A step-by-step guide to finding slack First you heed to find out what the symbols and terms mea n. You need to know the difference between a neap tide and a spring tide, and to understand the concept of high water, which is basically the highest point to which the water will get at a specific place. I had to pick out the nearest tidal diamond (these purple diamond-shaped marks on charts highlight areas where localised tidal streams have been calculated) to the dive site, which was fairly easy. In some cases the diamond might be quite far from the site, but this usually means there is very little change in the current in this area. When cross-referenced with its column in the chart, it will tell you in which direction the current is travelling and what the speed of the current is in knots at hourly times relative to the high water mark for a given port. Once you have found out the state of the current, you need to refer to a tide timetable. This can be found in an up-to-date almanac or in the local newspaper to find the time of high water at the standard port. Furthermore, the height of the tide is also given, which you then add to the depth shown on the chart to get the real depth. There is a bit of arithmetic involved, when you need to calculate times, but nothing too complicated. I practised a few times on different sites, then attempted to find out when slack water was going to occur on the day we planned to visit the site. The step-by step guide takes you through the process.

Step one: Find the dive site on the chart. (See photograph 1 ) Step two: Find the nearest purple tidal diamond to the dive site, which is purple diamond A. (See photograph 1)

Step three: Refer to the tidal stream table relating to the diamond. In this case, we look for column A. (See photograph 2) Step four: Find where the current is least. The left-hand number refers to the speed of the current during spring tide and the righthand column refers to the speed of the current during neap tide. The current is least at -5 hours on high water (0.0 knots on neaps and 0. I knots on springs).The second least is +2 hours high water (only 0.2 knots on a neaps, however 0.4 knots on springs). (See photograph 2) Step five: Find the time difference from high water at the designated port, which in this case is Plymouth i.e. -5 and +2 hours. (See photograph 2)

Step six: Refer to a local tide table or almanac. This will give you - on any specific day - high water and if you are on springs or neaps and will also give you the height of high water. (See photograph 3) Step seven: Calculate slack water at the dive site with respect to high water by either adding or subtracting the time difference found in step five. (See photograph 3) Step eight: In this case we are on neaps, which is the time of the month between a new moon and full moon, usually indicated by the small circles. The high water is at I I :23 and 23:38. Subtract the five hours or add the two hours to this time. In this case the two hours is a more suitable time for diving, which is I 3:23, rather than 06.23 or I 8:38 if you subtract the five hours. (See photograph 3)

Step nine: Check what the current is doing either side of the slack water. Here we can see that the speed is 0.2 knots one hour after high water, but it jumps to 0.6 knots at three hours after high water. The best time to get to the site is therefore an hour after high water. (See photograph 2)

Step ten: I finally worked out the best time, which was between I 3:23 and 14:23. As we are now in British summer time and the time displayed in the tide table is Greenwich Mean Time, I had to add an hour. After you have worked out the best time to get to the dive site, it is important that you then check the local weather conditions.

Get to grips with the terms: High water is the highest the tide will get in a 12-hour cycle at a particular location. Knot is a nautical mile per hour. Neap tide (neaps) occurs when the sun and moon are at right angles with each other. In simple terms, there is less water going in and out of the area, therefore there    will be less current. Spring tide (springs) occurs when the sun and moon are in line. In other words, more water is moving in and out of the area, therefore the current is stronger. Chart datum is effectively the lowest the tide will ever get. There are occasions when it may be lower, but these tend only to happen very rarely. Tidal diamonds are purple in colour and pinpoint an area where localised tidal streams have been calculated. Details from DIVE magazine.