77 Maritime LR3 Charon
I've built this huge, stock parts catamaran/rover for one purpose, to circumnavigate Kerbin in less than 24 hours and without refueling.
The engine is my strongest and most reliable to date, producing 1740 kN static thrust, using only 6 J-33 blowers. It's therefore very efficient.
Top speed on land is 144 m/s. Top speed in the water with tanks nearly empty is 81 m/s. It can easily traverse mountains and with tanks nearly empty it will happily climb