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'How To... Plan and Marshall a Dive' Page

The information expressed herein should be treated as opinion. No guarantee is given or implied that any advice on the web-site 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.

How To...

… 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 the influence of the Sun and Moon's gravity. Local ports like Portland and Southampton may get three or four tides due to a 'resonance' effect of local geography. There is never exactly two tides - a bit less each main high tide on average around 12 hours plus 40 to 50 mins later than the last depending on the location. This means that though many days have four tide table entries (2 highs and 2 lows), some have only three when the final one goes past midnight.

High & low tide levels vary. The biggest tidal range - i.e. highest highs and lowest lows - occurs on "spring" tides (nothing to do with the seasons). The smallest tidal range - i.e. lowest highs and highest lows - are on "neap" tides. Springs occur when the Sun, Moon and Earth are in a straight line which is easy to tell as this is at a new moon and full moon. Neaps are when the Sun and Moon form a right angle with the Earth, which gives a waxing or waning half moons. Roughly speaking, half moons are neaps, full or new moons are springs.

You can forecast this for any time in future: look at the moon phase calendar at:

Also, can get a moon phase calculator for personal organisers like 3Com PalmPilots (by Alex Garzia, from Pilot Gear HQ at and Psions. Or can look in many daily papers or daily 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 the currents are very low or zero. This happens as the current stops flowing one way and starts flowing the other. It seems logical that this is at high or low tide, but this is not the case. The direction, timing and strength of currents are not only affected by tides. Also it is greatly affected by the shape of the coastline, depths, resonance factors and so on.

Slack water can happen at very different times in places that are close together. 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 a 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". This usually means the time when water is at its lowest current and may well be less than a knot or so (therefore safe to dive) for some time either side of this.

This whole period is called the "slack window". Its length changes with spring or neaps. Springs cause more rapid current changes so the window is shorter (e.g. 20 mins on high spring for some sites). Neaps are gentle, therefore the window is bigger (maybe an hour for the same site on a 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

It is important to know high and low tide times. Even on drift dives you may need it to check that the currents are not silly and know when slips have enough water to launch/retrieve the RIB. Tide tables are published by many sources:

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. 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


Dive planning Any dive must suit the capabilities of all divers on the trip. Best approach is to get firm bookings very early so all divers are 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.

… Choose the best dive sites

Many factors affect your choice of dive site. The site itself is major one; dive books and other diver recommendations invaluable here. Hardboat skippers a great help. Look on our website too on the Wreck Search and Wreck Lookup pages.

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.

Force Description Sea Description Speed (Kts) Range (Kts) Forecast Description Sea State Waves (mtrs)
0 Calm Sea like a mirror 0 <1 Calm Calm 0
1 Light air Ripples with the appearance of scales are formed, but without foam crests. 2 1-3 Light Smooth 0.1
2 Light breeze Small wavelets, still short but more pronounced. Crests have a glassy appearance and do not break. 5 4-6 Light Smooth 0.2
3 Gentle breeze Large wavelets. Crests begin to break. Foam of glassy appearance. Scattered white horses. 9 7-10 Light Slight 0.6
4 Moderate breeze Small waves, becoming longer, fairly frequent white horses. 13 11-16 Moderate Moderate 1
Do not plan to dive on stronger winds than a Force 4              
5 Fresh breeze Moderate waves, taking a more Pronounced long form; many white horses are formed. Chance of some spray. 19 17-21 Fresh Rough 2
6 Strong breeze Large waves begin to form; white foam crests are more extensive everywhere. Probably some spray. 24 22-27 Strong Very rough 3
7 Near gale Sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind. 30 28-33 Strong High 4
8 Gale Moderately high waves of greater length; edges of crests begin to break into spindrift. The foam is blown in well-marked streaks along the direction of the wind. 37 34-40 Gale Very high 5.5
9 Strong gale High waves. Dense streaks of foam along the direction of the wind. Crests of waves begin to topple, tumble and roll over. Spray may affect visibility. 44 41-47 Severe gale Very high 7
10 Storm Very high waves with long over-hanging crests. The resulting foam, in great patches, is blown in dense white streaks along the direction of wind. On the whole the surface of the sea takes a white appearance. The 'tumbling' of the sea becomes heavy and shock-like. Visibility is affected 52 48-55 Storm Phenomenal 9
11 Violent storm Exceptionally high waves (small & medium sized ships might be lost to view for a time behind the waves). The sea is completely covered with long white patches of foam lying along the direction of the wind. Every-where the edges of the wave crests are blown into froth. Visibility is affected. 60 56-63 Violent storm Phenomenal 11.5
12 Hurricane The air is filled with foam and spray. Sea completely white with driving spray; visibility very seriously affected 64+   Hurricane   14

… Organise accommodation

Best source is old favourites. Can't beat digs everyone knows and likes - ask club members and other divers for recommendations.

