Tides and Tidal Currents — Height of Tide, Depth Under the Keel, and Slack Water

What the rule requires

Soundings on US charts are referred to Mean Lower Low Water, and MLLW is the same datum from which the tide tables predict the height of tide Bowditch Ch. 9 §904. Because the sounding and the prediction share a datum, they add. Actual depth equals the charted sounding plus the predicted height of tide, and underkeel clearance is that actual depth less the vessel's draft.

Heights of features — bridges, overhead cables, lights — are referred instead to Mean High Water, so vertical clearance shrinks as the tide rises. One tide, two datums, and the correction runs in opposite directions above and below the waterline. That single asymmetry decides more exam questions on this topic than anything else in the chapter.

Charted sounding 12 feet, predicted height of tide 3.4 feet, your draft 8 feet. What is the underkeel clearance, and what would the answer be if the question asked for depth of water?

Actual depth is 12 + 3.4 = 15.4 feet. Underkeel clearance is 15.4 − 8 = 7.4 feet. If the question asked only for depth of water the answer is 15.4 feet — the draft is not applied. Read which of the two quantities is being asked for; both appear as distractors in the same set of options.

Which book answers which question

The NOAA Tide Tables predict the times and heights of high and low water. The Tidal Current Tables predict the times of slack water and of maximum flood and ebb Bowditch Ch. 9 §905. Both publish those predictions for reference stations; for a nearby subordinate station you obtain the prediction by applying the tabulated time and height differences to its reference station. Before you apply anything, confirm the time zone of the table and whether daylight time is in force.

Vertical against horizontal

Tides are the periodic vertical rise and fall of sea level arising from the gravitational interaction of moon, sun and earth, and the principal lunar tide has a period of about 12 hours 25 minutes Bowditch Ch. 9 §901. Tidal currents are the horizontal flow of water that accompanies that rise and fall: flood is the incoming current, ebb the outgoing, and slack water the brief period of zero flow at the turn Bowditch Ch. 9 §903. The common device is flood fills, ebb empties — sound as far as it goes, and it says nothing about height, which is the whole point of keeping the two ideas apart.

Range: springs and neaps

Spring tides occur near new and full moon and have the greatest range; neap tides occur near first and last quarter moon and have the smallest range Bowditch Ch. 9 §902. The device in circulation is springs spring up at new and full moon; neaps nap at the quarters, and it is worth using — but it is lossy in one way that gets candidates marked wrong. What is greatest at springs is the range, not merely the high water. Springs run higher highs and lower lows; neaps do both less. A candidate who reads "spring" as "more water everywhere" will pick the wrong answer on any question about crossing a shoal at low water.

Two other things about a spring tide: it has nothing to do with the season, and the tabulated prediction already accounts for it. You do not apply a spring correction to a tide table.

Currents that are not tidal

Surface ocean currents are driven primarily by wind, with major gyres in each ocean basin; tidal currents are driven by the tide rather than by wind and reverse direction with the tide Bowditch Ch. 24 §2401. For working out the effect of any current on your track, the relation is that course steered plus the drift vector equals the course made good, and the triangle inverts to give the course steered needed to make a given track over ground Bowditch Ch. 24 §2402.

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Telling it apart

The criterion that separates every pair on this topic is whether the quantity is vertical or horizontal, and it decides which publication you open.

• Tide — vertical. Rise and fall of sea level, measured in feet. Answered from the Tide Tables: times and heights of high and low water. Misfiled case: a question asking when the water will be deep enough over a bar, which candidates sometimes take to the Current Tables because the word "flood" appears in the stem.
• Tidal current — horizontal. Flow measured in knots, reversing with the tide. Answered from the Tidal Current Tables: slack water, maximum flood, maximum ebb.
• Flood and ebb — the direction of that horizontal flow, incoming and outgoing. Neither term is a statement about the height of tide.
• Slack water — the brief zero-flow period at the turn of the current, and a Tidal Current Tables entry. A question asking for a time of slack is not a Tide Tables question.
• Surface ocean current — wind-driven, organised into the ocean gyres. It is the tidal current, not the ocean current, that reverses.

And the datum pair, which is the same distinction one layer down:

• MLLW — the datum for charted soundings and for predicted heights of tide. Add the prediction to the sounding.
• Mean High Water — the datum for charted heights of bridges and lights. Rising tide reduces the clearance.

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Working a question

You intend to cross a bar charted at 9 feet on the way into a subordinate station. Your draft is 7 feet 6 inches and you want 5 feet under the keel. The reference station tabulates high water at 1106, height 6.2 feet. The subordinate station differences are time +0h 34m and height −0.8 foot on high water. The tables are Eastern Standard Time; daylight time is in force.

1. Decide which publication. The question asks for depth of water, a vertical quantity. Tide Tables, not Tidal Current Tables.
2. Take the reference station prediction. High water 1106, height 6.2 feet.
3. Apply the tabulated differences for the subordinate station. Time: 1106 + 0h 34m = 1140. Height: 6.2 − 0.8 = 5.4 feet.
4. Correct the time zone before you use it. The tabulated times are standard time and daylight time is in force, so high water at the subordinate station falls at 1240 on the clock you are steering by. The height is unaffected by the clock.
5. Compute actual depth at that time. Charted sounding 9 feet plus height of tide 5.4 feet = 14.4 feet.
6. Subtract draft for underkeel clearance. 14.4 − 7.5 = 6.9 feet. That satisfies the 5-foot requirement, so high water gives an acceptable crossing.
7. Check anything overhead separately. If a fixed bridge lies inside the bar, its charted clearance is referred to Mean High Water, and crossing at local high water is the worst moment for it. The same 5.4 feet that helped you at step 5 works against you here.

