Applying Tide Corrections to Charted Depth

What the rule requires

The number printed beside your track is the depth at the sounding datum, not the depth at 1430 this afternoon. On a US chart that datum is Mean Lower Low Water, and MLLW is also the plane from which the tide tables predict the height of tide Bowditch Ch. 9 §904. Both numbers are measured from the same surface, which is the whole reason they may be added.

The chain has two links and the exam tests both:

1. actual depth of water = charted sounding + predicted height of tide
2. underkeel clearance = actual depth of water − draft

The trigger that switches this on is the datum the charted number carries. A sounding is referenced downward from MLLW, so the tide is added. Heights of features such as bridges and lights are referenced to Mean High Water, so vertical clearance shrinks as the tide rises. One chart, two datums, opposite signs.

Before you add anything, read the chart title. US charts show soundings in feet, fathoms, or metres, and the unit is stated there Bowditch Ch. 4 §403. A height of tide in feet added to a sounding in fathoms is not a small error; it is a grounding dressed up as arithmetic.

Then pick the right volume. 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. Only the first of those produces a number you can add to a sounding.

If the place you care about is a subordinate station, you do not read it directly. You take the reference station prediction and apply the tabulated time and height differences to it. Two operations, and candidates who apply one and forget the other lose the point on an otherwise correct plot.

Last, check the time. Confirm the time zone and whether daylight time is in force before you apply a prediction, because a prediction applied on the wrong clock is a prediction for a different state of tide. The principal lunar tide runs a period of about 12 hours 25 minutes Bowditch Ch. 9 §901, so an hour of clock error moves you a substantial way along the curve.

Range varies with the phase of the moon: spring tides near new and full moon have the greatest range, neap tides near first and last quarter the smallest Bowditch Ch. 9 §902. That is your judgement of when least water occurs — a spring low water — not a term in the calculation.

The chart shows 9 feet at your intended anchorage, the predicted height of tide is 3.2 feet, and you draw 4 feet. How much water is under the keel?

8.2 feet. Actual depth is 9 + 3.2 = 12.2 feet, then subtract the 4-foot draft. Both figures come from MLLW, so they add; the draft comes off the total, not off the charted sounding.

Telling it apart

The single criterion is which datum the printed number is measured from, and it decides the sign of your correction.

• Charted soundings — referenced to MLLW, measured downward. Add the predicted height of tide to obtain actual depth. Most often misfiled by treating the printed sounding as the depth available at the moment the question is asked.
• Charted heights and vertical clearances — bridges and lights are referenced to Mean High Water, not to the sounding datum Bowditch Ch. 9 §904. Clearance decreases as the tide rises. Most often misfiled by adding the height of tide, which produces an answer larger than the charted figure and a wheelhouse into a bridge span.

Tide Tables against Tidal Current Tables

The criterion is vertical against horizontal. Tides are the periodic vertical rise and fall of sea level Bowditch Ch. 9 §901; tidal currents are the horizontal flow of water that goes with that rise and fall, flood incoming, ebb outgoing, slack water the brief period of zero flow at the turn Bowditch Ch. 9 §903.

• Tide Tables — times and heights of high and low water at reference stations Bowditch Ch. 9 §905. The only volume that yields a height you can add to a sounding.
• Tidal Current Tables — times of slack water and of maximum flood and ebb. Nothing in them is a height. A candidate who lifts a time out of the current tables and treats it as high water has answered from the wrong book, and the distractor is usually sitting right there in the options.

Both volumes are built the same way: reference stations tabulated in full, subordinate stations reached by applying differences.

Working a question

You are berthing at a subordinate station and want the water at the pier face at high water. The chart title states soundings in feet. The sounding alongside is 11 feet. Your draft is 7 feet 6 inches. The reference station tabulates high water at 1116 standard time, height 4.6 feet. The subordinate station differences are +0h 34m on time and −0.5 feet on height. Ship's clocks are on daylight time.

1. Read the chart title and fix the unit. Feet, so a tide height in feet is compatible. Had it read fathoms or metres, the addition would be inadmissible until converted Bowditch Ch. 4 §403.
2. Decide which volume answers the question. You want a depth, so it is the Tide Tables. Slack water is irrelevant here even if the berth has a fierce current.
3. Establish the station type. The pier is a subordinate station, so the tabulated reference station prediction is raw material, not the answer Bowditch Ch. 9 §905.
4. Apply the time difference: 1116 + 0h 34m = 1150 standard time.
5. Apply the height difference: 4.6 − 0.5 = 4.1 feet. This is the predicted height of tide at your berth at high water.
6. Reconcile the clock. The prediction is standard time and the bridge clock is on daylight time, so high water occurs at 1250 by ship's clock. Nothing in the depth changes; what changes is when you must be there.
7. Add the height of tide to the charted sounding: 11 + 4.1 = 15.1 feet of actual depth Bowditch Ch. 9 §904.
8. Subtract draft for underkeel clearance: 15.1 − 7.5 = 7.6 feet.

