I go by two hours past mouth of Merrimac. It varies a half hour either way depending on moon, wind, and river volume. A new moon tide just keeps going until you think the river will dry up. A strong upriver wind blows water upstream against the tide and delays the outgoing. A flooded river seems to go out sooner and stay out longer. The real question is, what's everybodies favorite tide stage for shad at R.V.?