Answer
The horsepower to drive a boat needs to be eight times greater.
Work Step by Step
Let $h$ be the horsepower.
Let $s$ be the speed of boat.
$h=ks^3$
When speed is doubled,
$k(2s)^3=8ks^2$
Since $ks^3=h$, $8ks^3=8h$
The horsepower to drive a boat needs to be eight times greater.
