Tidal Heights and Secondary Ports

Prerequisites

Before this lesson you should:

• Understand why tides happen: the pull of the Moon and Sun, the difference between springs and neaps, and the meaning of high water (HW), low water (LW), range and duration.
• Know what chart datum is, and that on UK charts it is Lowest Astronomical Tide (LAT), so charted depths and drying heights are measured from LAT.
• Be able to read times and heights from a tide table or almanac, and to convert between UT (GMT) and British Summer Time.
• Be comfortable with simple arithmetic on times (hours and minutes) and heights (metres to one decimal place).
• Have worked through the Competent Crew chart and tide basics, and the Day Skipper lesson on tidal streams and tidal atlases.

The RYA Day Skipper shorebased syllabus requires you to calculate the height of tide at a standard port and at a secondary port, at any time, and to use these heights to find depth of water, clearance under a bridge or cable, and the time when a drying harbour or anchorage becomes accessible.

Learning Objectives

This lesson covers the tidal heights section of the RYA Day Skipper shorebased syllabus and the matching sections of the RYA Navigation Handbook (G6) and the Training Almanac. By the end you will be able to:

• Explain chart datum, drying height, charted depth, height of tide and clearance, and how they relate.
• Find high and low water times and heights for a standard port from the tide table.
• Construct and use a tidal curve to find the height of tide at any time, and the time at which a given height occurs.
• Choose between the spring and neap curve, or interpolate between them.
• Use the rule of twelfths as a quick check, and know its limits.
• Find the tidal data for a secondary port using time and height differences, interpolating correctly.
• Calculate the depth of water at a position and the clearance under a bridge or overhead cable.
• Work out when you can enter or leave a drying harbour or cross a bar.
• Recognise and avoid the common mistakes in tidal height calculations.

Why Tidal Heights Matter

Tidal streams move you sideways; tidal heights change how much water is under you. A boat that draws 1.6 m can sail safely over a bank at high water and be sitting on it, heeled over and waiting for the next flood, six hours later. Ports with a sill, drying moorings, river bars, shallow anchorages and low bridges all depend on the height of tide at a particular time. Skippers who cannot work this out reliably tend to either stay out of interesting places or end up aground in them.

The method is always the same. First you find the predicted times and heights of high and low water for the nearest suitable port. Then you convert those into a height at the exact time you care about. Finally you combine that height with the charted depth, drying height or clearance to answer the question you actually asked: is there enough water, or enough headroom, at that time?

Chart Datum, Depths and Heights

Every height and depth in this lesson is measured from a reference level. On modern UK Admiralty charts that level is chart datum, which is Lowest Astronomical Tide (LAT): the lowest level that can be predicted to occur under average meteorological conditions and any combination of astronomical effects. The sea will occasionally fall below LAT in strong offshore winds and high pressure, but rarely, and by small amounts.

• Charted depth is the depth of water below chart datum, shown in metres and decimetres (for example 5.4 is five metres four tenths). It is the least depth you should expect at that spot.
• Drying height is the height of the seabed above chart datum. It is shown on the chart as an underlined figure (for example 1.2 underlined means the seabed there is 1.2 m above chart datum and uncovers at low water by at least that much).
• Height of tide is the height of the sea surface above chart datum at a given time. It is what the tide tables and tidal curves give you.
• Charted vertical clearance under a bridge or cable is measured from Mean High Water Springs (MHWS), not from chart datum. The next sections explain why.

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The fundamental relationship for a sounding is:

Actual depth = charted depth + height of tide

For a drying height it is:

Actual depth = height of tide - drying height

If that second figure is negative, the seabed is above the water and the spot is dry.

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The Mean Levels in the Tide Tables

Tide tables and chart notes give average levels for each port, and you will use them in secondary port calculations and in bridge clearances:

• MHWS - mean high water springs: the average of the heights of two successive high waters during the periods of springs.
• MHWN - mean high water neaps.
• MLWN - mean low water neaps.
• MLWS - mean low water springs.
• MSL / ML - mean level, about halfway between mean high and mean low.

