Ex-Merchant Navy officer and Fellow of the Royal Institute of EXPERT ON BOARD Navigation, John Goode owned Southern Sailing School for 25 EXPERT ON BOARD years and is an RYA examiner ‘If reasonable care is taken when working out tidal heights, we can expect an accuracy of at How to work out least one foot – or better’

This gauge tells you the height of water over a marina sill, but would tidal heights you trust your own calculations? If calculating tidal heights doesn't come easily, John Goode offers help with the sums and encourages you to use the results with confidence PHOTOS: JOHN GOODE UNLESS OTHERWISE STATED OTHERWISE UNLESS GOODE JOHN PHOTOS:

In practice, if reasonable care is taken we can expect an accuracy of at least one foot – or better snook m m

he great thing about on a lightly manned yacht at accurate to within a metre or graha RIGHT: When the use of practical navigation is that is a grasp of basic principles and so is often quite sufficient. tidal heights is essential,

we don’t need to be red common sense in abundance. However, when more dependable PHOTO: it’s good to have them hot with numbers, or a This particularly applies when accuracy is called for we really where they can be seen Tcomputer genius, to get by. it comes to calculating tidal do need to have confidence in come easily, the least interpolation that could affect them at a glance. Pencilled on Whether we favour traditional heights, where simply casting a our calculations. If we haven’t, we have to do the better, so in practice. While the result of the chart or pilot book or electronic methods (or a knowledgeable eye over the tables we’ll be restricted to ‘playing it hopefully what’s shown over the a storm surge or strong wind we’re using is better than mixture of both), what’s wanted to obtain a prediction that’s safe’ by only going into shallow following pages will both clarify blowing for a long period into, or hunting for scraps of paper water near the top of a – a few basic concepts – and offer out of, an estuary isn’t so readily – or burrowing deep into or limiting our visits to easily some alternative prediction calculated, one factor that we the pages of an almanac entered (but often overcrowded) methods that can reduce the risk can easily allow for, but which is harbours that have well-marked, of arithmetical error. rarely taken into consideration, is deep-water channels. The more faith we have in our abnormally high or low barometric BST ability to predict the height of pressure. For every millibar above 1700 6m Nagging doubt tide (above what’s shown on the the average pressure of 1013mb, 1 1800 7 / m COURTESY OF UK Probably the main reason chart) at any stage of its rise or fall, charted depth will be reduced by 2 for not trusting (or using to the more interesting places we’ll 1cm ­– and raised by 1cm for every 1900 9m 1 full advantage) our mid-tide be able to visit – and the more millibar below it. 2000 9/2m predictions is the nagging doubt fun we’ll have. Finally, I’ve found it reassuring ABOVE: 2100 9m h

that we might get the sums over the years that in practice, if 30mb above O & ADLARD COLES wrong – especially if it means Less pressure reasonable care is taken when or below the average taking our keel anywhere near However, we don’t just need working out tidal heights, we can barometric pressure of the bottom on the strength of to come up with theoretically expect an accuracy of at least one 1013mb can lower/raise Probably the main reason for not trusting our tidal height predictions is them. For those to whom accurate ‘predictions’. We’ve also foot or better. So do try to depths by 0.3 metres the nagging doubt that we might get the sums wrong and end up like this juggling with numbers doesn’t got to be aware of any phenomena trust and use them.

30 www.yachtingmonthly.com summer 2012 summer 20129 www.yachtingmonthly.com 31 EXPERT ON BOARD EXPERT ON BOARD Tips to check and use your sums Straightforward standard port COURTESY UK Compare ‘calculated’ Standard ports are usually larger naval 1 height with ‘actual’ height or commercial harbours that have Extract times and heights of Low Water and High Water It’s important to always bear in mind that no their own comprehensive tide tables. This example uses the standard port of Milford Haven during a rising tide on the afternoon of 2 June –

