THREE decades after movie audiences were terrorised by Jaws , there is scientific evidence a great white shark really could munch a hole in a boat.

A team led by Stephen Wroe, an animal mechanics expert from the University of NSW, has come up with the best estimate yet of the power of a great white's jaws. The largest, females more than 6 metres long, bite with a force of up to 1.8 tonnes.

That is more than 20 times the maximum force of a human bite, about 80 kilograms of pressure. Even large African lions only bite with 560 kilograms of force.

"Certainly a big female great white could rip the bottom out of a wooden boat," Wroe says. After all, they have been known to seriously damage submarine sonar systems.

But the flaw in Jaws , Wroe suspects, is not the shark's power, but its nature. "They are surprisingly cautious predators. The perception that they are mindless, aggressive, feeding machines is something of a misnomer. They are actually quite timid."

The probable reason is that "their teeth are surprisingly delicate. Like very fine serrated blades, they are very well adapted to slicing through flesh but not so well adapted to slicing through bone. As far as we know they go out of their way to avoid biting anything that will break their teeth."

While sharks replace broken teeth, biting a hole in a boat would leave them "out of action for weeks, at the very least".

The sharks' normal hunting behaviour involves chomping chunks out of a seal or another marine mammal to disable it, and then retreating while it bleeds.

The tactic makes sense. "That's a very efficient and safe way to do it [make a kill]," Wroe says. The shark doesn't have to "get up close and personal" with its prey. "They do not have very big brains. Nonetheless, their behaviour is surprisingly complex and sophisticated."

It also explains why most great white attacks on people were really "test bites" to see if they were worth eating. "Of course, when you are talking about a tonne of shark, even a test bite can be fatal."