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 sea 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 tide – 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 UK OF COURTESY 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 COLES ADLARD & O 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 COURTESY 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 COURTESY 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: 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 AXINE 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.