Tide Gauges h

matter what the source of data, or method O By referencing these to their equally when tidal heights are about three-quarters of the way between the (higher) Springs and (lower) Neaps of calculation we use to obtain a tidal height When we pass a tide unique tidal curves and graphs, specific ranges. Note that all heights are above Chart Datum (below which all depths are shown on a chart) and prediction, it is just that – a prediction. gauge it can be useful height and time data can be quickly that the time zone is UT (GMT in old money), to which one hour should be added if working in BST. Whenever the opportunity arises, it’s to compare the (actual) and easily extracted without having to worth taking advantage of the many height of tide above Chart grapple with any interpolation. (charted) tide gauges around our shores. Datum shown on the gauge As well as being precise indicators of with our own (calculated) The tidal data illustrated what effect any extremes of barometric tidal height prediction. is reproduced from pressure, or storm surges, are having Any difference noted Reeds Nautical Almanac. on predicted tidal heights, they can also when we’re near the gauge Although not shown provide a very useful check on the accuracy can then be applied to our here, it also includes of our own calculations. calculated height for the special curves for places Located near the entrances of most major surrounding area – which where HW anomalies harbours and some smaller ones (although is especially good to know occur. Based on the more defined time of at the latter they are increasingly missing or when heading inbound LW, they are used exactly the same way. poorly maintained), they reliably indicate towards a harbour’s the actual height of tide above Chart Datum. shallower upper reaches Mark-up the graph

Check the calibration of the depth sounder's read-out 2 Before relying on a depth sounder’s to read from the bottom of the keel. But read-out, we first need to know how it has because an overall depth reading can be been calibrated. For some skippers (and used as a key navigational tool, it will be most charter outfits), the sounder is often a lot more beneficial if, as shown below, Mark up the Height considered to be not much more than an the sounder’s display is adjusted to read of High Water anti-grounding device and is calibrated from the waterline.

If we’re not sure what depth our echo sounder indicates, use a weighted line to check it out Mark up the Height of Low Water WATERLINE

TRANSDUCER

BOTTOM OF KEEL AXINE HEATH AXINE M In case there’s a sloping bottom, dip from both sides of the boat and use the average measurement to Write in the time of 1231 1331 1431 1532 1632 1732

Low Water 1832 GRAPHIC: GRAPHIC: compare with the sounder’s read-out

COURTESY UK After marking up the heights of High in the hourly time-boxes between them Note: When using the graph to establish A good reason to adjust to waterline depth and Low Water, join the marks together – working from either end towards the heights/times (as shown below) use the nearest GRAPHIC: 3 Just one example with a diagonal line. Then, after writing middle so that any ‘overlap’ is shown at Spring (–––) or Neap (– – – –) Curve. Here at Write in the time of how using waterline 1 in the times of High and Low Water, fill (the often less critical) mid-tide stage. Milford Haven there is mostly no difference. of High Water 3/2 h O depth can offer peace M AXINE HEATH of mind is when – 1 even in familiar local 5/2 How to use the graph waters – the visibility deteriorates suddenly. What is the predicted At what time will there be a As shown (far right), height of tide above Chart height of 4.5 metres above HEIGHT all we need do is ‘pencil’ 3.5m Datum in the Milford Haven Chart Datum in the Milford in the overall depth of OF TIDE area at 1500? Haven area? water on the relevant contour lines (here it’s Enter the time scale at Enter the top height scale at 3.5 metres above CD). 1500. Go up to the tidal 4.5 metres. Go down to the 1 Then, simply by 8/2 1 curve, then across to the diagonal line, then across CHARTED 13/2 maintaining our chosen 2.0m diagonal line, before to the tidal curve, before depth on the sounder DEPTH continuing up to the height continuing down to the time between contour lines scale. This shows 3 metres. scale. This shows 1600hrs. (or tacking between them), we can safely keep clear of the main Without the added complication of allowing channel or close the for a sounder’s bottom-of-keel offset, a depth shore to a depth that reading from the waterline can be a very useful suits our draught. navigational tool 1231 1331 1431 1532 1632 1732 1832

