A shallow bar, a drying ledge or a berth with limited depth can turn a straightforward passage into a costly mistake. Knowing how to calculate tidal heights gives you a defensible answer to a question every competent Coxswain or Master must be ready to answer: will there be enough water where and when I need it?
For AMSA assessments, the calculation matters, but so does your explanation. An assessor will want to hear that you understand chart datum, predicted heights, under-keel clearance and the factors that can make the actual water level differ from the prediction. The aim is not to produce a clever number. It is to make a safe operational decision.
Start with chart datum and the available depth
Tidal heights in tide tables are normally stated as a height above chart datum. Chart datum is the level from which charted depths and drying heights are measured. On Australian charts, it is generally set so that the tide will rarely fall below it, giving navigators a conservative reference point.
To find the depth of water at a charted position, add the predicted height of tide to the charted depth.
Depth of water = charted depth + height of tide
If a chart shows 1.1 metres and the predicted tide height is 1.7 metres, the predicted depth of water is 2.8 metres. If the feature is a drying height, treat the charted figure as a height above chart datum instead. A drying height of 0.8 metres with a tide of 1.7 metres leaves 0.9 metres of water over it.
That distinction is a common assessment trap. A charted sounding is depth below chart datum. A drying height is height above chart datum. Read the chart symbol and abbreviation carefully before reaching for the calculator.
How to calculate tidal heights from a tide table
Begin by identifying the correct location and date in the tide tables or approved electronic tidal information. Find the low water and high water times that bracket the time you are calculating for. You need four pieces of information: the low-water height, high-water height, low-water time and high-water time.
The difference between high water and low water is the tidal range.
Range = high-water height – low-water height
Say low water is 0.6 m at 0600 and high water is 3.6 m at 1200. The range is 3.0 m. If you need the predicted height at 0830, the tide has been rising for two and a half hours.
Where a tidal curve or height interpolation information is supplied, use that method first. It is based on the published prediction for that location and will generally be more appropriate than a rough rule. In an assessment, state which source you are using and show each part of your working clearly.
When you are required to estimate the rise or fall between high and low water, the Rule of Twelfths is a useful manual method.
Use the Rule of Twelfths carefully
The Rule of Twelfths assumes that the tidal range changes over six equal time periods. It estimates that the tide rises or falls by one twelfth of the range in the first hour, two twelfths in the second, three twelfths in the third, three twelfths in the fourth, two twelfths in the fifth and one twelfth in the sixth.
For the 3.0 m range in our example, one twelfth is 0.25 m. The estimated rise is therefore 0.25 m in the first hour, 0.50 m in the second hour and 0.75 m in the third hour.
At 0800, two hours after low water, the tide has risen by 0.75 m in total. The estimated height is 1.35 m:
0.6 m + 0.75 m = 1.35 m
At 0830, you are halfway through the third hour. The third-hour rise is estimated at 0.75 m, so half of it is 0.375 m. Add this to the rise at 0800:
0.6 m + 0.75 m + 0.375 m = 1.725 m
Rounded sensibly, the estimated tidal height at 0830 is 1.7 m above chart datum.
The calculation is only as good as its assumptions. The Rule of Twelfths is an approximation and works best with a reasonably regular semi-diurnal tide and a six-hour interval between low and high water. It is less reliable where the interval is noticeably different, where there are strong local tidal effects or where tides are unequal. Use the official tide information and local knowledge available for the job, rather than forcing every situation into the rule.
Convert tidal height into a passage decision
A predicted height of tide is not the final answer. You must apply it to the controlling depth on the route, then allow for the vessel and conditions.
Using the earlier example, a charted depth of 1.1 m plus a calculated tide height of 1.7 m gives 2.8 m of predicted water. A vessel drawing 2.2 m appears to have 0.6 m under-keel clearance. Whether that is acceptable depends on the operation.
A slow displacement vessel entering a sheltered berth may have a different margin from a vessel crossing a bar, operating in swell, carrying extra load or manoeuvring at speed in a confined channel. Squat can increase draft, especially in shallow water. Heel, pitch, roll, wave action, density changes, trim and an inaccurate position can all reduce the margin you thought you had.
Good passage planning sets a minimum safe depth before departure. Rather than asking, “Can I just get through?”, decide what under-keel clearance is required for that vessel, route and weather forecast. Then calculate the earliest safe entry time, latest safe departure time or safe tidal window.
Check whether you are using the right port
Not every location has its own full set of daily high and low water predictions. Some locations are secondary ports, where heights and times must be derived from a nominated standard port using published differences.
Do not simply use the nearest town name or assume the tide is identical around the corner. Estuaries, river entrances, islands, shallow banks and narrow channels can alter both tidal time and height. A difference of even 20 or 30 minutes can matter when you are timing a bar crossing, clearing a sill or meeting a berth window.
If secondary-port corrections apply, first calculate or obtain the corrected high and low water times and heights for the location. Then work between those corrected values. Keep your work in sequence. Applying a correction after calculating an intermediate height can give you a misleading result.
Predicted tide is not observed tide
Tide tables provide predictions, not guarantees. Atmospheric pressure, wind setup, storm surge, river flow and local weather can cause the observed water level to be higher or lower than predicted. Prolonged strong offshore winds can reduce water levels in some locations, while onshore winds and low pressure may increase them.
This is why a prudent skipper does not plan critical clearance down to the last centimetre. Check current weather, notices to mariners, local conditions and, where available, a nearby tide gauge. If the margin is tight, delay, choose another route or obtain local advice. Commercial decisions should be based on the vessel’s safety management procedures and the operating limits that apply.
Present the calculation clearly in an assessment
When an assessor gives you a tidal-height question, speak through the method in order. Identify the chart datum reference, read the high and low water data, calculate the range, determine the rise or fall at the required time, then apply the result to the charted depth. Finish by explaining the under-keel clearance and the operational allowances you would make.
Avoid giving a bare answer such as “2.8 metres”. A Master or Coxswain is expected to show why that depth is available and whether it is safe for the vessel. If a figure looks wrong, stop and check the basics: have you confused a drying height with a sounding, used the correct day, mixed up local time, or calculated from the wrong tidal port?
Tidal calculations become much less intimidating once you practise them against realistic chartwork and passage-planning scenarios. At True North Marine Training, one-on-one revision can focus on the exact tidal, chartwork and oral-question areas that need sharpening before your assessment. Build the habit now: calculate conservatively, write down your assumptions and give yourself enough water to manage the unexpected.
Understanding tidal conditions is a key part of conducting safe and professional coastal passages. For more information on coastal navigation please use this link: https://truenorthmarine.com.au/coastal-navigation-aus-version/