"The trouble with kayak crossings of the Cook Strait is not the distance itself but the Strait's notorious tides and winds, and the interplay of the two." Conrad Edwards.
View of the bottom of the North and top of the South Island of New Zealand showing the stretch of water to be kayaked.
This is the intended route between the islands. West to east then east to west.
I cannot under estimate the Cook Strait as it one of the most dangerous and unpredictable straits in the world. I need to consider many factors when I cross:
tide
currents
navigation
weather conditions.
Tidal Streams
A tidal stream is the horizontal motion associated with the flood and ebb of the tide (in and out) whereas the tide is movement in the vertical direction. There is usually a period when flow stops, called slack water, before the flow changes direction. Interestingly slack water does not necessarily occur at high or low water.
Tidal streams can be calculated similarly to tides. It is simpler however to compare the stream and tide at a port, say Wellington, and determine the time difference between high and low water at the port and the ebb or flood of a stream in a particular area, i.e. the Cook Strait.
The tidal streams in Raukawa Moana are incredibly unreliable. Often they run from 8-10 hours in one direction but there have been times when one stream will run for 18 hours or more. The reason for this is that occasionally the opposing stream is very weak or hardly noticeable at all, so the one stream just keeps going.
Neap Tide
Moon sizing at Apogee and Perigee. It is 13% larger in the sky at Apogee.
Perigee, moon closest to Earth. Apogee, moon furthest from Earth.
Neap tide is a half (quarter) moon. Image source: www.muvuca.net 24.jpg
Tide table.
Graphs that show, Wind speed, Swell and Chop size, Frequency of waves, Current direction, and Tides at the bottom. You can see on the right side that high and low tide are getting closer together. Those are the Neap tides.
I want to do the double crossing, so the best possible conditions for the crossing is really important. The best days for crossing is two or three days after a neap tide as this means there is less discrepancy between high and low tides - less tidal currents to contend with. If the moon is in apogee (greatest distance from the earth) it has less pull too.
It is not just the neap tides I need to worry about however. Also the more local tides on the day are a large aspect of whether a crossing is possible. The aim is to have it at slack tide all the way across the Strait, so that there is a lot less flow to deal with.
Based on the phases of the moon, the best days to cross are:
- Saturday 2 April to Monday, 4 April
- Saturday 16 April to Monday 18 April
- Saturday 30 April to Tuesday 3 May
- Sunday 15 May to Tuesday 17 May
- Monday 30 May to Wednesday 1 June
- Tuesday 14 June to Thursday 16 June
- Wednesday 29 June to Friday 1 July
The moon is in apogee on May 18 and June 15 so if the weather is calm, either of those days would likely be excellent for the double crossing. The June and July dates will be getting a bit cold and are a much less likely proposition.
Currents
This is Julian Thompson from GNS Science talking about why currents are so strong.
Navigation
Navigation is made harder by the tides in Cook Strait because they flow perpendicular to the traveling vessel (me in my kayak). A compass heading will give you a direction but currents may sweep the boat one way or the other and it could take a lot longer. What I will do is use a the Transit Method. However taking a compass bearing for a start will be necessary because of starting before light and not being able to see the other side of the Strait for the first couple of hours. Then when the Brothers Islands are visible will take two points and line them up and make sure I keep them lined up so I know I am not drifting off my course. That is the general idea.
Weather Conditions
Probably the most frustrating thing is going to be wind. It is definitely going to cancel a few days. It is very variable and can gust up to 50% over the average on the weather forecast. Wind can make the waves much larger if stream and wind are in the same direction. And it kicks surface water up if it is blowing against waves.