I am either mad or brilliant · 4:33am Apr 25th, 2015
So I was thinking about the vikings and the raids they did: all the murdering, raping, burning, pillaging they did to people who'd never done anything to them; and then my mind can't help wondering how to make them 10X more dangerous. 
Their strength was in their longships which were fast, could land anywhere without dock or pier, and could be rowed in waist high water, allowing them to strike up rivers hundreds of miles inland. Problem was, that they had fairly limited cargo capacity, so you'd row for 3 weeks, hit one place, and then row back; probably needing to leave a lot of what you won behind. Stealing herds of cattle and putting them on your ship would most likely be out of the question.
Now the Chinese really had better fleets than the rest of the world for almost all of history; not many sea invasions, but they have a lot of big lakes and rivers where they've fought naval battles with thousands of ships. Apparently at one stage, they made a square floating island castle that was 1,000 feet on all four sides, and about 100 feet tall! Now this sounds awesome, and I kept thinking that if you had something like this slowly drifting down the coast of wherever you're invading, and used as a base for a fleet of longboats that can launch raids along the coasts rivers; so once you take your plunder, instead of needing to row three weeks back to Norway, you just row a few hours out to your sea fort, drop your stuff off, and then go back out tomorrow; so then you can do like 20 raids a year instead of just one.
Problem is, I couldn't find any detailed schematics or pictures of the castle boats or how they worked; and I'm fairly sure traditional (western) shipbuilding wouldn't support something like that, so I've needed to essentially redesign this thing from scratch in my head, including how to steer the thing. It's been an interesting couple of days; and the thing is turning out to be a rather awesome hot air balloon/ ship/ submarine/ fortress/ warehouse/ palace/ fishing trawler/ monument to insanity. 
I believe the big castle of a ship was the Battle of Chi Bi, where Wu forces led by Cao Cao chained their ships together.
I could be wrong but it should be right.
For the record; the "hot air balloon, submarine" part of my idea probably wouldn't work.
I'd been thinking how hot air balloons go up and down to catch different air currents going opposing directions, and I was thinking of a huge ship that could take on or release sea water to go up and down, adjusting the depth of underwater 'sea masts' to catch different ocean currents. Using wind power, you can adjust your sails on a sailboat to an angle to use any wind except heading directly against the direction you wanted to go, so I figured if you could just have two different options for your 'current' you could go in whatever direction you want.
Turns out your ship would need to be able to dip about 300 feet down to catch a secondary tide, rather than the 50 or so feet I was thinking you could dip your ship; and the surface lever tends to go in the same direction as the wind. On the upside, the winds and currents all more or less go in giant circles around the various oceans,
upload.wikimedia.org/wikipedia/commons/9/9b/Corrientes-oceanicas.png
So it's plausible you could just head around in a giant circle each year; raiding and trading whoever lived on the coasts of both continents that formed the edges of the sea you were in, before wintering in a harbor to maintain your ship.
Well... From your additional information and further consideration, I would think something like that would be implausible solely for the size.
Granted having a ship cum submarine is a plausible one but with the mix of lift from your hot air masts, it would be quite... Ineffective. I say that for three reasons:
1. The balloons would easily give away your position even if you submerged unless you somehow have a large enough ballast tank to sink it with but even then the size of the ballast tanks alone would prove to be very hindering in terms of mobility. Unless you can deflate it all the way, it's not plausible as much.
2. Inflating and deflating the balloon masts. Mind you I only know of the traditional way of gaining and lowering altitude with hot air balloons would be via the flame. Which presents the problem when over sea, on a large vessel that by theory be able to submerge. The amount of lift needed would be quite large which would need to include a large amount of hot air balloons.
3. Movement. Yes, as you have said you can float freely about however, without an easier way to propel the roving fort, one will very easily fall into a preplaced ambush or even a natural hurricane off shore.
Docking as well but that would fall under the latter problem category.
3014671
I think this is an example of the Castle Ship:
chinaculture.org/img/2005-06/21/xinsimple_270602211600792427337.jpg
Known as a "Lou Chuan," It was used as a command ship and doesn't appear to carry any armaments (apart from people). It's purpose seems to have been as an assault ship or as a troopship, due to mentions of it carrying many soldiers and horses. The crew was , from one account, "manned by more than 3000 men." I haven't found any descriptions of the actual size apart from a height of 33 meters.
The construction of such a ship capable of circumnavigating earth would be extremely difficult in any time period where raiding is a practical means of survival, not to mention the task of maintaining secure docks around the globe for resupply and maintenance.
The title of the blog post is also misspelled.
(The edits are coming I swear)
3016151 You mean brilliant? Meh, it happens. Because strokes of genius insanity.
3016151
I couldn't find any pictures, of it, or a detailed description, which is why I have to invent one myself.
