Really old secret projects (1800s and earlier)

[Bottleship]

Ivanov "three-keel air-swimmer" ("трёхкильный аэроплав"), Russia, 1853

 

[Bottleship]

Projects of ships with turrets, 1850-1860s, Cowper Coles:
Raft with gun:

Gun ship:

First project of full-size ironclad, ten double-barrel turrets (rebuilt of wooden ship-of-the-line)

Known about later projects with single-barrel turrets, 4000 ton with six 68-pdr guns, later - five 7-inch RML guns, later - four 9-inch RML guns.
Unknown British six-turret twelve-gun ironclad project (1860s?):

 

[publiusr]

Like something from ONE PIECE, some of these designs. With modern materials, you might have even more elaborate sailpunk.

 

[Bottleship]

Weird ships from medieval manuscripts:
Drill ship (da Vinci?)

Valturio manuscript:

 

[cattalonX]

独家揭秘丨“泰兴”号，考古发现的最大中国古代沉船？

mp.weixin.qq.com

Chinese trade ships in the 18th and early 19th centuries
The largest is about 60 meters long

 

[Grey Havoc]

On a tangent, regarding the Roman navy:

View: https://www.youtube.com/shorts/bly_3It5FcY

 

[Grey Havoc]

View: https://www.youtube.com/watch?v=LDoIhUPyaxo

 

[Nik]

Wow...

FWIW, the infamous circular Russian river-monitors seem to have been inspired by that formidable English treatise...

 

[Dilandu]

FWIW, the infamous circular Russian river-monitors seem to have been inspired by that formidable English treatise...

Sigh.

1. They weren't.
2. They weren't river monitors, they were coastal defense ships.
3. They weren't exactly "infamous", because they done their main function - deter enemy ships from attacking important ports and harbors - perfectly well. Their whole concept was basically caused by poor state of Russian Black Sea shipyards by 1870s; after Crimean War, Black Sea was demilitarized, so Russia have no reason to spend much on military shipbuilding here. So after limitations were removed, the shipyards on Black Sea were in rather outdated condition and struggled to build modern ironclad warships.

The Popovka's were the solution. Due to their simple hull shape - all frames and plates are the same - they could be constructed even by unexperienced workers. And they could carry much more guns & armor in limited displacement than "proper" ironclad of same size. Considering that Ottoman Empire have a massive ironclad fleet in Med, and Russia just started to rebuild Black Sea fleet, unorthodox solution was the only way to deter Ottomans.

 

[T. A. Gardner]

A lessor known Confederate submarine project of the ACW was the Bayou St. John submarine found in 1878 in the bayou it is named for.

On display about 1890.
Today it's on display at the Louisiana state museum

Today a picturesque waterway, Bayou St. John once harbored a Civil War submarine

Imagine this. It’s Bayou Boogaloo and you’re on the raft you built with your buddies. You plunge your oar into the waters of Bayou St. John and feel it scrape

www.nola.com

Nothing is known of the designer(s) or builder(s) and the sub was scuttled towards the end of the Civil War and likely any documentation was destroyed at the same time. But the sub itself does still exist so there's that...