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Useful Information from CPA

The information expressed herein should be treated as opinion. No guarantee is given or implied that any advice on the CPA website is necessarily correct. Nor might it best suit other divers and clubs due to regional and personal differences. Diving is a risk sport. All advice herein should be validated with advice from your own diving club, governing body, or approved published material before being adopted.

Plan a General Dive Trip

Understand tides and currents
In the English Channel, we get roughly two main tides per day as water swooshes up and down the channel under influence of Sun and Moon’s gravity. Local ports like Portland and Southampton may get three or four tides due to ‘resonance’ effect of local geography. Never exactly two tides – a bit less each main high tide on average around 12 hours plus 30 to 50 mins later than last depending on location. Means that though many days have four tide table entries (2 highs and 2 lows), some have only three when final one goes past midnight. High & low tide levels vary. Biggest tidal range – i.e. highest highs and lowest lows – occur on "spring" tides (nothing to do with seasons). Smallest range – lowest highs and highest lows – on "neap" tides. Springs occur when Sun, Moon and Earth are in a straight line. Can easily tell as this gives new moon and full moon. Neaps are when Sun and Moon form a right angle with the Earth, gives waxing or waning half moons. So roughly speaking, half moons are neaps, full or new moons springs. Can forecast this for any time in future: look at a moon phase calendar. Also, can get a moon phase calculator for personal organisers like 3Com PalmPilots (by Alex Garzia, from Pilot Gear HQ) and Psions. Or can look in many paper diaries! Springs and neaps also vary in how extreme they are. Some springs have bigger range than others. Called "high springs" – currents will be strongest and for a while after viz poorest. Neaps are best for wreck dives etc. Viz normally best when coming off neaps. Generally better on "flood tides" too (water rising/coming into shore) as brings settled water in from deeper areas. "Ebb tides" (water falling/going back out to sea) can sweep silt from shallows.

Understand slack water
Slack water is simply the time at a given location when currents are very low or zero. Happens as current stops flowing one way and starts flowing the other. Seems logical that this is at high or low tide, but not the case. Direction, timing and strength of currents not only affected by tides. Also greatly affected by shape of coastline, depths, resonance factors and so on. Means that slack water can happen at very different times in places that are close together. Also means slacks are rarely at high or low water. They are always relative to high or low water though. Slack times thus expressed as a number of hours before/after high/low water at given port (doesn’t have to be a local port), e.g. "3 hours after High Water Portland", or "5 and a half hours before Low Water Dover". Usually means time when water is at lowest current. May well be less than a knot or so (therefore safe to dive) for some time either side of this. This whole period called "slack window". Its length changes with spring or neaps. Springs cause more rapid current changes so window is shorter (e.g. 20 mins on high spring for some sites). Neaps gentle, therefore window bigger (maybe an hour for same site on good neap). Important to be able to estimate this as affect duration of dive or whether have time for two waves or not.

Find out tide times
Important to know high and low tide times. Only way to predict slack water for wreck dives. Even on drift dives may need it to check currents not silly and know when slips have enough water to launch/retrieve RIB. Tide tables published by many sources. Coastal towns and dive shops sell little booklets/leaflets of local tides. Admiralty Chart suppliers (e.g. Kelvin Hughes) sell admiralty tide tables. Brian Charles (JBC hardboat) gives tables to regular customers the year before & they include springs/neaps curves. If need tide times a lot, and especially well ahead, best is to get hold of computer programme to calculate them. I use The Thompson Partnerhip’s "AutoTide" on a Windows PC. Shows tide graphs or generates tables for any time hence at any European (in my version) port, adjusted for DST if need be. Admiralty also produce a tide calculator based on "Simple Harmonic Method", but my version had very poor usability (may be better now though?). Again, try Kelvin Hughes or similar for this. Even possible to get them for modern personal organisers now. If you have a 3Com PalmPilot, get the stunningly good fully functional freeware tide calculator from Walt Bilofsky (sincerest compliments to Walt if he ever reads this). Also, for the common ports we use, look at the Tide Tables on this web site for useful weekend listings. If you only have tide times for one port, others can be derived approximately. Many tables include listings of Dover Tidal Differences. Show how much time to add/take way from Dover high waters for any other port. Only then need this listing and Dover tide table to get rough HW anywhere. South Coast version of this in our Tide tables section and full European Excel spreadsheet version available from the Downloads Page. If only have table for, say, Portland, but want to know Devonport, can convert to Portland to Dover , then Dover to Devonport. For rough planning, Dover Tidal Differences and a single tide table all you need. Get more accurate results with proper tables for the port though.

