12 minute read
Overfalls
The Rough Guide
Clive Loughlin analyses the causes and effects of rough water – and explores some notorious areas
There be dragons
What constitutes ‘rough water’ is a matter of personal perspective. A cracking sail for one might be a terrifying experience for another that would put them off sailing for life.
Good skippers need to take the capabilities and natures of their crew into account in addition to the physical considerations of the rough water itself. A crew that has been briefed that “It’s going to be rough for about an hour until we round that headland, but then it will be a lot smoother”, will be a lot happier and have confidence in their skipper.
ABOVE
Portland Bill is a notorious spot for overfalls although you can sometimes avoid them by hugging the shore
Do your research
Areas of rough water should not take anyone by surprise. They are all well documented on the UKHO charts, the Reeds Nautical Almanac and pilotage guides.
I am surprised and disappointed that they do not currently feature prominently on some electronic charts. Casual mariners who currently rely solely on electronics, and especially those using A to B auto routing software, are likely to experience some unpleasant surprises.
Passage planning is a lot more than a legal requirement to be ticked off on the back of an envelope.
Careful studies of tidal streams will not only alert you to potential hazards but can also significantly reduce your passage times.
The Almanac provides good guidance on the well-known rough water areas such as the Portland Race, and pilot guides provide additional detail and salutary anecdotes. Specialist guides that provide detailed tidal stream data can be obtained for some areas, and I find these reassuring if I am considering the sort of short cuts for which ‘local knowledge’ is advised.
In this article I will cover some of the causes of rough water and illustrate these with some examples from personal experience.
A bit about waves
Waves are energy, and they get this energy from the wind. Although the wave moves across the sea there is no overall forward motion of seawater.
After a wave has moved over a stretch of water all the water ends up in much the same place as it was before the wave arrived. However, under the wave itself there is a circulation of water (see fig above) that extends to about half the wavelength below the crest like an inverted teardrop.
The height of waves depends on the wind speed and duration, and the ‘fetch’ or distance that it has travelled.
Duration
There is a lot more to weather than just wind speed. A force 6 (24kts) that has been blowing for 5 hours will create waves that are 1m high and not of too great a concern, but after 36 hours the height will have increased to over 4m.
Fortunately most sailing weather forecast apps also include wave height and direction, so although we cannot do much about the weather, we are blessed with technology that will at least give us a good idea what is coming our way.
Fetch
Wind direction is also key. Normally a trip heading to the West Country from the Solent involves a SW wind on the nose and a boisterous passage. On my last trip to Brixham we had a good breeze (F5) and most importantly it was from the north. This meant it was coming off the land, and with a fetch of just a few miles there was no time for waves to
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Anatomy of waves showing the circulation system.
BELOW
Opposing currents have a disproportionate affect on wave height and steepness build up. We had smooth seas, 9.5 knots over the ground, and no tacking for 100nm!
Waves against current
Most sailors will be familiar with the rough seas associated with wind, or more correctly, wave against tide conditions. The current essentially compresses the wavelength and this leads to a dramatic increase in wave height and steepness.
As an example of the disproportionate affect of waves against current, if a wave of wavelength 15m, which will be travelling at 10kn, meets a 2kn current its wavelength will reduce by 50% while its height is increased by 75%. This is a huge change (see fig below).
Steepness
Wave height is not in itself a concern, what really matters is how steep the waves are. Steepness is defined as the ratio between wave height and wavelength. Once this ratio is greater than 1/7 the wave will break.
Learning curve
I have only experienced truly horrendous wind against tide conditions on two occasions. The first was off the west coast of Scotland at the end of a very tiring crossing from Canna in a Force 7. We were seeking shelter in Armadale on Skye, but this meant a passage up the Sound of Sleat between Skye and the mainland. The ebb tide can hit 8kn in this area but we ‘only’ had about 5kn against us.
The second time was heading west out of the Solent in the Needles channel under similar conditions.
In both cases the boat pitched up and down by 30 degrees and everyone was hanging on to anything they could get hold of. To add insult to injury, forward progress to more sheltered waters was tediously slow.
Both of these events happened early in my sailing career, and I say that not as an excuse but to reinforce that they were so awful I have made very sure not to repeat them.
Shallow water
In deep water the speed that a wave travels is entirely dependent on its wavelength with long waves travelling faster.
When a wave enters shallow water, with a depth less than half a wavelength, the speed becomes entirely dependent on depth. The bottom of the teardrop circulation system begins to drag on the seabed and this slows the wave down, which in turn leads to a reduction in wavelength and an increase in height and steepness in much the same way as an opposing current. As the wave progresses into the shallows the wave gets steeper and steeper until it breaks. This is great for surfers but not so good for sailors.
Bars
If waves encounter a contrary current at the same time as a reduction in depth then the effects combine dramatically. This can most often be experienced in the approaches to harbours and estuaries.
Chichester Harbour is a large expanse of shallow water with a narrow channel just 1 cable (0.1nm) wide connecting it to the sea and a shallow bar with depths of 2 m or
ABOVE LEFT
St Catherine’s Lighthouse, Isle of Wight
BELOW
Skirting the overfalls – calm water very close to rough water. less guarding the approach. On a spring ebb the current can reach 6 knots and should not be attempted in a strong onshore wind.
Many harbours and estuaries have bars across their entrance, and they should be treated with respect. They are all the more dangerous because they represent the gateway to a safe refuge. After a testing passage it can be tempting to seek sanctuary by ducking into a harbour – but the approach to sanctuary could actually be the greatest danger of all.
Overfalls
Overfalls are areas of turbulent water caused by a sudden change in depth.
