Seeking a market niche somewhere beyond luxury, Rolls-Royce Motor Cars is offering the discriminating buyer its most expensive model yet: a $340,000 convertible.

To Chief Executive Chris Woodwark, an equivalent of the new Bentley Azure cannot be found on four wheels: “For a customer purchasing this vehicle there can be no comparable choice of car. The true alternative would probably be a boat or private plane.”

Driving a Bentley Azure is in fact a bit like steering a boat, because the huge car with the gas-guzzling 6.75-liter engine doesn’t exactly take sharp corners easily.

The new car, to be introduced Tuesday at the Geneva Motor Show, combines what is vaunted as impeccable British craftsmanship with an eight-cylinder turbocharged engine that delivers 350 horsepower at a top speed of 150 mph.

It also gets poor gas mileage: 9.9 miles per gallon in the city, 22 mpg in highway driving.