The following plot shows PHRF handicaps for a variety of common sailboats. Note that the Bénéteaus are split up, since there is a big difference between the faster First series and the slower Oceanis. The curve was a least-squares fit to the data assuming that average speed is proportional to the square root of the length.
handicap = H0 ( 1/sqrt(Length) - 1/sqrt(L0) )
The fit results are
H0 = 2580
L0 = 74.3 feet
L0 is the length where the average handicap should be 0. H0 gives a rough prediction for speed: Speed = 3600 * sqrt(Length) / H0. The handicaps are from www.phrfne.org