RESISTANCE AND POWERING
The total resistance (or drag) of a boat like "Spirit of Australia" or Ken’s new boat is made up of five major components: the pressure (or form) drag of the planing surfaces, the frictional drag of the planing surfaces, the pressure drag and frictional drag of the rudder, and the aerodynamic drag. These forces typically depend on the square of the speed; i.e. if you double the speed then the drag due to that component goes up by a factor of four. The power required therefore typically depends on the cube of the speed; i.e. if you double the speed then the power required goes up by a factor of eight. This means that, for boats travelling at record-breaking speeds, even though the areas are small the forces and power required are high.
The following calculations, for "Spirit
of Australia" and Ken’s new boat (which have similar dimensions), give an idea
of the forces and power required at 500 km/h (a little below the current world record) and
at 600 km/h (well above the current world record).
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One of the interesting things about the drag is the high
contribution of the rudder (both pressure and frictional) to the total drag. There must be
sufficient rudder in the water to be able to steer the boat at speed but, conversely, it
needs to be as small as possible to minimise the drag, and so there needs to be a
compromise. This led to the now-famous incident where Ken asked Professor Tom Fink (his
technical adviser in 1978) how he could increase the speed of "Spirit of
Australia". Tom recommended that he cut 65 mm off the bottom of the rudder. Ken promptly cut off 75 mm with an oxy-acetylene torch, and went out and set the current world record. |
Phil Helmore
Lawry Doctors
Thank you to Lawry and Phil for sharing their knowledge and giving us their time to produce the above information.