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Paul Caffyn's Planning for a Strait crossing

From http://www.seacanoe.org/Crosplan.htm which is no longer on-line, so 2011 text copied below, and copy of original here. This page linked from Cook Strait kayak crossing.

Notes by Paul Caffyn

Firstly congratulations to Dave [Herrington] for a well planned and executed double crossing.

Strait crossings are much more committing than coastal cruising for three main reasons:

• straits are generally subject to strong tidal stream flow and some are subject to both strong current and tidal stream flow.
• crossings take the paddler a long way out from the security of shore
• straits between high land masses, are subject to strong winds where the air stream is funneled between the two land masses.

The two essential elements of a successful strait crossing are:

The notes below apply not only to straits but also to channels subject to tidal stream flow.

Planning

The three important sources of information regarding straits are:

• The relevant volume of the 'Pilot'
• The relevant marine chart
• A set of tide tables or Nautical Almanac.

In the case of Cook Strait, which is a good example to discuss as it is subject to strong tidal stream and current flow, plus funneling of the wind between the North and South islands, page 86 of the 'New Zealand Pilot' (1971 edition) has a lengthy section on the strait with information on the tidal streams and current. It describes the worst areas for tidal violence, for example: 'As the tidal streams in the strait are rapid, especially off Cape Terawhiti where they attain a rate of 5 knots and upwards at springs, when the wind opposes the tidal streams a turbulent sea is raised, which with very heavy gales may be dangerous even to large vessels...... High water on the western side of Cook Strait occurs about 5 hours later than on the eastern side, so that when it is high water on one side it is nearly low water on the other.'

Further reading of the descriptions of the eastern and western sides of the strait allow a full picture to be built up of the tidal stream activity. The important slack water tidal stream times are detailed reference Wellington, and it is important to note that more often than not, they do not correspond to high or low water tide times on shore.

Since tidal stream strength is strongest during spring tides and weakest during neap tides, it is important to consult the Nautical Almanac or set of tide tables to pick a period of neap tides (minimal tidal range).

The marine chart often has more detailed information than is contained in the 'Pilot'. A diamond symbol, with a alphabet letter, in the strait will be referenced in a tidal stream table on the edge of the chart. Tidal stream direction (in degrees) and strength (in knots) are given at hourly intervals plus the slack water times reference a main port or secondary port.

Start and finish points for a crossing need to be researched. Where a strait is long with relatively straight coastlines, for instance Foveaux Strait or Shelikof Strait between Kodiak Island and the Alaska Peninsula, the choice is the strongest 'land to land' straight line crossing. Where a strait has capes or reefed headlands jutting out seawards into the narrowest part, it is best to look for a longer crossing as these headlands/capes invariably have violent tidal stream activity in the way of overfalls, races and rips. On a calm day, the paddler may be able to fight through such areas, however with any weather tide effect, wind blowing against tidal stream, these areas need treating with great caution and respect.

Two Approaches to a crossing

There are two different means of executing a crossing.

The big ferry glide

The tidal stream strength is calculated and an angle of drift is allowed during the course steered for the crossing. Dave Herrington worked out a course correction of 20 degrees for tidal stream drift during his north to south crossing. This enables the paddler to leave shore at slack water and cross while the tidal stream is flowing in one direction.

Caffyn's cunning slack water mid-strait method.

Ever since my first strait crossing, Cook Strait in 1979, I have used this method with great success. It involves calculating the time it will take me to make the crossing, saythree hours for a 12 mile crossing. I subtract 1.5 hours from a daytime tidal stream slack water time, and set this as my start time. Thus I should be mid-strait when the tidal streams turn. Having calculated the course to be steered, I steer that course for the three hours, with no allowance for drift until I have virtually completed the crossing. Maintaining the same course means a dog-leg course is paddled. For instance in Cook Strait, I start with the last 1.5 hours of the north- going tidal stream which by mid-strait drifts me well north of my compass line on the chart. Drift is then minimal for the short period of slack water, after which the south-going tidal stream drifts me back south so that I should arrive at my aiming point. This advantage of this technique are:

• Only one compass course is necessary with no need for drift allowance; very handy if visibility deteriorates with fog or mist
• The paddler misses the full mid-tidal stream strength both mid-strait and on both sides of the strait.
• If wind lifts during the crossing, then the weather tide effect is minimized.

Optimizing Favorable Conditions

To ensure success, it is important to optimize both favorable weather and tidal conditions.

• As noted above, the optimum tidal stream conditions are during neap tides, when tidal stream activity is at its weakest.
• Favorable weather conditions are not as easy to predict and will not always coincide with neap tides.
• It is necessary to observe the weather maps and wait until there is a very weak pressure gradient across the Cook Strait area.
• Cold front situations should be avoided like the plague for Cook Strait, as the NWly winds preceding the front funnel through the straight while a abrupt change to southerly winds accompanies the passage of the front across the strait. In 1979, even though I was superbly fit after the 1700 mile North Island trip, I waited seven days for favorable conditions to cross Cook Strait.