. At this stage in the war the German military high command upheld at least the veneer of honor. Germany was winning and they could afford to fight fair. My speculation is this would not have gone over well with Dönitz or the U-Boat captains and crews. It may seem strange to draw a line between firing on civilians in a city and firing on civilians in a boat, but my speculation is they would have.
In the end, the goal was not to harm the US, the goal was to prevent it from projecting power to Europe. They were doing that just fine sinking ships, and the US was doing a fine job letting them.
