To run 70mph in a fountain the boat needs to be further up on plane with the nose higher, a high speed turn would leave less boat in the water to catch. if the boat was trimmed down and an aggressive move was made then the boat is more likely to roll. Most people would not think that a turn at 40mph is dangerous and more mistakes are likely to be made.
Check out the following link, starting on page 47. This tells of the typical handling characteristics of step hulls and how to run them properly.
http://www.fountainpowerboats.com/co...al05online.pdf
Here is an excerpt:
HULL STEP TECHNOLOGY HAS CHANGED THE WAY WE NEED
TO DRIVE!
WHILE SMOOTH OPERATION HAS ALWAYS BEEN THE KEY TO
SAFE HIGH SPEED OPERATION, BEING SMOOTH AND USING
COMMON SENSE IS EVEN MORE CRITICAL WITH TODAY’S
FASTER, MORE TECHNOLOGICALLY ADVANCED HULL
DESIGNS.
THE NEW TECHNOLOGY HAS ALLOWED LARGER, LONGER,
HEAVIER BOATS TO TRAVEL AT MUCH FASTER SPEEDS, WITH
STANDARD HORSEPOWER. THE SIMPLE FACT IS THAT
THINGS HAPPEN MUCH FASTER AT HIGHER RATES OF
SPEED, AND LARGER BOATS SIMPLY CARRY MORE ENERGY
ONCE IN MOTION.
INCORRECT TRIM SETTINGS OR IRRATIC TURNING MANEUVERS,
WHILE STILL DANGEROUS AT SLOW SPEEDS, CAN BE
DISASTROUS AT HIGH SPEEDS.
AT HIGHER RATES OF SPEED, THERE IS LITTLE OR NO WARNING
BEFORE THE BOAT REACTS TO RAPID TURNS OR INCORRECT
AND/OR ABRUPT TRIM CHANGES.
COMMON SENSE IS YOUR BEST DEFENSE!
NONE OF US WOULD TAKE A NEW CORVETTE OUT, RUN IT UP
TO 60MPH, GRAB THE WHEEL AND MAKE AN ABRUPT 180
DEGREE TURN ON THE WHEEL. THE SAME CONCEPT
APPLIES TO BOATS.
KNOW YOUR LIMITATIONS, AS WELL AS THE LIMITATIONS OF
THE EQUIPMENT, AND DRIVE WITHIN THEM.