The decision that carries this problem is step 3 and step 4 together: the difference is applied to the reference station figure, and the clock correction is applied after. Candidates who reverse that order, or who apply a low water difference to a high water prediction, arrive at a clean-looking number that is simply wrong.

Inverting the triangle for a course to steer

You want to make good 090°T. Your speed is 10 knots. The current sets 180°T, drift 2 knots.

1. Identify what is known and what is being solved for. You have the course made good you want and the drift vector; the unknown is the course steered Bowditch Ch. 24 §2402.
2. From the fix, lay the intended track 090°T.
3. From the same point, lay the current vector in the direction of set, 180°T, for the drift over the interval you are using — 2 miles for one hour.
4. From the end of that current vector, swing your speed for the interval — 10 miles — to cut the track line.
5. The direction from the end of the current vector to that intersection is the course steered. It falls to the north of 090°, about 078°T: you offset into the current, not away from it. The distance from the fix to the intersection along the track is your speed over ground, slightly under 10 knots.

Same track and speed, but the set is 000°T at 2 knots. Which way does the course to steer move?

South of 090° — you offset into the set again, so roughly 102°T. The correction always opposes the set; a candidate who works one example and memorises "steer north of the track" will get the reciprocal case wrong. Solve the triangle, do not recall the answer.

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Where candidates lose the point

Subtracting the height of tide from the charted sounding. The words "lower low water" read as though the chart already shows the deepest case, so the tide must come off. It comes on: the sounding and the prediction are both measured from MLLW, which is why the operation is addition.

Answering with actual depth when the question asked for underkeel clearance. Both numbers will be among the options. The draft appears in the stem for a reason.

Treating a rising tide as extra clearance under a bridge. More water under the keel is fewer feet under the span, because the charted height is referred to Mean High Water while the sounding is referred to MLLW.

Opening the Tide Tables for slack water. Slack water and maximum flood and ebb are Tidal Current Tables entries; high and low water are Tide Tables entries. Any option offering to find one in the other publication is wrong on the face of it.

Losing an hour to daylight time, or applying the differences to the wrong reference station. Time-difference errors produce an answer that is exactly one hour or exactly one tabulated interval from the correct option, and the exam writers put that value in the list.

Reading "spring tide" as a season, or as high water only. It is a phase relationship — new and full moon — and what it maximises is the range.

Confusing the course steered with the track made good. In the vector relation, course steered plus drift gives course made good; the heading you hold is not the line you travel over the ground unless the drift is zero.

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Check yourself

The charted sounding at a berth is 14 feet. Predicted height of tide at your ETA is 2.1 feet. Your vessel draws 9 feet 6 inches. How much water do you have under the keel?

Actual depth is 14 + 2.1 = 16.1 feet; underkeel clearance is 16.1 − 9.5 = 6.6 feet. The sounding and the prediction share the MLLW datum, so the height of tide is added, then draft is taken off.

You need the time the current turns in a narrow entrance so you can transit with steerage but minimum cross-set. Which publication, and what are you looking for?

Tidal Current Tables, and the entry is slack water — the brief period of zero flow at the turn. Times of high and low water in the Tide Tables answer a different question: the vertical one.

It is full moon. A candidate concludes that low water will be higher than usual and the bar will be easier to cross. Where is the error?

Full moon puts you near a spring tide, which has the greatest range. High water is higher and low water is lower than on an average tide, so a low-water crossing is worse, not better. Neap tides, near first and last quarter, have the smallest range.

Your reference station shows low water 0518, height 0.6 foot. Your subordinate station differences are −0h 12m and −0.3 foot on low water. What is the subordinate prediction?

Low water at 0506, height 0.3 foot. Differences are applied to the reference station prediction, and the low water differences go with the low water figures. Then check the time zone and whether daylight time is in force before you use 0506 as a clock time.

A fixed bridge lies on your route and you have the charted vertical clearance. Your air draft is close to it. Do you transit at high water or at low water, and why?

Low water. The charted height is referred to Mean High Water, so vertical clearance decreases as the tide rises. This is the one computation on the topic where the tide works against you.

What distinguishes a tidal current from a surface ocean current?

The driving force. Surface ocean currents are driven primarily by wind and form the major gyres of each ocean basin; a tidal current is driven by the tide and reverses direction with it.

You are making good 10 knots through the water and want to track 315°T. The current sets 045°T at 3 knots. Describe how you find the course to steer.

Lay the intended track 315°T from the fix. From the same point lay the set, 045°T, for 3 miles — one hour of drift. From the end of that vector, swing 10 miles to cut the track line; the direction from the end of the drift vector to the intersection is the course to steer, and it lies to the west of 315°T because the offset opposes the set. The fix-to-intersection distance along the track is the speed over ground.