The diagram below shows these reference levels in relation to chart datum.

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A spring tide is large, with HW high and LW low; a neap tide is small, with a smaller range. In the UK springs follow about a day or two after the new and full moons, and neaps about a week after springs.

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Sources of Tidal Height Data

You will meet several sources. They all give the same kind of information.

• Admiralty Tide Tables (ATT), the official publication, gives daily HW and LW times and heights for standard ports (and a long list of secondary ports with differences), plus tidal curves for the standard ports.
• Nautical almanacs, such as Reeds Nautical Almanac and the RYA Training Almanac used on the course, give the same kind of predictions for the ports in their area, usually one page per standard port with the curves printed alongside, and with secondary port differences tabulated on a separate page.
• Electronic sources, such as the UKHO EasyTide service, give predictions for many more ports, and are fine for planning, but the Day Skipper exam and good practice both require you to be able to do the calculation by hand from the almanac.

Always check which time zone the table uses. The Admiralty tables and many almanacs are in UT (GMT), with BST an extra hour in summer. Reeds and the Training Almanac state clearly at the top of each page whether times are UT or local; a one-hour mistake here shifts your answer by roughly a third of the rise or fall, and it is the single most common error in tidal calculation.

Standard Ports

A standard port is one for which tidal predictions are published in full, day by day, from detailed harmonic analysis of long observations. Portsmouth, Dover, Plymouth (Devonport) and Milford Haven are all standard ports. For each standard port the table gives, for every day:

• The time and height of each high water.
• The time and height of each low water.

The page also gives the port's mean levels (MHWS, MHWN, MLWN, MLWS) and one pair of tidal curves, a spring curve and a neap curve, specific to that port.

The tidal curves are drawn for one particular port, because the shape of the tide differs from place to place. Some ports have a long stand of high water (the Solent), some have a double high water (Southampton), some rise fast and fall slowly, and some have steep curves like Dover. You should always use the curve for the standard port, never a curve borrowed from another place.

The Tidal Curve

Why a curve rather than a straight rule

Tide does not rise and fall at a constant rate. At HW and LW it is almost stationary (the stand), and it moves fastest midway between them. A tidal curve is a graph of the height of tide against time, drawn from HW. The horizontal axis shows hours before and after HW. The vertical scale is a height scale, left blank because the heights depend on the particular day, so you build the scale yourself with the diagonal method below.

Step by step: find the height at a given time

1. Write down from the tide table the time and height of the HW and the LW on either side of the time you want, for the standard port.
2. Work out the time you want in relation to HW, in hours before or after. Always use the HW you are closest to, but any HW will do as long as you count the hours correctly.
3. Work out the range: HW height minus LW height. Decide whether it is nearer to a spring range or a neap range; the almanac gives the mean ranges. Springs are the heavy curve, neaps the dotted or lighter curve (check the key on each page).
4. On the height scale mark the HW height on the top line and the LW height on the bottom line. Join the two marks with a straight line. This is your diagonal.
5. Find the time from HW on the horizontal axis. Go vertically up to the curve you chose.
6. From the curve go horizontally across to the diagonal.
7. From the diagonal go vertically down to the height scale and read the height of tide.

For a time outside the HW to LW sequence, such as half an hour after LW, you can use the same curve: the horizontal axis normally runs from 6 hours before to 6 hours after HW, so LW is about +6.

Choosing between spring and neap

If the range is close to the mean spring range you use the spring curve; if close to the neap range use the neap curve. In between, use whichever curve is closer, or draw both and interpolate the answer in proportion. The curves differ only slightly in most ports, so a small error here changes the answer by perhaps a tenth of a metre, but at Dover, or anywhere with an unusual shape, you may need to be careful.

If the HW or LW heights shown are lower or higher than the mean values (surge, or big pressure change), the predictions still use the same method; you simply add your own safety margin, covered below.

