Video: Californian inventor takes water bike out for a spin

Everyone knows that the correct number of bikes to own is n+1, where n is the number you already have. But how do you choose +1? If you live by a lake, maybe you need a water bike like this, the Schiller X1.

Pedal-powered watercraft are nothing new, of course, but with a cruising speed of about 9mph, this Schiller X1 is rather faster than the pedalos on your local lake.

Schiller claims it's the most advanced water bike ever made, and while a few human-power wizards (see below) might dispute that, it certainly has a bunch of clever features.

The Schiller X1 uses twin oscillating propellors so there's no need for a rudder, and has a Gates Carbon drive belt to avoid the obvious problems of running a chain in wet conditions. It can even go in reverse.

Company founder Judah Schiller says the X1 can be assembled and dismantled in ten minutes and packs down small enough to fit in the boot of a car, though being he's from Marin County, California, he may be thinking of the trunk of a Lincoln Continental and not the boot of a VW Golf.

Here it is in action:

Before you decide this is n+1, you better check with the bank manager. The base model costs $6,495, or about £4,000 and the limited-edition Founder's model, which comes with a guarantee that Schiller will retro-fit any performance-improving technology developed in the next two years, is $8,775 (about £5,400).

Full details from Schiller Bikes.

Hydrofoil waterbikes

Impressive as the Schiller X1 is, if you genuinely want to go fast, you don't want hulls in the water dragging you down: you want hydrofoils.

The fastest human-powered watercraft ever built is the MIT Decavitator, which used a propellor in the air for thrust and hydrofoils to lift it out of the water as much as possible, Ridden by Mark Drella, Decavitator reached a speed of 21.28 mph in 1991, despite sounding like a particularly cheesy Dr Who baddie.

Here's the Decavitator in action:

And here's Olympic gold medallist Steve Hegg aboard the first incarnation of Decavitator's great rival, Flying Fish, which used a propellor in the water and therefore ended up a bit slower, at 19.21 mph.