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 qualifications match the dive - club member qualifications are on file with the Club Secretary / D.O. Also ensure that medicals are in date else no dive (again, on record with Club Secretary). For guest divers, the dive marshall should ask to see qualifications/medicals, for self protection in case of incident, but some organisations (e.g. PADI) don't demand medicals, so valid qualifications and current organisation membership is OK here. If in doubt about capabilities, ask to see normal logbook.

… Marshal a General Dive

… Estimate journey and dive timings

Road journeys need to be based on experience, though computerised journey planners help a lot.

Often not bad getting down there as typically early Sunday morning start. Allow more time in high summer though for weekender traffic. If meeting for breakfast somewhere, make sure there's a clear departure time else time slips away very easily. Allow an hour once at site for preparing RIB, kitting up and launching. Sea trips are very variable. Depends on sea-miles, boat, tides and weather conditions. In practice, rare for our club to need more than half an hour to get to a site. If shotting a wreck, need to allow tons of extra time, even once on site: finding wreck may be hard, shotting it can take several attempts, and slack may be early. An hour is typically plenty even on the RIB (and if goes well can do it in ten mins flat). Normally need less with a good hardboat skipper.

… Ensure right safety equipment aboard

Ensure certain equipment is available. If using hardboat and it doesn't have Oxygen (always should but check), take the Clubs O2 kit. Check full well in advance. Ditto for a First Aid kit. Club stuff is normally kept with the RIB anyway. All safety equipment should be checked regularly. For RIB dives, take spare cylinder of air in case stage cylinder needed (or if bottle not full). Other kit standard: VHF radio, flares, fire extinguisher, anchor, spares… Diver recall signals a good idea too (underwater firecracker to tell divers to surface such as incident with other divers).

… Arrange meeting points and times

Make sure everyone knows where/when to meet. Make clear that if not there on time, others may have left. Can't afford to wait for people as delays cascade and ruin the day for everyone else.

… Make sure all dive details known

For hardboat dives, the ideal way is to issue an information sheet to all divers with everything they need to know on it. Things to make clear:

… Find out what the weather may do

More than a week out, hardly worth trying.

Four or five days ahead, several sources.

… Decide if the weather is a problem

Most important safety factors are sea swell and surface viz. High seas make people ill and often less disciplined. Greater risk of injury when thrown around on boat. Most importantly can be exceptionally hard to see surfacing divers. Launch and retrieve likely to be hazardous too. Beware diving in more than Force 4 with a typical RIB. However, swell also influenced strongly by wind direction and combination of wind and tide directions. If wind coming off the land, swell normally far less than if coming in off the sea. If diving in the lee of land too, lessens swell. If wind going in opposite direction to tidal flow, waves kick up higher and if wind flowing with tide, waves ease off. Can also get bigger waves if deep depressions in Atlantic sending swells in. Main thing to note is whether conditions likely to get better or worse? Will wind lesser or strengthen, will direction swing for better or worse? Will tide go more with wind, or more against it? Try to guess impact of all this then can use to judge whether safe to stay out longer or head for shelter early. Waves not the only problem. Fog highly dangerous as divers easy to lose and navigation harder. Cold weather very bad for poorly protected divers, and/or after cold water dives and/or long high speed runs back to shore as all increases risk of hypothermia. Sudden precipitation normally only a problem if causes problems with visibility or cold.

… Decide whether to use SMBs

Decide for yourself first (see "…Know when to use a Surface Marker Buoy") but then if on hardboat, talk to skipper. May be special circumstances that change things. If any debate use SMBs anyway, and if any risk of tangling, at least use DSMBs. Check whether if skipper says "don't need SMBs" that means truly no surface marker at all (rare) or DSMBs ok instead (far more common, and much safer). Don't be lulled into carelessness by calm sea. Tracking bubbles is very hard even in good conditions, and sea can easily worsen during a dive making tracking impossible.

… Buddy-pair the divers

Can be hard to do well. Several factors to consider. Pairs must have right experience. Do any divers need looking after? Buddy them with experienced divers if so. After this, try to get pairs with similar air consumption. Also better to have pair using same suit: both semi or both dry rather than one of each. Try to arrange it so that at least one of each pair has redundant air supply if poss (twins or pony). Ideally have at least one of every pair with either computer or at least ascent rate warning (e.g. good watch or D-Timer) but make sure that if only one computer between two that both divers crystal clear how to dive defensively that way. Think about diving style: some divers swim like torpedoes, others bimble. Try to match. If anyone has a camera, pair them with a bimbler to avoid frustration. And don't forget comfort factor. Ask if diver happy diving with your buddy suggestion and try to re-juggle if not. Getting best balance quite difficult sometimes. In practice often easiest to start with 'obvious' buddy pairs then see if can fit rest around this. If that no good, start swapping obvious pairs around. Odd numbers mean there will be a threesome. See "… Dive safely as a threesome" for advice and ensure divers fully briefed on how to do this safely. Finally - most important point: if can't make safe pairings for everyone, no choice but to stand some divers down. Try make sure that another site available and all get two dives in the end to avoid mutiny, but ultimately safety over-rides all other considerations.

