it could not produce a consistent performance.

The best it could achieve was nowhere near

what we needed, and some readings even

showed the knobbly producing negative thrust

– it was trying to drag me into the Tamar! We

also experienced a major problem with the

wheel suddenly starting to spin faster and

the engine revs rising. When I backed off the

throttle, I quickly found the wheel slowing

down and the revs dropping too low. If this

was what it was going to be like when I was

attempting to hydroplane, I would never be

able to maintain a steady thrust. Graham

explained that what we were seeing here

was cavitation.

Cavitation happens when forces acting on the

water, such as those produced by a propeller or

our Suzuki’s rear wheel, cause rapid changes in

pressure. This can lead not only to areas of high

and low pressure, but also to a void forming

for an instant – almost like a vacuum bubble.

When the water rushes into the void it does so

with enormous force, and when those tiny jets

of water smash into a metal propeller they can

cause pitting on the surfaces of the blades. For

us, cavitation would mean the tyre tread, or the

sand tyre paddles, plunging into the water as

the wheel revolved and trying to push against

a void, or an area of low pressure, where they

would encounter little or no resistance. That

would let them push through really easily,

allowing the rear wheel to spin faster and

causing the engine revs to shoot up. It would be

like the rear wheel skidding on ice or someone

burning rubber doing doughnuts on a race

track – a lot of noise and energy expended

without any forward thrust being generated.

In short, cavitation would leave me dead in the

water and I would sink.

The hope was that, in deeper water when

we would have good forward momentum,

158  hydroplaning motorbike

Above: We had plenty of gear with us to work out our

thrust measurements – everything bar the kitchen sink

. . . and the bathroom scales!

the rush of water would prevent any voids

from forming and we wouldn’t have a problem

with cavitation. Maintaining the momentum

and keeping the rear wheel at the optimum

depth would help. Again, it was all coming

down to balance.

Cavitation aside, it was clear that the knobbly

probably wasn’t going to do the job for us,

so we fitted the sand tyre to the rear wheel

instead. The difference was amazing. We were

seeing up to 100 kilograms of thrust, more than

enough for our purposes. That made the choice

of rear tyre very easy and we could finally drag

the test rig back up the slipway. I returned the

scales to the gentleman who had kindly lent

them to us. They were a lot drier than I was, but

nothing could dampen my spirits at that point.

I was going to rip across that water!

Right: The knobbly tyre

produced a fine plume

of spray but a very

disappointing thrust reading.