Reading NOAA Tide and Current Predictions for Trip Planning (MLLW, Slack Water and Bridge Clearance)

On NOAA Tides & Currents, open Tide Predictions and pick a station by state, by name, or on the map. Choose the one closest to where you'll actually be, on the same side of any inlet or narrow pass. A station at the ocean beach can be off by an hour or more, and by feet of height, from one a few miles up a creek.

The tide table lists each high and low for the day, with time and height. Check:

• Time zone and daylight saving time shown on the page
• Units (feet or meters)
• Datum, which should be MLLW for planning with charts
• Whether it's a reference station or a subordinate one (predictions estimated from a nearby station); subordinate stations give highs and lows only

The daily graph helps you see when the water is rising fastest, roughly in the middle hours between low and high.

NOAA defines Mean Lower Low Water (MLLW) as the average of the lower of each day's two low tides over the national tidal epoch, and states it's the reference datum for NOAA predictions and nautical charts. That means:

• A charted depth of 4 feet is the water you'd typically have at an average lower low tide.
• A predicted tide of +2.0 ft adds about 2 feet to the charted depth.
• A negative tide (say -1.0 ft) means the water is lower than the chart datum, so subtract that from charted depths. Negative tides are most likely around new and full moon.

Water under you = charted depth + predicted tide height (minus the effect of any strong offshore wind or high pressure).

Write down your boat's draft with the engine or drive down (and the depth needed to plane or to get off a trailer). For any shallow spot on your route, anchorage or marina entrance, add the predicted tide at the time you'll be there to the charted depth and leave a safety margin. Shoals move; charted depths in sandy areas may be old.

Open Current Predictions and find a current station for your channel, inlet or pass. The table shows:

• Slack times (the current turning)
• Max flood and max ebb times and speeds, in knots
• The direction of flood and ebb in degrees (true)

Some stations show predictions at a specific depth; use the one nearest the surface for boat handling.

• Inlets and narrow passes: aim to cross near slack or on the flood; avoid the strongest ebb against an ocean swell (our inlet guide covers why).
• Slow boats and sailboats: a 2-knot current with you or against you can be a big part of your speed. Time long legs to ride the current.
• Docking in a river or tidal marina: note which way the current will be running when you come back, and plan to approach into it (our docking guide covers how).
• Fishing: many anglers time trips around moving water; just remember the current also changes how your boat drifts and how the anchor holds.

NOAA charts list bridge vertical clearances above Mean High Water (MHW), a different datum from the depths (NOAA's bridge-clearance update for the Lower Mississippi confirms charted clearances there were referenced to MHW, with a special river correction; read the clearance note on your own chart or in the Coast Pilot). In practice:

• At an average high tide, you have about the charted clearance.
• At an above-average high tide (spring tides, onshore wind, storm surge), you have less than charted.
• At low tide you usually have more, by roughly the difference between MHW and the current water level.
• Many bridges have clearance boards (gauges) on the fender system showing actual clearance; trust those over math. NOAA's Coast Pilot describes them as vertical number scales that read from top to bottom and show the actual clearance between the current water level and the lowest point of the bridge over the channel, normally on the right-hand pier or abutment, on both the upstream and downstream sides.
• The Coast Pilot confirms the convention: bridge and overhead cable clearances are in feet above mean high water unless otherwise stated. The Great Lakes volume (Coast Pilot 6) is an exception and uses Low Water Datum, so read the note for your area.
• Overhead power cables are a separate danger for sailboats and boats with tall antennas or outriggers. The Coast Pilot notes that charted cable clearances are for the lowest wire, that weather, ice and time can reduce them, and that electricity can arc to a mast without touching it, so the Coast Guard and OSHA advise a safety buffer below power lines. When a chart or Coast Pilot gives a "safe vertical clearance" for a cable, use that figure, not the plain clearance.

Know your boat's air draft: the height from the waterline to the highest point (antenna, light, tower, radar, mast top) with any fold-down gear in its up position. Leave a margin, because waves and wakes lift the boat.

Many tidal ramps are fine at mid and high tide and unusable near low:

• The end of the ramp may be exposed, and trailers can drop off the end
• The tow vehicle may have to back in too deep to float the boat
• Mud and algae on the lower ramp are slippery (our ramp traction guide covers that)

Check the low tide for your launch and return times. If you're coming back at low water, scout the ramp the first time at low tide or ask the ramp operator. Our solo launch guide covers the ramp routine.

When you anchor in tidal water, figure the scope for the highest water you'll see while anchored, not the depth when you drop it. Example: 8 feet at low tide with a 5-foot range means about 13 feet at high water, plus the height of your bow roller above the water. Also check you'll still float at the next low tide, and that the boat swinging 180 degrees when the current turns won't put you on a shoal or another boat. Our anchor scope guide shows the math.

If you need the height between high and low, the old rule of twelfths gives a rough estimate on a regular, twice-daily tide: in the six hours from low to high, the water rises about 1/12 of the range in hour one, 2/12 in hour two, 3/12 in hours three and four, 2/12 in hour five and 1/12 in hour six. It doesn't work well for irregular or once-a-day tides (common in the Gulf); use NOAA's graph instead.

Many NOAA stations also show observed water level next to the prediction. If the observed line is running a foot under the prediction because of a strong offshore wind, plan for that, and expect the same at your shallow spots. In some busy ports, NOAA's PORTS system shows real-time currents and water levels too.

Write down for the day: low and high times and heights, slack and max current times at your inlet, and your bridge and ramp limits. Add them to your float plan so your contact knows when you planned to cross.