Worked example: height at a standard port

Portsmouth. You want the height of tide at 1630.

• HW 1400, 4.8 m.
• LW 2012, 0.9 m.
• Range 4.8 - 0.9 = 3.9 m. Mean spring range at Portsmouth is about 3.9 to 4.0 m, so use the spring curve.
• Time from HW: 1630 - 1400 = 2 h 30 min after HW, so +2.5.
• Diagonal from 4.8 m at the top to 0.9 m at the bottom.
• Up from +2.5 hours to the spring curve, across to the diagonal, down to the scale: about 3.4 m.

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Check it with the rule of twelfths: after 2 hours three twelfths of the range have gone, and after 3 hours six twelfths, so at 2.5 hours about 4.5 twelfths (37.5 percent) of 3.9 m, or 1.5 m, has fallen, giving about 3.3 m.

Finding the time for a given height

Often you want the reverse: at what time will there be enough water? Draw the diagonal as before. From the required height on the scale go across to the diagonal, then vertically up to the curve, then across the curve to read the time axis (before or after HW). Convert to clock time by adding or subtracting from HW time.

Example: you draw 1.6 m and the bar at the harbour entrance has a drying height of 0.4 m. You need 0.5 m under the keel, so you need the tide to be at least 0.4 + 1.6 + 0.5 = 2.5 m. With HW 1400 at 4.8 m and LW 2012 at 0.9 m, go across from 2.5 m to the diagonal, up to the curve and read the time: about 3 hours 25 minutes after HW falling, so the water falls below 2.5 m at roughly 1725. Add a margin of safety and aim to be through the entrance by 1630.

The Rule of Twelfths

The rule of twelfths is a quick approximation that treats the tide as a smooth curve over six hours, assuming the duration between HW and LW is six hours exactly. The range is divided into twelve parts and the tide moves by:

Remember it as 1, 2, 3, 3, 2, 1.

Using the Portsmouth numbers (range 3.9 m, one twelfth is about 0.325 m):

The same table applied to the 1630 question: 2.5 hours after HW is halfway between 3.8 m and 2.8 m, so 3.3 m, close to the 3.4 m from the curve.

Limits of the rule

• It assumes six hours between HW and LW. The real interval is usually 5 to 7 hours, and at some ports (Southampton, Poole, the Solent generally) it is far from a sine curve. If the interval is 6 hours 25 minutes, divide it by six to get the length of each "hour" and apply the rule to those.
• It assumes a smooth curve with no stand. Dover's curve and Poole's double high water break the assumption.
• It is good for a quick check and for pilotage where an error of 0.2 to 0.3 m is not critical; for decisions on a narrow margin use the curve.
• The first and last hours move the least, and the third and fourth the most. The danger zone for rising tide is therefore the first and second hours after LW: water arrives slowly, so waiting a little longer for the flood can leave you with very little extra depth, but the third hour brings nearly a quarter of the range.

The rule is the Day Skipper's workhorse when there is no almanac on hand. Many experienced skippers keep the numbers 1, 2, 3, 3, 2, 1 in their head.

Secondary Ports

There are far more places you will want to visit than there are standard ports. A secondary port is one where tidal predictions are not given day by day. Instead the almanac gives differences from a named standard port, and you apply them to the standard port's predictions. The assumption is that the shape of the tide at the secondary port follows that of the standard port, just shifted in time and scaled in height.

A secondary port entry in the almanac gives:

• The name of the standard port to use (chosen because its tide has a similar character and timing).
• Time differences for HW and LW, given for specified standard port times (for example HW at 0000 and 1200, and at 0600 and 1800), and similarly for LW.
• Height differences for MHWS, MHWN, MLWN and MLWS.
• The secondary port's own tidal curve: this is normally the standard port's curve.
• Sometimes a mean level or seasonal correction, and notes on the character of the tide.

