Jet Boats

This week's challenge is to build a jet boat to go round a slalom course, and over a speed test. The boat must be driven by a jet, not a propeller.

The Hairy Hogs are motorbike racers from Northern Ireland. Conn Kelly, Darrell Hannah, and Alistair Nethercote are playing in maroon suits, and their expert is Alan Pickard. Their plan is to use an impeller pump - the pump will suck in
water, and blow it out the back end. With a directional output nozzle, it should be very manoeuvrable.

Team Ragbo - Glynn Myers, Colin Bater, Nick Mellor, and expert Alex Wallace - are playing in lime green. They're recycling merchants from Todmorden in northern England. This team is going to use a centrifugal pump. This type of
pump takes in water through a hole in the middle, accelerates it up, then spins it out to provide the driving force.

Both teams will need to find a good, strong, sturdy hull. Ragbo's, especially, will need to be able to carry a lot of weight, as neither centrifugal pumps nor the engines to power them are particularly light. They do, however, have the
easier build; the impeller pump will have to be constructed from scratch.

The judge reckons that Ragbo's boat is very top-heavy, and they'll need a good boat. They have no fewer than three hulls available - one is not too manoeuvrable, one is too heavy, and one is flatter than the expert would like. No such hull for the Hogs, they find a nifty little number (albeit with holes) and set to work.

The engines and pumps come out of a muddy heap in time. The Hogs' impeller came from an air conditioning unit, and is about the size of a hand. They still need to construct a tight fit around the impeller, and let the Venturi effect speed up their jet. Ragbo have a large cast-iron pump, which means they'll have to use the V-shaped hull otherwise the craft will tip over; that's going to create extra drag and cause them to go slower than they might.

The Hogs have a large petrol engine; Ragbo a smaller, diesel motor that isn't going to be as powerful. Ragbo also have the heavier craft, but their centrifugal design is new design - it could yet pack a huge punch. The Hogs have had to compromise when making their pump, using metal clips to hold the driving arm in place. Oh, and they'll have to hole their boat below the water line to fire the jet out. Ragbo may have started slowly, but they catch up by dint of very hard work. It's close, but Ragbo just manage to complete their build in the time allowed.

First up is the sprint leg - all they've got to do is put the pedal to the metal and go like the clappers. The Hogs go first, and the back is low in the water. The ballast might be wrong, and there's some smoke coming out of the back. They finish in 1:47.4. Ragbo's boat takes forever to get going, but when it move, it really moves. The boat's big, but it's stable, and finishes in 1:49.5.

Then there's an obstacle course: through a channel of decreasing width, round a slalom section, then a roundabout, and home. Ragbo go first, and they run into problems thanks to their back-to-front steering. As he approaches the slalom, the boat's radiator rather explodes, and he can't really fix that while on the move. The Hogs simply need to finish the course to progress, but they looked a bit unstable last time out. They've made some changes so it can go faster, and complete the course in 2:28 to win the week.