Now I can't find the website I was on though, maybe I was mistaken? Oh well, too late to stop now.
3014715
To clarify; I was never intending to 'float' this with hot air (or helium etc) as the amount of hot air you need to lift anything is just plain ridiculous. (Theoretically, if you had an ultralight metal zeppelin, made by Ferric, whose 'balloon' you could completely pump empty of any air, so you had no mass instead of 4/5ths of the mass or air; you might have a steady airship if it was big enough, but that is a totally different story.)
The idea was inspired at how hot air balloons navigate; the wind might be heading north, but if you go up a mile then another current is heading south, so by going up and down you can go wherever you want. One of my first ideas for navigating a super ship was having underwater wooden 'sails' that could catch ocean currents, and the ability to pump water in and out of this ship to raise or lower the masts, to catch different currents.
This would not work. You'd have to be able to drop 100 yards to get a secondary current, the current is typically too slow, and several other issues. It was just an idea I was enjoying playing with in it's half-baked form. As for methods that might actually work to move a ship.
Sails would probably be ineffective. Taking the total surface are of the ship/ sails, (even with your fleet of longboats towing) and divide that by the amount of your ship that would be underwater, you'd have about one tenth of the ratio as a longboat has, which probably isn't going to get you that fast.
Belive it or not, I think oars could get you moving, not ones out the sides in pairs, like a longboat, but out the back like you see in a barge or a raft, which can be 3 X more efficient, but aren't used as much because putting more oars out the back would need a wider deck, displacing more water and making you go slower again. However, the more you make the boat with equal dimensions, rather then long and thin, the cargo capacity dramatically increases.
If you've got your 1,000 square ship, a fleet of lets say 120 longboats towing and 10,000 guys 'rowing' this would give you about 60% of the proportionate thrust of a longboat being rowed by 50 men. If you were able to hitch some horses to walk in circles, spinning a giant gear that moved a propeller, you might be able to reach the proportionate rowing power of a longboat. Given the wind wouldn't be helping you as much, you're probably still in for a longer journey to wherever you're invading, but if you raid and catch 20,000 slaves, you could potentially have a much faster ride back.
Another mode of propulsion I'm thinking about is include sharks; who cannot swim backwards. If you had tunnels in your ship that a shark could swim through but not turn in, used a hunk of meat to lure them in, then threw some kind of net over their face so they would have to tow you, it might work... for a while. Sharks sometimes congregate by the hundreds, so if you floated over one of these, started tipping meat in the water and caught 100 great whites, you might be going rather fast.
Your ship would sail about 8 miles off the coast, just on the edge of the horizon, and from there launch your longboats which could cover that distance in about an hour, weather permitting. You use your cavalry to capture all the herds you can, and bring them down to the beach. Your longboats all have a gangplank at the front of them, either for boarding another ship, or making a ramp when you land on the beach. One ship lands on the beach, and extends its ramp, a second ship comes up behind it, and fastens it's gangplank onto the back of the first one, and so on, quickly assembling a bridge from the shore to your capital class ship, and you herd the cattle/ captives/ loot on board.
Making your ship fire and hurricane resistant would be more complicated designs; I might explain them when they're move developed, but for now I've only been thinking about this for less than a week. Usually I think about my schemes for longer before posting anything online, sorry.
3017685 it's ok, this is the brainstorming stage after all. If you want ridiculous sized ships, look up the game Warship Gunner 2. It might be a game but it's actually the closest one can get to an actual working ship plan.
3017892
I looked over warship gunner, but I'm not sure it's what I'm looking for. These are more traditional ships, just a bit bigger; I'm contemplating completely rethinking the idea of a 'ship' so it doesn't even have a hull... which might be a bit foolhardy for someone without a degree in physics and only been interested in shipbuilding this year.
Though I am tickled pink that the game makers put in a version of Project Habakkuk
You know it's a bit ironic: I've been thinking through the physics of a floating island fort, and I remembered from my brief time of playing Warhammer Fantasy online that the dark elves had giant floating fortresses for raiding called Black Arks, so I looked them up to see what info there was too them. What keeps them floating? Magic. What propels them? Giant sea monsters.
And I roll my eyes in disdain.
Then I think about my floating raiding castle; from the perspective of someone whose never seen it before, but sees what looks like this giant stone castle racing towards them with no visible means of propulsion; and any ship that gets near it is torn apart from underneath. What are they going to say?
"Magic and sea monsters."
Let's say you're one of these 'viking raiders' and someone asks you how your vessel works. You don't want to go into the details of explaining it, and want to generate a reputation of power and prestige around yourself; so what do you say?
3017892
I will retract what I said earlier. Looking at Wargunner 2 is exactly the inspiration I needed, though not for the mega-craft I'd been thinking of.