USCG Exam Prep · 🌊 Interactive Trainer
Tides & Currents — Height and Clearance
Will you clear the bar, or fit under the bridge? Tide work is just interpolation — once you can read the curve, the table problems fall out.
Tested on the Navigation General module on every USCG deck exam. Binnacle School has 213 Navigation General questions in the bank.
What it is
Tide and current reduction predicts the water level and stream at a place and time. From the Tide Tables you find the high and low water at a reference station, apply a subordinate station's time and height differences, and interpolate the height at any moment between tides. The Current Tables do the same for the speed and set of the stream.
How the USCG tests it
Navigation General problems ask for the height of tide at a given time, the corrected time and height at a subordinate station, the current velocity between slack and maximum, and the classic application questions: will a vessel of a stated draft clear a charted depth, and will her mast clear a bridge. The trainer lets you scrub a live tide curve and then solves these with worked numbers.
Key concepts
Between high and low water the height follows a half-cosine curve; the Tide Tables' Table 3 (and the 'rule of twelfths') approximate it from the range and the duration of rise or fall.
Most places are given as time and height differences (or ratios) from a reference station — apply them to the reference high and low water to get local predictions.
Available depth = charted depth (referenced to the chart datum, usually MLLW) + height of tide; subtract draft for under-keel clearance. A rising tide adds depth.
Vertical (overhead) clearances are referenced to mean high water (MHW): available clearance = charted clearance + (MHW − height of tide). A rising tide REDUCES overhead clearance.
Worked example
Low water 1.0 ft at 0600, high water 9.0 ft at 1230. Roughly what is the height at 0930 (mid-way)?
Mid-duration is the mean of high and low: (1.0 + 9.0) / 2 = 5.0 ft. The half-cosine curve passes through the mean at the midpoint, which is also where the tide is changing fastest.
Practice Tides & Currents hands-on
The interactive trainer generates a fresh problem every time and checks your work against exam-accurate math. Free with a Binnacle School account, on web and iOS.
Frequently asked questions
How do you find the height of tide at a specific time?
Find the bounding high and low water, then interpolate using the Tide Tables' Table 3 (a half-cosine of the range over the duration of rise or fall). The 'rule of twelfths' is a quick hand approximation of the same curve.
What datum are charted depths and bridge clearances measured from?
Charted soundings use the chart datum (mean lower low water in the U.S.), so the actual depth is the sounding plus the height of tide. Overhead bridge clearances use mean high water, so a higher tide reduces the clearance under the bridge.
What is a subordinate station?
A location given in the Tide or Current Tables as time and height differences from a nearby reference station. You apply those differences to the reference predictions to get local tide or current values.