If the same question had put a fixed bridge over the approach, step 7 reverses. The charted clearance is referenced to Mean High Water, so at the moment you have most water beneath the keel you have least air above the masthead. The correct answer moves clearance down, and the option that adds 4.1 feet to the charted figure is the trap.

Same berth, same 11-foot sounding, but the question asks for the depth at low water and the predicted height is −0.4 feet. What do you report?

10.6 feet of actual depth, and 3.1 feet under a 7-foot-6 keel. The operation is still addition; a negative predicted height simply produces a sum smaller than the charted sounding. Do not switch to subtraction — you will subtract twice and arrive at 10.6 by luck or 11.4 by error.

Where candidates lose the point

• Answering with the charted sounding. The distractor is the number printed on the chart, which feels authoritative because it came off the chart. It is the depth at MLLW; the question asked what is there now, and the height of tide has to go on before the answer is a depth Bowditch Ch. 9 §904.
• Reporting actual depth as clearance. Having worked 15.1 feet correctly, the candidate picks 15.1 when the stem asked for water under the keel. Draft is the last subtraction and the option list nearly always offers both numbers.
• Subtracting the tide "to be safe." Conservatism is a fine habit in the wheelhouse and a wrong answer on a multiple-choice paper. Soundings are referenced from a low-water datum, so the tide is additive; safety margin is built into the datum, not into your arithmetic.
• Adding the tide to a bridge clearance. Attractive because the candidate has just spent five questions adding the tide to soundings, and momentum carries over. Vertical clearances are MHW-referenced and shrink on a rising tide.
• Using the reference station value unchanged. The subordinate station appears in the stem, the reference station is where the printed height lives, and the candidate reads the height without applying the tabulated differences. Both the time difference and the height difference must be applied; one of the distractors is usually the uncorrected reference figure.
• Answering one hour out. Standard time against daylight time, or a table read in the wrong zone. Check the zone before you apply the prediction, and expect the paper to offer the shifted time as an option.
• Mixing units. A tide table in feet against a chart in fathoms or metres. The chart title states which unit is in use, and it takes three seconds to confirm.
• Opening the wrong volume. A stem that mentions a strong flood tempts the candidate into the Tidal Current Tables, which predict slack water and maximum flood and ebb and no height at all Bowditch Ch. 9 §903.

Check yourself

The chart shows 14 feet in the channel. The predicted height of tide at the time of your transit is 2.7 feet. Your vessel draws 5 feet. What is the least water beneath your keel?

11.7 feet. Actual depth is 14 + 2.7 = 16.7 feet; underkeel clearance is 16.7 − 5. The 16.7 figure will appear as an option, and it answers a different question.

A fixed bridge on your route carries a charted vertical clearance of 40 feet. The tide is rising toward high water. What happens to the clearance available to you?

It decreases. Heights of bridges and lights are referenced to Mean High Water rather than to the sounding datum, so rising water reduces the air draft available. Any option that adds the height of tide to the charted 40 feet is wrong on its face.

Your destination is a subordinate station. The Tide Tables print high water 0908, height 5.2 feet, for the reference station. What do you do before using those figures?

Apply the tabulated time and height differences for your subordinate station to the reference station prediction, then confirm the time zone and whether daylight time is in force. Reference station numbers used raw are a standard distractor.

You need the time of maximum ebb through a narrows. Which publication, and does it give you a height of tide?

The Tidal Current Tables, which give times of slack water and of maximum flood and ebb. They do not give heights. A height of tide comes only from the Tide Tables, and the two are separate questions even when both concern the same narrows.

Charted sounding 4 fathoms, predicted height of tide 3.0 feet, draft 8 feet. What is the first thing you check?

The chart title, to confirm the unit of the soundings. Charts are published in feet, fathoms, or metres, and the height of tide must be brought into the same unit before it is added. Adding 3.0 to 4 and calling the answer 7 is the failure this question is built around.

You want to transit a shoal patch on the day of least water. New moon falls on the 14th, first quarter on the 21st. Which is the shallower low water?

The 14th. Spring tides occur near new and full moon and have the greatest range, which puts low water lowest; neap tides near the quarters have the smallest range. The height of tide still has to be added to the charted sounding for either day — phase tells you which day to work, not what the depth is.

High water at your berth is predicted for 1140. It is now 1430 and you are computing depth for an 1500 departure. Why can you not simply use the high water height?

Because the tide is falling away from that peak — the principal lunar tide has a period of about 12 hours 25 minutes, so the water level at 1500 is well down the curve. The prediction you apply must be the height of tide at the time in question, taken from the correct station and the correct clock.