  We numerically study the Loewner driving function U_t of a site percolation
cluster boundary on the triangular lattice for p<p_c. It is found that U_t
shows a drifted random walk with a finite crossover time. Within this crossover
time, the averaged driving function < U_t> shows a scaling behavior -(p_c-p)
t^{(\nu +1)/2\nu} with a superdiffusive fluctuation whereas, beyond the
crossover time, the driving function U_t undergoes a normal diffusion with
Hurst exponent 1/2 but with the drift velocity proportional to (p_c-p)^\nu,
where \nu= 4/3 is the critical exponent for two-dimensional percolation
correlation length. The crossover time diverges as (p_c-p)^{-2\nu} as p\to p_c.
