198
T116 Last Resource
pressures on the back of the blades can become so low that the water
there starts to ‘boil’, forming minute bubbles of vapour. This
effect called cavitation, disrupts the pressure around the blade and,
ultimately, reduces the efciency of the entire propeller. To make
matters worse, where the bubbles collapse—generating high transient
pressures—the surface of the propellers becomes pitted and eroded.
ln some experimental craft, erosion has been so severe that pro—
pellers have lasted little more than a day or so of high—speed
operation.
To solve the cavitation problem, ‘supercavitating’ propellers have
been designed. These propellers, which operate at the high speeds
demanded by hydrofoil and other high—speed craft, encourage rather
than discourage the creation of cavitation bubbles. They avoid
erosion and loss of power because the bubbles swirl away from the
immediate surface and collapse downstream of the blades.
Because of diîculties With cavitation in propellers and trans—
mitting power
to them, hydrofoil designers have studied alternative
ways of providing the necessary thrust, including air propellers, jet
engines and water jets. The most promising of these various
systems seems to be the water jet (see gure 24). ln one successful
craft, the Tucumcari designed by Boeing, a 3,200—h0rsepower gas
turbine engine drives a water—jet pump. Water is sucked through the
rear foil struts and up into the pump, which in turn forces jets of
water out through nozzles in the hull. At speeds of more than 40
_
knots, the craft pumps 100 tons of water per minute. A smaller
water—jet propulsion unit driven by a diesel engine provides the
thrust for bull—borne manœuvres. Launched in july 1967, the
71-foot craft, designated PGH—2 (Patrol Gunboat Hydrofoil), is
operated by the US. Navy.
Tire submm‘ine deterrent
ln keeping with man’s aggressive nature, the most startling
development in marine vessels started in 19 54 with the launching of
the Nautilus, the United States’ rst nuclear submarine. lt came
close to completion on july 20, 1960, with the successful underwater
launching of two Polaris missiles by the George Washington. Within
the space of these few years, the submarine, already a potent
weapon, had become a decisive factor in international conict.
The rst Polaris (AI) missiles had a range of 1,200 nautical miles
but in later versions this was improved to 2,500 nautical miles. Since
T116 Last Resource
pressures on the back of the blades can become so low that the water
there starts to ‘boil’, forming minute bubbles of vapour. This
effect called cavitation, disrupts the pressure around the blade and,
ultimately, reduces the efciency of the entire propeller. To make
matters worse, where the bubbles collapse—generating high transient
pressures—the surface of the propellers becomes pitted and eroded.
ln some experimental craft, erosion has been so severe that pro—
pellers have lasted little more than a day or so of high—speed
operation.
To solve the cavitation problem, ‘supercavitating’ propellers have
been designed. These propellers, which operate at the high speeds
demanded by hydrofoil and other high—speed craft, encourage rather
than discourage the creation of cavitation bubbles. They avoid
erosion and loss of power because the bubbles swirl away from the
immediate surface and collapse downstream of the blades.
Because of diîculties With cavitation in propellers and trans—
mitting power
to them, hydrofoil designers have studied alternative
ways of providing the necessary thrust, including air propellers, jet
engines and water jets. The most promising of these various
systems seems to be the water jet (see gure 24). ln one successful
craft, the Tucumcari designed by Boeing, a 3,200—h0rsepower gas
turbine engine drives a water—jet pump. Water is sucked through the
rear foil struts and up into the pump, which in turn forces jets of
water out through nozzles in the hull. At speeds of more than 40
_
knots, the craft pumps 100 tons of water per minute. A smaller
water—jet propulsion unit driven by a diesel engine provides the
thrust for bull—borne manœuvres. Launched in july 1967, the
71-foot craft, designated PGH—2 (Patrol Gunboat Hydrofoil), is
operated by the US. Navy.
Tire submm‘ine deterrent
ln keeping with man’s aggressive nature, the most startling
development in marine vessels started in 19 54 with the launching of
the Nautilus, the United States’ rst nuclear submarine. lt came
close to completion on july 20, 1960, with the successful underwater
launching of two Polaris missiles by the George Washington. Within
the space of these few years, the submarine, already a potent
weapon, had become a decisive factor in international conict.
The rst Polaris (AI) missiles had a range of 1,200 nautical miles
but in later versions this was improved to 2,500 nautical miles. Since