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nook f ‘The more faith Secondary port interpolation S m Hal raha TIDE we have in our Providing tide tables and graphs for every and heights at the more widely spaced harbour would take up too much space standard ports are given. Interpolation is calculations, the in the almanac. To overcome this, only then required to obtain heights and times G PHOTOS: more interesting the ‘differences’ to the LW and HW times at the many secondary ports in between. places we’ll be Standard port 1 heights and times able to visit’ First, extract and write down the designated standard port times and heights. Because interpolating and applying the differences can be confusing, a neat and logical layout – It's important to have faith in our calculations when as suggested below­ – is essential. drying out, particularly in the Channel Islands where the tide can rise or fall 2m or more in an hour

The simple way to interpolate differences If your mental arithmetic’s not that hot, it’s best to do any workings on a separate bit of paper. There’s usually no need to get over-complicated with a calculator or graph paper. For most purposes we can use two parallel lines – Secondary port differences drawn freehand and divided into hours/ 2 The differences are annotated minutes by eyeball. In this example we in the almanac in the standard want to find the time difference to apply format (right). For example, the HW to the day’s HW time of 0833. time differences shown here tells us that when HW at the standard port occurs at either midnight (0000) or midday (1200), HW at the secondary port would be 35 minutes earlier – but when it occurs at 0600 or 1800 it would only be 5 minutes earlier. If HW at the standard port occurs somewhere in between these times (here it’s 0833), Interpolation Draw two parallel lines and mark off, at we’ve got to interpolate to find the difference. The same concept applies to step-by-step either end of the top line, the Standard the LW time difference – and the HW and LW height differences. Port’s HW times (0600 and 1200) that straddle 0833hrs. On the bottom line, Apply to standard port curve directly below the HW times, mark off the time differences (–05 and –35 mins) 3 Note: Time differences apply to UT times at the standard port. If you're working in BST, wait until AFTER the calculations (right) have been completed Divide the top line into a six-hour time before adding the The -18 minute scale and the bottom line into any one-hour correction difference (bottom convenient scale of minutes (here it’s at right) is now 10 minute intervals) Right: With the applied to our secondary port’s calculation differences established and applied to the straightforward calculation, the results are then applied to the graph of its designated standard port (right), To find the difference to apply to the which is marked up Standard Port’s HW time of 0833, simply and used in exactly run a pencil between the two lines. Here, With confidence, we can enjoy more the same way as HW at the Secondary Port occurs challenging places where others fear shown on the 18 minutes earlier LOW to tread. Here, an adventurous spirit previous page WATER and a boat with a lifting keel help!

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The rule of Tidal heights at a glance twelfths During a six-hour period the The classic rule of twelfths tide will rise/fall at the following For accuracy and ease of use, my favourite source of tidal height (and stream) data over the past The Rule of Twelfths can be used to calculate intermediate times rate from Low/High Water: and heights between High and Low Water without having to three decades has been the Yachtsman’s Tidal In the 1st hour 1 Atlases, published by Adlard Coles. Based on the refer to tidal curves or graphs. It’s often used by those with /12 of its range time and height of HW Cherbourg (which is all we pocket-sized tables issued by many yacht clubs, but which In the 2nd hour 2 only give times and heights of local HW and LW. /12 of its range need from the almanac), they are annotated the In the 3rd hour 3 same way as any other tidal atlas – by marking While the ‘rule’ can produce acceptable results if the /12 of its range up the pages six hours before and after High area to which it applies has a reasonably symmetrical In the 4th hour 3 /12 of its range Water. Thereafter, we can extract ‘instant’ tidal six-hour rise and fall of tide, it can be complicated to Hoo Ness YC In the 5th hour 2 heights at every Standard and Secondary Port use in practise. Taking the height of LW from HW (to A fairly /12 of its range tables give HW get the range), then dividing the result by 12 before we're likely to visit, for each hour of the 12-hour The ideal symmetrical In the 1st hour 1 and LW times adding up the required number of twelfths and /12 of its range tidal cycle, without any interpolation. combination: a tidal curve and heights for applying them to either LW or HW heights can The atlases are based on the times and heights suitably scaled is required Chatham call for quite a bit of mental arithmetic. of High Water Cherbourg because of its central chart and the tidal

reference position, suitably high and heights applicable GRAPHIC: symmetrical curve. It is also relatively free from to everywhere on it the fluctuations in mean that can be available at a glance