Find out slacks and currents
Many diving books quote slack water times for dive sites with nasty currents. This is easiest to use. If not, best way is to use marine charts for that dive site. Charts have scattered points with letters in a diamond to label "tidal streams". Streams tend to flow roughly parallel to coastline though some (e.g. round Portland ) very much more complex. For these need a tidal stream atlas showing more precise streams. For normal sites, find the letter for the stream most likely to apply to your dive site (one nearby or a similar distance from coastline). Chart will have a tidal stream table on it showing how the current strength and direction varies hour by hour relative to HW for both springs and neaps. Note: this HW can be any port – must look which it is. Don’t assume local port. By finding section of this stream with currents less than a knot or so (direction rarely important here), can see where slack window is relative to HW at the port quoted. Can thus estimate when and how wide the slack window is for any given site at any date in future. Fairly good estimates but they are approximate, so need to be on site early in case slack is early.

Match dive to divers’ capabilities
Any dive must suit capabilities of all divers on trip. Best approach is to get firm bookings very early so all divers known. Can then look in club records, ask to see qualification logs or personal logbooks to assess experience and safe diving limits for all. From this information, can then plan best dives for that group. Often impossible to get firm names far in advance though, so next best thing is to plan dive for likely takers then publicise acceptable minimum diving quals/experience/recency for planned dive. Only accept those people meeting this. Last ditch approach is to accept divers below this standard but do two sites: first as plan, then another to suit the few others. Many things to consider. Safety primary of course. What maximum depths permissible for qualification level? (e.g. SAA limits are: Trainee 10m; Elementary Diver: 20m; Open Water Diver: 35m; Club Diver & above: 50m). Even if qualified, what about experience? May be qualified to 35m but what’s the deepest they’ve done so far (may only be 20m, so 35m is too risky). Check recency too – don’t plan to let someone anywhere near their max limits without them building up depth first (but hard to ensure), e.g. if done many dives to 50m but not dived beyond 25m for six months, then straight into 40m plus is dangerous: need a few 30 and 35m dives to get ‘dived up’ to it. Another major issue is deco diving: our club doesn’t, but think about guests who may. This affect slack windows. If anyone doing deco, need to have a longer slack and or plan to have all doing deco on DSMBs at same time so boat can track divers. Are there Nitrox divers, and if so what depth limits can they handle? Consider equipment – if some in semi-drys beware of deeper longer dives at either end of the season. Diver attitude is important too: depth/recency etc may all be OK, but perhaps some of the party easily spooked by gloomy wrecks – if so, avoid known turbid wreck sites. Think of fitness too, if anyone in party not as fit as should be, avoid planning dives with awkward currents or enforced long swims etc. And last but not least, what style of diving suits the party best – are they all wreck fanatics, or scenic divers? Drifts or bimbles? Deep and exciting or shallow and relaxed? Marine life or dramatic landscapes? Hunters or conservationists? … Normally impossible to please everyone, but some parties have very strong bias so try to pick dives that suit.

Choose the best dive sites
Many factors affect choice of site. Site itself is major one: dive books and other diver recommendations invaluable here. Hardboat skippers a great help. Look on the web too. Long range planning can also account for tides and ‘typical weather’ but little else. Day-before planning can take into account: likely weather & changes; experience etc. of divers going (see "… Match dive to divers’ capabilities"); springs/neaps and whether a drift or wreck; recent visibility; how many waves of divers; sea state; temperatures, current RIB reliability; road trips and transport problems; and slips/launching issues. Several things to watch. If weather lumpy or will go lumpy during day, avoid sites with long sea trips. If wind offshore, can stay closer inshore to avoid worst. Be aware that ‘wind against tide’ makes bigger waves than wind running with tide, so lumps may change as tide changes even if weather same. In general avoid winds higher than force 4 in open sea. Work out launch retrieve times and compare to local tides & slip info to check slip has enough water when needed. If several waves of divers, don’t choose sites a long way from slip and/or pick-up point. Will divers have 2 cylinders? If not, need single dive, or two shallower ones, or get back to air station easily at lunchtime. If weather not good, pick site where can do lunch/S.I. in sheltered bay.

Organise accommodation
Best source is old favourites. Can’t beat digs everyone knows and likes – ask club members and other divers for recommendations. If a hardboat trip, skippers always know good places to stay. If no joy here, refer to one of the many B&B guide books. Another good source is tourist info centres: they’ll pick ones that meet your needs and can even make bookings for you. Ring directory enquiries to find T.I. centres. Some accommodation available on the web now too. Do a search or two and see what you get. Things to check before booking include: plenty of car parking space; somewhere to dry suit if poss; proximity/time to get to boat; and make sure can get breakfast early enough to get to boat on time. Many digs do packed lunches if you ask too. If booking on behalf of others, make sure they pay you full amounts in advance to avoid problems later.

Check the divers’ qualifications and medicals
Make sure divers quals match dive – club member quals on file with secretary/D.O. Also ensure medicals in date else no dive. Again, on record with club sec. For guest divers, marshall should ask to see quals/medicals, for self protection in case of incident, but some orgs (e.g. PADI) don’t demand medicals, so valid quals and current org membership ok here. If in doubt about capabilities, ask to see normal logbook.

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