How to apply the differences

The differences are tabulated at only a few points so you almost always have to interpolate. Time differences vary with the time of HW at the standard port (a proxy for the spring or neap state and the time of day). Height differences vary with the height of HW or LW at the standard port (a measure of how spring or neap it is).

1. List the standard port HW and LW times and heights for the day.
2. Interpolate the time difference between the two tabulated times that bracket the standard port time.
3. Add the time difference to the standard port time to get the secondary port time.
4. Interpolate the height difference between the neap and spring differences, in proportion to where the standard port height sits between its MHWN and MHWS (or MLWN and MLWS).
5. Add the height difference (usually negative, so it is subtracted) to get the secondary port height.
6. Do this for HW and for LW separately. Then use the standard port tidal curve with the secondary port times and heights, as in the standard port method.

Interpolation here is simply working out proportion. If the standard port HW is a third of the way from the 1200 column to the 1800 column, then the time difference is a third of the way from the first figure to the second.

Worked example: a secondary port

The figures below are illustrative, not taken from a real table, but are typical of a real calculation. The standard port is Portsmouth. The secondary port is "Examplehaven".

Standard port predictions for the day (Portsmouth):

• HW 1400, 4.4 m.
• LW 2012, 1.1 m.

Portsmouth levels: MHWS 4.7, MHWN 3.8, MLWN 1.9, MLWS 0.8.

Examplehaven differences:

Step 1: HW time. The standard port HW is at 1400, which is two hours after 1200 and four hours before 1800. It is one third of the way from 1200 to 1800. The difference moves from -0040 towards -0020, a change of 20 minutes, so one third gives 7 minutes. Time difference is -0040 + 0007 = -0033. Examplehaven HW is 1400 - 0033 = 1327.

Step 2: HW height. The Portsmouth HW height of 4.4 m lies between MHWN (3.8) and MHWS (4.7), a span of 0.9 m. It is 0.6 m above MHWN, which is two thirds of the way to springs. The height difference moves from -0.4 (neaps) to -0.6 (springs), a change of 0.2; two thirds is 0.13. Height difference is -0.4 - 0.13 = -0.53, say -0.5. Examplehaven HW height is 4.4 - 0.5 = 3.9 m.

Step 3: LW time. Standard port LW is 2012, which lies between 1700 and 2300. It is 3 h 12 min after 1700 out of 6 hours, 0.53 of the way. The difference moves from -0010 to -0030, a change of 20 minutes, 0.53 of which is 11 minutes. Difference is -0010 - 0011 = -0021, say -0020. Examplehaven LW is 2012 - 0020 = 1952.

Step 4: LW height. Portsmouth LW 1.1 m lies between MLWS (0.8) and MLWN (1.9), a span of 1.1 m. It is 0.3 m above MLWS, 0.27 of the way to neaps. The height difference moves from -0.2 (springs) to -0.1 (neaps), a change of 0.1, 0.27 of which is 0.03. Difference is -0.2 + 0.03 = -0.17, say -0.2. Examplehaven LW height is 1.1 - 0.2 = 0.9 m.

Step 5: Result. Examplehaven: HW 1327, 3.9 m. LW 1952, 0.9 m. Range 3.0 m.

Step 6: Height at 1630. Time from HW: 1630 - 1327 = +3 h 03 min, call it +3 h. Use the Portsmouth tidal curve with a range of 3.0 m, nearer neaps than springs. Reading the curve at +3 h gives about half the range gone, so about 2.4 m. The rule of twelfths agrees: after three hours the tide has fallen six twelfths of 3.0 m, which is 1.5 m, and 3.9 - 1.5 = 2.4 m.

Notice that the secondary port's HW falls 33 minutes before the standard port's and its range is smaller. Using Portsmouth's height at 1630 (3.4 m) instead of Examplehaven's (2.4 m) would overestimate the water by a full metre.