5 HIGH WATER M prevalent at other more familiar Standard Ports. Easier to clock the Rule of Twelfths One twelfth AXINE HEATH Conveniently, in summer months the times given Two twelfths 4 in the almanac equate to British Summer Time. Fortunately, we can eliminate most Putting this concept into practice of the above number crunching by is easy. We only need to divide the Three twelfths Regrettably, the area of coverage is limited 3 HALF TIDE comparing the Rule of Twelfths to range by two, and the result by to what’s shown on the covers of the three If you sail in these Three twelfths the face of a clock. By extending two again, to mark up the gauge – 2 atlases illustrated here, plus one that’s solely areas, the atlases imaginary horizontal lines from and substitute the 12 and 6 o’clock Two twelfths dedicated to the Channel Islands and another do away with the its hour-marks to an adjacent tide points on the clock with the times of One twelfth 1 that combines them all. need to interpolate LOW WATER gauge, the gradually accelerating HW and LW. Copying the illustration 0 and decelerating rise or fall of tide below (in ink) will provide a handy The rate of rise or fall of tide over CHART DATUM over six hours will automatically be tidal clock that can be quickly six hours relates directly to the Example: Find the height of tide Fill in the atlas’s time boxes divided in accordance with the ‘rule’. pencilled-in whenever it’s needed. hourly marks on the face of a clock 2 on each page for the six at all ports, on 16 September hours before and six times 2156 Get the time and height of HW Cherbourg on after Cherbourg’s HW time of 2256. Construct our own simple HW HEIGHT 1 16 September (here from Reeds Almanac). tidal height clock 4.8m Cut out or hw HEIGHT 0830 photocopy this In this example (right), the times 0930 of HW (0830) and LW (1440) template 3.8m are pencilled-in on the clock 1030 for your Here, on the HW-1 page, we face – and the heights of own use subtract one hour and write HW (4.8m) and LW (0.8m) TIME 1135 2.8m 2156; on the HW+1 page we on the . add an hour, and so on. The tide gauge is marked 1240 TIME up by halving the 4.0m range 1.8m (2.0m) and then halving it again 1340 1440 (1.0m). By adding the 2.0m to the 0.8m LW height of 0.8m gives a half-tide LW Mark up the TIDAL HEIGHTS – PORTS & PLACES height of 2.8m above Chart Datum – CHART DATUM 3 scale by pencilling-in (the same on every page) with 1.0m divisions between half tide the intermediate hours on the the HW height of 5.9 metres. and HW or LW. clock it’s prudent to show any time lw Because there’s rarely exactly six ‘overlap’ at (the often less critical) CHArt datum hours' rise or fall, when marking up mid-tide stage.

Websites Chartplotters and websites and apps For those with access to the Internet, either at home or afloat, there are a variety of tidal height apps and programs RESULT for PCs We can now instantly available. extract tidal height predictions at all of Shown here is the places we’re ever Admiralty EasyTide, which likely to need. Here, offers predictions for 6,000 ports at 2156 (one hour worldwide and up to 50 years ahead. before HW Cherbourg) Electronic chart predictions using a C-Map MAX chart, the nearest tide we have 2.0 metres The Yachtsman's Atlas not only These days, most electronic chartplotters icon (off Stansore Point) is clicked to bring Thanks to above Chart Datum at tells you the height of tide at any use software that can instantly display up a display showing a rolling, tabulated n Southern Sailing School, Southampton Great Yarmouth. port, it also gives relevant tidal the predicted height of tide above Chart 24-hour tidal curve and the current height Tel: 01489 575511 stream rates on the facing page Datum. Here, in Southampton Water of 2.67m at 1200hrs. Website: www.southern.co.uk

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