I have a very-VERY stupid and crazily expensive idea:

[hider=Ymheodr(Emperor)-class Fast Battleship]
[img]http://images.wikia.com/starblazers/images/6/64/Battleship_greatyamato.jpg[/img]
A battleship with submarine-like streamlined design, underwater cannons (against submarines of course :lol) and a [b]FREAKIN' WATER-BOILING JET ENGINE![/b]
Yup, somebody had the [b]SHINING IDEA[/b] to build jets on water. Not waterjets. Those are completely sane. It's a giant boiler which uses the steam for propulsion through the waters like a classic jet does! The engine needs 10 minutes of preparation but when ready by my minimalistic estimates it can accelerate the 20,000 ton warship at 1.5G and the top speed of 100 knots! This is the only warship in the world which requires seatbelts! 
[b]FOR THE EMPEROR'S GLORY![/b]

EDIT: Of course it has yet to be built but when it's done it'd possess the single most massive jet engine know with roughly 15 meters exhaust diameter.
...Given if it won't blow up during the test run.
:lol :lol :lol :lol
[/hider]
Gnomes, step up your game right now!
:lol:lol:lol:lol:lol:lol