Anyone planning a circumnavigation of the Isle of Wight has the overfalls off St Catherine’s point to consider. The depth reduces from about 40m down to 15m and can produce very rough conditions,
especially on a west-going spring ebb against a strong SW wind.
The rough water created by overfalls is often very tightly defined and can normally be spotted, at least in daylight, by an alert crew. If travelling against the current it is then a relatively simple matter to skirt around the hazard keeping the boat in calm water while the turmoil is no more than a few boat lengths away.
The danger comes if the boat is riding the favourable stream, in which case it is very easy to get sucked in, and before you know it the crew are wondering where all the rough water has suddenly come from.
In both cases a bit of diligent passage planning can avoid the hazard altogether. A planned diversion made early enough leads to a very minor increase in overall passage length, and it is very satisfying for the skipper to draw their crew’s attention to the white horses that have been effortlessly avoided.
Races
Races are areas of turbulent water caused where two currents meet in shallow areas around headlands and in constricted channels.
Portland Bill off the south coast of England is one of the most notorious.
I was once on the dockside in nearby Weymouth, following a very easy, blue-sky sail from the Solent, when a Westerly Centaur arrived and tied up alongside. After a brief interlude consuming whisky in their cockpit, the crew disappeared below and started passing up all their soaking-wet bedding to be draped unceremoniously over the boom.
Apparently they had just been through the race.
The Portland Race is a ‘perfect storm’ location for rough water with the currents flowing in the main Channel meeting the currents diverted down either side of Portland Bill.
The southern tip of Portland Bill extends south over Portland Ledge with depths of just 10m over an extensive area that extends about 1.5nm from the headland, and an additional shallow area called the Shambles to the east of the Bill.
The position of the rough water depends on the direction of flow and state of tide. Being displaced to the east by an east-going flood tide and to the west on the ebb.
If the confluence of strong currents and the rising seabed were not enough of a hazard this area is also popular with lobster pot fishermen. The prospect of being anchored by your propeller in such a dangerous area does not bear thinking about, and for this reason no passage through the race at night should be considered.
The race can be avoided by passing 3-5nm south of the Bill. There is also an inshore passage between 1 and 3 cables off the Bill that offers relatively smooth water.
While planning challenging passages I like to consult every reliable source of information I can lay my hands on, and for this area my go-to sources are the Alamanac, Shell Channel Pilot and Peter Bruce’s ‘Tidal Streams between Portland Bill and St Alban’s head’ which happens to be waterproof; presumably as a testament to the practical experience on which it is based.
St Alban’s Race
St Alban’s Race is another area that does its best to impress. The merging of currents is not such an
ABOVE
Dramatic conditions off Portland Bill
BELOW
Currents around Portland Bill merge on the Ledge to create a ‘perfect storm’
BELOW RIGHT
The shallow ledge off St Alban’s Head creates another Race
issue as it is at Portland Bill, however there is the St Alban’s Ledge that extends some 4nm from the Head. As with Portland there is an inshore passage within 2 cables of the Head.
Timing is everything for coastal passages in this area and if heading east I like to hole up in Chapman’s Pool before leaving on the early turn of tide that flows south along the west coast and rounding the Head at slack water or on the start of the flood.
Those heading west can hole up in Poole, Studland Bay or Swanage waiting for the tide to ebb. St Alban’s Head may not have quite the notoriety of Portland Bill but it can boast the added danger, if any were needed, of the Lulworth Cove firing range immediately west of the Head.
The Alderney Race
The Alderney Race lies between Alderney and Cap de la Hague on the French mainland, and is characterized by very strong tidal streams that regularly exceed 5 kn. These currents, combined with an extensive shoal area with just 20m depth, make this a no-go area for sailing boats under certain states of tide and especially with an opposing wind.
On one trip we had been overnight in Cherbourg and had intended to get to the Cap de la Hague just before the tide turned south to take us to our destination of St Peter Port on Guernsey. Our departure got delayed and we arrived at the Cap after the ebb tide had become established. There was also an F4-5 from the SW.
When you can see rough water from 3nm away you can be confident it will be even worse than it looks. If we had pressed on we would have had a very fast and very rough passage tacking our way to Guernsey. I decided to continue due west on an easy close reach, leaving Alderney and the Casquets to port before heading south.
Corryvreckan
Whirlpools are best viewed from a high-speed tourist RIB of significant dimensions. The only one I have personal experience of is the notorious Gulf of Corryvreckan that lies in the narrow (0.5nm) channel between the islands of Jura and Scarba off the west coast of Scotland.
The tidal streams in this area are always interesting and passages between the islands often include dramatic changes in the boats heading over which you have little or no control. These streams are so strong that fighting them is pointless and the best solution is simply to go with the flow and give all granite a wide berth.
The Gulf of Corryvreckan is something else, on the ebb water flows eastward from the open sea and is relatively undisturbed, however the flood tide heading west has been disturbed by the varied topology as it flows up the Mull of Kintyre.
The Gulf includes a huge hole that drops to below 200m and also a pinnacle just north of the centre of the channel that rises to just 27m below the surface. When the strong west-going flood tide hits this pinnacle it is forced upwards and creates whirlpools and waves up to 9m high.
I have sailed through the Gulf many times, but always on the east-going ebb, and on one memorable occasion our passage was accompanied by a rainbow and leaping porpoises, in a pastiche of hippy artwork.
ABOVE LEFT Corryvreckan : Whirlpools and large steep waves await the unwary.
ABOVE RIGHT
The Gulf Of Corryvreckan
BELOW LEFT
The Alderney Race – best avoided under certain conditions.
Clive Loughlin is an RYA Yachtmaster Instructor and regular Sailing Today contributor. Clive teaches for First Class Sailing in the Solent area.