Practical notes on secondary ports

• Always check the almanac's remark about which standard port to use. Some secondary ports have differences from a port that is geographically distant because the tidal character is similar.
• When the standard port times of HW fall exactly on a tabulated column you do not need to interpolate. When they fall outside the range of the table, use the nearest column rather than extrapolating.
• Do the HW and LW calculations independently. The time difference at HW is not the same as at LW, so the duration of rise and fall will differ between the two ports.
• If the secondary port is a river or estuary the tide curve shape may be quite different (a shorter rise and longer fall). The almanac note will tell you if so. In such cases the rule of twelfths would mislead you.
• Standard port predictions in the almanac cover the same days as the secondary port; if HW at the secondary port falls on a different calendar day from the standard port because of the time difference, make sure you use the right day's figures.

Tidal Heights and Safety Margins

The prediction is the average expectation. The sea does not read the table. Always add a margin to your calculation:

• Meteorological effects. A strong onshore wind and low pressure can raise the water level by half a metre or more (storm surge); high pressure and offshore winds can lower it. A rule of thumb is that a fall in pressure of 1 hPa raises sea level about 1 cm. Treat predicted heights as more reliable for calm, settled weather.
• Datum and chart updates. Soundings may have changed since the last survey, particularly in bars, estuaries and sandy channels.
• Squat, swell and heel. A moving boat sinks slightly in the water; wave motion lowers the bottom of the keel in the troughs; a heeled yacht draws more. Allow extra when in swell.
• Other margins. A typical safety margin under the keel for pilotage in sheltered water is 0.5 m; offshore or in swell, 1 m or more. At a bar with swell, much more.

Never rely on a tight margin calculated from predictions alone. Check the actual depth with the echo sounder as you go, and when approaching a shallow place on a rising tide, you will have the benefit of the tide coming to your aid.

Clearance Under Bridges and Cables

A bridge or overhead cable presents the reverse problem to a shoal: the more water there is, the less headroom.

Charted clearance is measured from MHWS

The clearance given on a UK chart, and the one shown in the almanac, is the height of the lowest part of the structure above MHWS. So at any time when the tide is lower than MHWS you have more headroom than charted, and at times when the tide is higher than MHWS (the top of a spring tide with a surge) you have less.

The formula is:

Actual clearance = charted clearance + (MHWS height - height of tide now)

where MHWS height is the figure for the nearest port (given in the almanac or on the chart notes).

Air draught

Your own height from the waterline to the highest point of the boat is the air draught: mast, masthead fittings, VHF aerial, wind instruments, radar reflector and the radar dome if fitted. Measure it, do not guess from the brochure. The boat's trim and load alter it by a few centimetres, and a crowd of crew on one side will lower it slightly in very narrow cases.

Always allow a margin: at least 1 m in calm conditions, and more if there is wash from other vessels, swell, or the possibility that you will be pitching. Never go under a power cable without checking its charted clearance, as electricity can arc across a gap.

Worked example

You plan to pass under a bridge with a charted clearance of 16.0 m above MHWS. At the nearest port, MHWS is 4.7 m. Your air draught is 15.2 m. You want to pass at 1630, when the height of tide is 3.4 m.

• Actual clearance = 16.0 + (4.7 - 3.4) = 16.0 + 1.3 = 17.3 m.
• Margin = 17.3 - 15.2 = 2.1 m. This is acceptable.

If you needed to pass at 1330 in the same tide, when the height of tide is 4.7 m, the clearance would be 16.0 + 0 = 16.0 m, a margin of only 0.8 m. That is less than the recommended 1 m, so you would wait until the water fell, or lower the mast.

On a very high spring tide, with HW at 5.2 m and a margin of 16.0 + (4.7 - 5.2) = 15.5 m, you would not fit under with 15.2 m air draught at all and would risk the masthead aerial.

