Plausability check: floating castles

:confused: I'm wondering the technical probability and viability of what's practically mobile sea fortresses with 15th to 17th century technology.

First, a slab-like concrete ship, since concrete was already known by the romans and it floats. I believe it would probably look somewhat similar to the japanese atakebune or tekkõsen. The propulsion system still eludes me.

The second would be a catamaran-style hull inspired by the Tessarakonteres of Ptolemy IV, propeled by waterwheels which in turn are powered by horizontal windmills. I chose horizontal windmills because they can be enclosed inside towers for greater protection during battle, but I'm wondering if the ship would actually be able to move at all. The crude drawing attached is meant to represent this ship.

Is it possible?

Ps: I'm sticking to solely the technical aspects because this is meant for a fantasy setting, and it isn't in ASB because of my personal guideline is to refrain from magic justification unless necessary.

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The problem with concrete is that a ship's hull needs flexibility as well as strength which wood has and ferro-concrete has but basic concrete not so much.

That said I am fairly surely that wood, perhaps with some fire resistant cladding would probably do for your floaty fortress. I expect it would move like a pregnant sow that has been over eating and has tired feet but I doubt it would be much more sea worthy than a brick anyway so likely something to be kept in a lake or close inshore and used to show off to the neighbours.
 
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Concrete might indeed not be the best choice, not least because it doesn't work well for structures that size without reinforcement. But wood should actually be doable. There are historical examples of ships approaching this size, and they were seagoing (admittedly, for a given value of 'seagoing' in many cases - the practical limit for wooden hulls seems to be around 60-70 metres).

The question is - why. It's not going to be terribly practical. Propulsion will be a problem. In any kind of wind, the tower will act like a sail, putting strain on the hull, and I don't think you can get enough windmill surface or cram in enough paddlewheels to counteract that. Going to sea would be extremely hazardous. It would probably be limited to inland waters, and avoid large lakes for fear of being caught in storms. The hull will need regular maintenance and repairs, which would require some kind of beaching apparatus - hard to envision with a behemoth like that. I could see it as a kind of vanity project or feasibility study, something like the Lake Nemi ships or Cleopatra's floating palace, but not as a regular feature of battlefleets. You'd lose too many before you even made contact with the enemy, and it's hard to see what good this thing would do in a fight.
 
The limiting factor on the size of a wooden ship has more to do with the size of the frame than anything else; the generation after the end of the Napoleonic wars were wooden hulls on iron framework and could considerably exceed the limits of naturally grown knees.

In theory you could get some fairly enormous clinker built ships, the skin is the strength component, but there are natural size limits there too- Henry Grace a Dieu being one of the largest successful at almost, reconstruction suggests, three thousand tons modern measure. Otherwise they tend to work too much to be secure in a seaway.

A wooden skin on a cement frame, now, that might work- could push into the five to six thousand ton range of the largest clippers and the last of the line of battle; masts can be, as the last of the cogs' masts were, composite, and should be enough without the complex and inefficient staffing about with windmills.

The tendency of the frame to break rather than bend, though, that's the worry- as an operational solution, have the ship's ballast made up of concrete joists; when one breaks in use, dump it in the ballast and pull out a fresh one.
 
The problem with concrete is that a ship's hull needs flexibility as well as strength which wood has and ferro-concrete has but basic concrete not so much.

That said I am fairly surely that wood, perhaps with some fire resistant cladding would probably do for your floaty fortress. I expect it would move like a pregnant sow that has been over eating and has tired feet but I doubt it would be much more sea worthy than a brick anyway so likely something to be kept in a lake or close inshore and used to show off to the neighbours.

Concrete might indeed not be the best choice, not least because it doesn't work well for structures that size without reinforcement. But wood should actually be doable. There are historical examples of ships approaching this size, and they were seagoing (admittedly, for a given value of 'seagoing' in many cases - the practical limit for wooden hulls seems to be around 60-70 metres).

Flexibility AND strength, huh? I admit that this was mostly an attempt at a dwarven-looking ship without much practical concern.

The question is - why. It's not going to be terribly practical. Propulsion will be a problem. In any kind of wind, the tower will act like a sail, putting strain on the hull, and I don't think you can get enough windmill surface or cram in enough paddlewheels to counteract that. Going to sea would be extremely hazardous. It would probably be limited to inland waters, and avoid large lakes for fear of being caught in storms. The hull will need regular maintenance and repairs, which would require some kind of beaching apparatus - hard to envision with a behemoth like that. I could see it as a kind of vanity project or feasibility study, something like the Lake Nemi ships or Cleopatra's floating palace, but not as a regular feature of battlefleets. You'd lose too many before you even made contact with the enemy, and it's hard to see what good this thing would do in a fight.

Does the attached image's propulsion looks doable?

It's not meant to be a regular in any case. Its purpose is to be a flagship and convincent distraction anyway. The lack of a suitable dry dock is a big problem though, hadn't thought of that...

The limiting factor on the size of a wooden ship has more to do with the size of the frame than anything else; the generation after the end of the Napoleonic wars were wooden hulls on iron framework and could considerably exceed the limits of naturally grown knees.

In theory you could get some fairly enormous clinker built ships, the skin is the strength component, but there are natural size limits there too- Henry Grace a Dieu being one of the largest successful at almost, reconstruction suggests, three thousand tons modern measure. Otherwise they tend to work too much to be secure in a seaway.

Wouldn't wooden ships with iron framework exceed 17th century technology?

A wooden skin on a cement frame, now, that might work- could push into the five to six thousand ton range of the largest clippers and the last of the line of battle; masts can be, as the last of the cogs' masts were, composite, and should be enough without the complex and inefficient staffing about with windmills.

The tendency of the frame to break rather than bend, though, that's the worry- as an operational solution, have the ship's ballast made up of concrete joists; when one breaks in use, dump it in the ballast and pull out a fresh one.

That sounds somewhat promising, but would the crew manage to replace a concrete joist during a voyage?

I'm now thinking of a 80m catamaran, based on sails, wood skin over cement frame, munz metal sheathing and an armored upper deck based on turtle ships. Does that sounds more (relatively) sensible? I don't plan for it to move during battle, only to go where necessary by its own power.

Would ships like this be sea worthy enough for, say, the Mediterranean?

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