… Keep everything moving quickly

So easy for time to trickle away and end up missing slack, only doing one dive or getting home really late. Make sure all know key timings. In particular if stopping for breakfast, make sure a departure time agreed stick to it. At launch site, marshall should make sure everyone doing something useful all the time. Needn't be high-pressure/hassly, just watch for anyone ambling around looking useless or simply chatting and find them something to do. Good idea to ask questions rather than 'command', e.g. "is your kit ready for launch?" rather than "get your kit ready". Delegate lots: e.g. get others to set up the RIB rather than fall into trap of thinking 'quicker for me to do it as I know how' - others will never learn this way.

… Estimate the viz from the boat

Once in deeper water, look down into the sea where the water shaded from reflections (e.g. close against side of boat). In the UK, generally the blacker the better. If water looks green or milky, viz likely to be poor. In early summer can often see particles of plankton. Prolific fine plankton usually worse than sparse big globs of the stuff. Allow for sky conditions. Bright sunlight shows up particles more so dull conditions can make viz seem better than it is. Watch bubbles from props. Poor viz makes even bubbles near surface look green or brown. Good viz if can see even deep bubbles and they just look darker rather than green or brown tinted. This often gives useful guide to viz on dive, but beware that surface viz not a guaranteed guide to viz at depth. Crud can form layers in water at any depth, so good surface viz may yield to awful viz at, say, 15m; and even if poor viz at surface, can occasionally be good at bottom, though will be darker than normal for depth.

… Brief the divers

Check echo sounder for max depth around dive site and tell divers. Describe site as far as poss. Describe major features (drawings ideal but rarely possible on UK boats), suggest good routes round them for best tour and forward dive profile (i.e. deep first, working shallower). Advise of any hazards: poor viz (see "…Estimate the viz from the boat"), unexpected drop-offs; wreck holds that may be entered accidentally, nasty current traps and so on. If diving at slack, state when slack window coming to an end and whether likely to be sudden or not (springs or neaps). Based on current dive depths, dive type, slack window and previous dives (if any), set maximum dive time by which diver to be back on surface. If any threesomes, remind of correct drill (see "…Dive safely as a threesome"). Decide if (D)SMBs needed (see "…Know when to use a Surface Marker Buoy") and ensure divers have right kit. If doing drifts in little/no current, tell divers what compass bearing to follow (roughly) so all go in same direction. If doing wreck, describe whereabouts on wreck shot seems to be and what direction best to go in. Make sure divers and skipper all agreed whether divers can/should come back up shot line. If more than one boat on site, agree special pickup signal so skipper can tell his divers from others. Ensure divers know if boat might use Thunderflash diver recall signals and tell to surface immediately (within ascent rate & deco obligations) when they hear the bang. Also note recall signal that may be used if diving with SMBs - club standard normally four strong tugs on SMB line.

… Watch for incidents in the making

Lots of little signs can indicate potential trouble. Watch divers kitting up. Any divers slow or making mistakes - check if dived recently. If not buddy with experienced diver and limit max depth. Watch for anyone glossing over missing accessories (e.g. knife, watch, SMB…), or minor equipment failure. Look out for sea-sickness. Check sufferers carefully and insist on buddy check as these are ones who will miss things. Keep an eye on anyone looking flustered/rushed for same reason. Watch out for unaccustomed buddies who don't talk to each other. Insist they talk through dive plan together.

… Record important info before dives

Rarely done but it is an important thing to have an emergency contact phone number for every diver. Imagine having a serious incident and not knowing who to tell… (the skipper may insist on divers logging numbers anyway). The DO can provide a laminate list of club member contact numbers to take with you. DO or secretary has these personal details on file too.

Dive slate It is also important to note colour of divers' kit. If divers swept away, rescue services need to know what to look for. Note hoods, suits, BCDs, cylinders.

If marshalling from the surface, best to make a note of when each pair goes in (wax pencil on plastic best). If diving too (as often the case) ensure that skipper/cox will note times in. Tell divers when you expect them up and raise alarm as soon as cause for concern.

Note specially if anyone diving on Nitrox or even Trimix and what is the mix.


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