When working out the time

To find the time at which a particular clearance exists, work backwards: find the minimum clearance you need, subtract the charted clearance and add MHWS to get the maximum height of tide that gives that clearance. For the bridge above with a 15.2 m air draught and 1.0 m margin, you need a clearance of 16.2 m, so the tide must be at most 4.7 - 0.2 = 4.5 m. Then use the tidal curve in reverse to find the times at which the tide is below 4.5 m.

Drying Heights and Drying Harbours

Many south coast and east coast harbours dry out: a boat will take the ground at low water. Such places can be convenient, but you need to know the following.

How to calculate access

The depth over a drying spot is the height of tide minus the drying height. For a boat drawing 1.5 m with a 0.5 m margin, the tide must be at least the drying height plus 2.0 m.

Worked example: the entrance to a creek has a drying height of 1.2 m. You draw 1.5 m and want 0.5 m clearance. You need a height of tide of 1.2 + 1.5 + 0.5 = 3.2 m. With HW 4.8 m at 1400 and LW 0.9 m at 2012, the diagonal technique says the tide will be above 3.2 m between roughly 2 h 15 min before HW and 2 h 15 min after HW, a window of about 4.5 hours around HW, depending on the shape of the curve. At neaps, with HW perhaps only 3.0 m, you would never get in at all.

Practical points for drying harbours

• Choose your berth, and check the type of seabed on the chart or pilot book: soft mud is kind to the hull, but rock or a steep gradient is not.
• Make sure the boat has a firm place to take the ground: a tidal grid, a wall or a bilge-keel arrangement. A fin-keel yacht may need legs or fenders against a wall, and must lean towards the wall.
• Know how long the boat can stay afloat on the ebb: a fall of 0.5 m takes an hour or more in the middle of the cycle, but only about 20 minutes on a steep ebb.
• Allow for neaps: the neap HW can be 1.5 to 2 m lower than the spring HW. A harbour accessible every tide at springs may be accessible only near HW, or not at all, at neaps. Neap tides can leave you stranded as readily as springs.
• Make sure you have a scope of warp and lines that allow the boat to rise and fall without hanging by a line.

Tidal Height and Tidal Streams Together

In pilotage and passage planning you use both tidal heights and tidal streams. Remember the following:

• Tidal streams do not change direction at HW and LW. In many places the stream continues to flow for an hour or more after HW. Do not assume slack water is at HW or LW; use the tidal stream atlas or diamond table.
• The greatest rate of stream often occurs when the height changes most quickly, around mid-tide, which is why it is often best to enter a harbour on a flood near HW rather than at mid-tide.
• Tidal gates, such as headlands and narrow channels, often require arrival at a specific time, which can be chosen by combining the stream and the height of tide for any bar.

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Using the Training Almanac and Handbook

The RYA Navigation Handbook (G6) teaches the same tidal height method under the heading of tidal curves and secondary ports, with examples for Dover and Portsmouth. The key points to remember when using the books are:

• Find the right page for your standard port; the standard port tidal curve is printed with the predictions or on the facing page.
• Check the time zone at the top of the page. Convert BST to UT before using the table if the table is in UT.
• Use the diagonal on every occasion; do not read the curve directly, because each day has a different range.
• Write the working in a systematic layout so you can check it afterwards. In the exam, working matters.
• Give heights to one decimal place (0.1 m) and times to the nearest five minutes or so; any greater precision is false accuracy.

Common Mistakes

• Using the wrong time zone. Forgetting that BST is UT + 1 hour. Check the heading on the table and write the zone beside every time.
• Measuring time from the wrong HW. The curve is symmetrical around HW. Make sure you count hours before or after the right one.
• Not using the diagonal. Reading the tidal curve directly as if the height scale were fixed. Always draw the diagonal.
• Using the wrong curve. Using the spring curve for a neap tide, or another port's curve for the standard port.
• Forgetting interpolation. Using the nearest column for secondary port differences, which can be wrong by 15 minutes or 0.3 m.
• Adding instead of subtracting. Mixing up the sign of differences. Write the sign next to each number, and check that the result is reasonable.
• Confusing clearance datum. Bridge clearances are measured from MHWS, not chart datum. Depths are from chart datum. Do not mix them.
• Forgetting the margin. Treating the predicted height as exact. Always allow a margin for weather, squat and swell.
• Assuming tidal stream and height change together. Slack water is not necessarily HW or LW.
• Rule of twelfths on an awkward port. Applying the rule to a port with a long stand or double HW.

