Remember, if you are outside and the sea conditions are deteriorating, the break will be even worse with following breaking seas easily reaching 6' tp 10'. The dividing line between Nantucket sound and the Atlantic is Monomoy Island in Chatham. This wildlife sanctuary bar of sand is upwards of five miles long, the southern tip half reaching Nantucket from Chatham Light. Striper shoals on the north end and Pollock Rip. Pollock Rip, located off the southern end of Monomoy is considered the most dangerous navigable water north of Cape Hatteras.
Tidal currents can exceed three knots and standing waves, rips, can be more than ten feet high.
Three to six feet is usual. This is because a lot of water rounds Monomoy and the depth changes from as much as 150' to as little as 6', suddenly. The boater should exercise due caution in this area. Most boaters come here for the Striper fishing which is fairly consistent in the rips. The prevailing wind being Sou'west, the Islands are a beat to windward for the sailor in the Sound and an easy run back to the mainland.