Heading back south down the ICW (Inter-Coastal Waterway) through southeastern VA and NC, the shallow water in “the ditch” (the affectionate name for the ICW) becomes a problem. Nothing like the condition in Deltaville shown above with the recent Noreaster. In the ditch you must pay particular attention to the mariner’s Rule of Twelfths. Although this rule has its primary application in estimating the depth of water at certain times during the day, there are many lesser known applications….which we are happy to explain.
The Rule of Twelfths is based on the common tidal occurrence of two high tides and two low tides per 24 hour daily cycle and the generally sinusoidal pattern of tidal change. That being assumed, in the six hours between low and high water, the hourly depth changes are related to the tidal range or the difference in the tidal height and are: 1st hour is equal to 1/12 of the tidal range. 2nd hour is equal to 2/12 of the tidal range. 3rd hour is equal to 3/12 of the tidal range. 4th hour is equal to 3/12 of the tidal range. 5th hour is equal to 2/12 of the tidal range. 6th hour is equal to 1/12 of the tidal range.
So, an example of common tides, and problems encountered along the ICW in NC: if the tidal range is 3 feet, and we leave Swansboro NC at 0600 (that’s 6 AM for non-boaters), will High ZZ’s be able to make it thru the notorious trouble spot at the New River Inlet, approximately 12nm southwest of Swansboro, assuming High ZZ’s normal cruising speed of 6kts? Given that the water depths at low tide at the New River inlet are reported to be <6ft, and High ZZ’s draws at least 5ft 6in, this is a very common problem for High ZZ’s…given also that water <6ft deep is an all too frequent phenomenon along the ICW, which is supposed to be maintained by the Army Corps of Engineers at, you guessed it, 12ft MLLW. So, continuing on…the high tide at the inlet occurs at 0300 and @ 6nmph, High ZZ’s will reach the New River inlet ~ 0800, 5 hours after high tide. So 1/12 of a 3ft tide is 3 inches. So @0800, after 5 hours, the tide will have dropped 3+6+9+9+6= 33 INCHES!!! Holly crap! We will have water something <6ft +3 inches….
We did make it past the inlet, but only just. And that was only the beginning of, yes, you guessed it, 12 ICW trouble spots in our 60nm (5*12) trip from Swansboro to Carolina Beach that day, and 12 separate calculations (well some were no brainers) in our nearly 12 hour trip. Twelve hour trip you say? But the 6nmph speed of High ZZ’s should have been only a 10 hour trip to go 60nm? Ah, but there are other “rule of 12’s” that extend travel times. You see there are 12/3, or 4 restricted height bridges that must lift or swing open along this section of the ICW to allow sailboats to pass through. And, one might think, for ease of movement, that there would be 12 opportunities to open these bridges (every 5 minutes or so like stoplights). But sadly, no; some only open once on-the-hour, some on the hour and half-hour. And wouldn’t you know it, for some, it takes exactly 12 minutes longer than an hour or a half hour for High ZZ’s to travel between them at her 12/2, or 6nmph speed. So big deal you say! Just goose it a bit, hammer down, petal-to-the-metal… between bridges. Well 12 minutes in hour is 20% faster… or equal to our 7.2kt hull speed. Too fast to maintain for hour without risking an overheated engine. Twelve minutes in a half hour…impossible! And so to make the next hour or half-hour opening we just slow way down; so that makes the trip 12 hours long. Two weeks and 700+ miles down the coast we are at Cumberland Island GA enjoying the warmth of the deep south. A rough ride overnight from Southport NC to Savannah GA and a long day in 4-6ft swells from Wassaw Sound to Saint Simons GA and we are 2/3 of the way to Vero Beach FL. We will hang in FL for Thanksgiving, make a quick trip to San Diego for Xmas, and then High ZZ’s will head to the Bahamas early in January…for more warmth (and rum)!