Summary

• Charted depths and drying heights are measured from chart datum (LAT). Actual depth is charted depth plus the height of tide; over a drying height it is the height of tide minus the drying height.
• Bridge and cable clearances are measured from MHWS. Actual clearance equals charted clearance plus (MHWS minus current height of tide).
• At a standard port, find the HW and LW on either side, work out the range, choose spring or neap, draw the diagonal, and read the height from the curve.
• The rule of twelfths (1, 2, 3, 3, 2, 1) gives a quick check, but assumes a six-hour smooth curve.
• At a secondary port, apply interpolated time and height differences to the standard port predictions, separately for HW and LW, then use the standard port's curve.
• Always check the time zone, use the correct HW, and add a safety margin for weather, swell and squat.
• Tidal streams are independent of heights; do not assume slack at HW and LW.

Check Your Understanding

1. On a UK chart, what datum are charted depths and drying heights measured from, and how is a drying height shown?

Answer: From chart datum, which on modern UK charts is Lowest Astronomical Tide (LAT). A drying height is an underlined figure on the chart, giving the height of the seabed above chart datum.

2. A charted depth is 2.5 m and the height of tide is 3.2 m. What is the actual depth?

Answer: 5.7 m (2.5 + 3.2).

3. At a standard port HW is 4.8 m at 1400 and LW is 0.9 m at 2012. Describe how you find the height at 1630.

Answer: The time is 2.5 hours after HW. Range is 3.9 m, close to springs, so use the spring curve. Draw a diagonal from 4.8 m to 0.9 m, go up from +2.5 hours to the curve, across to the diagonal and down to the height scale. The answer is about 3.4 m.

4. What are the rule of twelfths fractions, and what is one major limitation?

Answer: 1/12, 2/12, 3/12, 3/12, 2/12, 1/12 of the range in each successive hour. It assumes a smooth six-hour curve, so it is unreliable at ports with a stand, a double high water or an interval very different from six hours.

5. Why do you need to interpolate when using secondary port differences?

Answer: The differences are tabulated only for a few standard port times and for mean spring and neap heights. The actual HW or LW on the day usually falls between the columns, so you must work out the proportion to find the right difference.

6. How would you find the time difference for a secondary port when the standard port HW is at 1400, with differences of -0040 at 1200 and -0020 at 1800?

Answer: 1400 is one third of the way from 1200 to 1800, so the difference changes by one third of 20 minutes (about 7 minutes) from -0040 to -0033. The secondary port HW is 1400 - 0033 = 1327.

7. A bridge has a charted clearance of 16.0 m. MHWS is 4.7 m and the current tide height is 3.4 m. What is the actual clearance, and what datum is the charted figure measured from?

Answer: Charted clearance is measured from MHWS. Actual clearance is 16.0 + (4.7 - 3.4) = 17.3 m.

8. You draw 1.5 m and want 0.5 m under the keel. A bar has a drying height of 1.2 m. What height of tide do you need to cross it?

Answer: 1.2 + 1.5 + 0.5 = 3.2 m or more.

9. Why should you always use the standard port's own tidal curve, and the diagonal?

Answer: The shape of the tide is particular to each port, so the curve must be the one printed for that port. The diagonal scales the curve to the particular day's HW and LW heights; without it you would read a height from a fixed scale that does not match the range.

10. Give three reasons why a predicted height of tide may not be exactly what you find.

Answer: Atmospheric pressure (about 1 cm per hPa), wind and surge, and swell or squat. Also, an old survey or a wrongly applied time zone can make the calculation wrong.