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Crossing 'the Ditch' - how, when, where? (Why is easy)

by John Martin, Island Cruising Association on 30 Jun 2014
Crossing the ditch - the Bay of Islands beckons SW
New Zealand has some of the greatest cruising in the world, but many Australian sailors are put off by the horror stories they have heard about how difficult that crossing can be. Here, John Martin of the Island Cruising Association gives a few simple rules to achieve a pleasant and successful crossing - and to be rewarded with some dreamy cruising grounds.

Yes, making the choice to 'Cross the Ditch' will place yourself first up in New Zealand with it’s fantastic cruising and as the Cyclone season tails off towards the end of April you’re perfectly placed for getting to the SW Pacific and enjoying those trade wind breezes and a downwind run home.

There are a lot of myths surrounding the passage to New Zealand from Australia. Most are the result of inexperience and lack of knowledge. I’ve done the trip personally only once and had a great trip. Too little wind would have been the only complaint.

But I’ve listened to the experiences of many that have made the passage, both good and bad and have successfully weather routed more than a few across from as far north as Brisbane all the way south to Tasmania.

Weather is the governing factor when making this passage and your strategy will vary depending on your departure point. What doesn’t change is the nature of weather in this region. In the South West Pacific, Tasman Sea and the Australian continent the weather systems migrate from west to east, low follows high follows low in a never ending succession.

Summer to winter however sees a subtle change in these patterns as they move toward the equator in the winter months and south toward the pole in the summer months. This affects the wind on the periphery of the high pressures systems.
The effect a high pressure system has on the winds at its edge depends on two factors, the centre pressure and the latitude of the centre. A 1020 centre pressure high will give 15 to 20 knot at the perimeter of the system. As the centre pressure increases so do the winds along its top and bottom edge.

This area of wind is the squash zone between the high pressure cell and the inter tropical low in the tropics and the low pressures south of the high in the roaring forties. The latitudes of these squash zones is dictated by the latitude of the centre. This also dictates where the band of Westerly’s lies on the bottom edge of the high. It’s these westerly winds that we’re looking for to have a successful passage across the Tasman.

As the high pressures migrate east the direction of the winds change. In the southern hemisphere the winds around a high pressure system rotate in an anticlockwise direction. As the high presents itself, usually after a frontal band associated with the departing low pressure system, the winds on the front edge on the lower hemisphere of the system will be SW. This is often what’s known as a disturbed flow in the wake of the trough or front.

Imagine sweeping your arm across the surface of your pool, the water that follows in your arms wake is roiling with eddies and a rolling motion. The same happens with a disturbed SW flow. The air tends to roll, as it does so the air rises and cools causing showers and squally conditions. Then falls and starts the cycle again picking up moisture as it warms again closer to sea level.

As you travel further south the effect and intensity of the frontal bands associated with each low are magnified. The majority of low pressure cells during October and November migrate along under Australia and tend to pop north east in the Tasman, cross NZ and migrate to the east. During summer however low pressure cells often form off the warmer water traveling down the Australian east coast in the EAC (East Australian Current) the intensity of these low depends on a number of factor starting with the temperature of the sea in the north and the temperature and wind velocity in the upper atmosphere as it travels south.

Looking at the reports from boats sailing across the Tasman over the last few years the best time seems to be early in the year, January and February. The highs have moved south and are stable and the time frequency of the migration is at its longest, usually around 6 to 8 days at this time of year. As we are talking about sailing from Australia to NZ in this scenario you will be moving from West to East and going with the systems, this will have the effect of prolonging the time you are in the system. For more information, go to the http://www.islandcruising.co.nz/!website.

In the weeks leading up to your passage we need to be looking at the speed that the systems are coming through. An 8 day cycle is easier to plan for and we want to be looking at where our start point is going to be. Many people making the jump across the Tasman are inflexible when it comes to departure point and often have to wait considerable periods to get a good window. The net effect of this of course is a rising impatiens and leaving regardless, next thing you know, you’ve got another horror story.

Departing in Jan /Feb and being a good cruiser (good cruisers never go to windward) we’re going to be looking for Westerlies and our departure point is going to be dictated by how far south the centre of the high is. If they are crossing the coast around Sydney latitude then leaving from Brisbane is going to be impractical unless you are prepared to leave before the high presents, usually a NW wind and dive as far south as you can before tacking over to starboard as the front goes through.

A better scenario may well be Eden on the south eastern NSW coast, also a clearance port, putting you on the lower hemisphere of the high and those blessed westerlies. How far south you go on departure will dictate the strength of these winds.



Next to look at is the arrival point in New Zealand. If my departure was from Sydney or Eden I would consider Nelson as an arrival port. Remembering the rotation and the direction of the winds in relation to where the centre is, if you were to consider Opua or Auckland and a consequent route around North Cape you would likely be running into those cursed Easterlies on your approach to New Zealand. A second consideration is the distance, Eden to Nelson is the shortest distance across the ditch.

So to summarise, we are looking for the following:

* A high pressure system
* Light to moderate SW to W winds (dictated by the centre pressure of the high)
* A frequency of around a 7 to 8 day cycle
* No nasty lows forming to the north
* A good lookout for lows approaching from the south.

Tips and Tricks, things to look out for:
We use PredictWind for our weather information as well as weather routing and this has worked very well over the last several years. Once you’ve made the choice to depart keep updating your weather and remain flexible. If there is a low developing in the South Australian bight and this is likely to affect you adversely if you carry on your initial track then go to plan B. This may mean heading further north and making Opua your destination. Good weather routing and the ability to get the right information on passage is essential.

About the Island Cruising Association:
ICA offers an ever growing knowledge-base of cruising resources and information specific to Extended Coastal (New Zealand and a developing section on Australia) and Offshore, with an emphasis on the South West Pacific. A wide range of fun events, training, practical demonstrations, on the water preparation and back up to assist cruisers to get out there. Membership in the Island Cruising Association is NZ$45.00 per year and membership gives you access to the entire knowledge base.


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