Crossing Bars

On our way back from Tasmania we stayed stuck in the Port of Eden for quite a few days. Although beautiful, this is not the best place to be stuck as the wharf is ancient and catamarans are not permitted on it. No not even to take water, so having to rely on a water maker in a port is never a good idea particularly as some of the fishing boats were spilling diesel. This port is famous for its mussels but even those were not available to eat, so a disappointment all round.

Why you may ask, did you not take the risk to head North as there is the port of Bermagui just 40 miles North and you could head in there. Having alternative ports is indeed good practice in a passage plan. However, sometimes going into a protected harbor can actually be more dangerous than sticking it out at sea due to a large number of ports on the East coast of Australia having bar crossings. For the cruiser unfamiliar with the bar crossings an alternative port can be a port too far as shown in these movies.

What is a bar?

No not this sort of bar….

Coastal bars are shallow banks formed by the movement of sand and sediments. They build up at the seaward entrance of coastal rivers and lakes. They cause waves to become steeper and, in some cases breaking as they approach the bar. Some bars are stable so leading lines and beacons help a boat to enter, but other bars are renowned for constant movement.

We have just published a movie documenting our trip from Southport to Tasmania and this blog is a back up for the movie. Our movie documents how we take the decision to interrupt our passage south to Pittwater by pulling into Tuncury-Forster also known as Cape Hawke Harbour.

IMG_4724

We have an unpredicted southerly system meeting the northerly system, which we are sailing to get down to New South Wales and we want to spend a comfortable night out of the head on winds.

So this blog represents some background skipper’s knowledge all sailors and fishermen should consider having.

What is the analysis preceding our decision to go into Cape Hawke Harbour?

Firstly, one shouldn’t come into a harbor over a bar on an ebb tide. General advice is to only come in with the tide on a flood. This may be good advice, but as the sun is setting, for us not  possible.

The tides at Cape Hawke are roughly the same as the tides for Sydney (Alan Lucas Cruising the NSW Coast) so we consulted the time tables for Sydney.

These can be downloaded here Tide tables for Sydney

We usually download all the relevant tide tables for the areas we cruise in, prior to departure. However, the Navionics charts on our Ipad have usually also quite accurate data and do not require internet connection.

From the tide tables we see that we are on an outgoing tide, but luckily the wind is not against the tide. So would it be ok to go in?

tides Tuncury-Forster

How do we calculate both the height and the strength of the tidal stream for a given time? 

Well of course you can just go to a website that gives you the information if you have internet connection:

tides forster tuncury ww2.jpg

However, internet connection is not always a guarantee. So here is the manual method which uses the rule of twelfths. The rule of twelfths gives you an approximation of where the tide is at any given point in the day. It is not totally accurate, but on the whole will give you the information you need.

We can see in the tide timetables that we have a high tide at 13.21hours of 1.69m and a low tide at 20.00hours of 0.36m in Sydney. I actually know from the internet that there is a 7 minute difference between Cape Hawke and Sydney, but lets just ignore that as we are looking for broadly accurate information.

The time difference between high and low tide is 6 hours and 39 minutes or 399 minutes.

The height difference between high and low tide is 1.33m or 133cm.

We now divide the time difference is 6 equal parts of 66minutes approx.

And we divide the height difference in 12 parts of 11cm approx..

We are expecting Impi to arrive at the bar around 17.30 hours. That is 4 hours and 9 minutes or 249 minutes after high tide.

In the first hour (or 66 minutes) after high tide, the water levels will go down by about 1/12 or 11 cm.

In the second hour after high tide (or 112 minutes) after high tide, the water level will go down by 2/12 or 22 cm in addition to the 11. cm so in total, 33.3 cm.

In the third hour after high tide (or 178 minutes), the water will go down by 3/12 or an additional 33cm or 66cm in total.

In the fourth hour after high tide (or 246 minutes), the water will go down by another 3/12 or an additional 33cm or 99cm in total.

As you can see it is during the third and fourth hour that theoretically most of the water will rush out of the inlet.

Impi is arriving at the end of the fourth hour/start of the fifth hour, so we know that the rush of the water out of the inlet should be starting to come down as only 2/12 of the water will rush out.

Finally in the sixth hour only 1/12 of the water will rush out.

So the overfalls which occur when most of the water rushes out to sea, should have calmed down. These overfalls or standing waves can be dangerous when there is an onshore wind.

We also calculate that we will still have an approximate 70cm above chart datum (the lowest recorded point on a chart).

As you can see there is a slight difference with the internet calculation.

Our chart shows us that the lowest recorded depth is 3.3m so for Impi with a draft of 1.3m, we should be ok. However, there is always a danger when there are big waves, especially at ebb tide with opposing wind, that the waves bottom out and that a boat hits the ground as the water level can be less than chart datum.

Which other considerations does one need to take into account?

Due to the geographical shape of an inlet, the general rule of twelfths may not apply as to the strength of the current. Slack tide, which occurs when the tidal stream is zero, can differ considerable from high and low water, and in certain areas, the outgoing tide may run for up to three hours after the water level has started to rise, and the flood may run for three hours after the water has started to fall. We experienced this a lot in the Tuamotu islands.

Therefore it is advisable to consult a guide book if you have one and/or phone VMR (Volunteer Marine Rescue) in the relevant ports to have their view on the waves and the tides today.

In our case at Cape Hawke, they give us the go ahead, so off we go! The only caution VMR gave us was: ‘Mind the jet skis!’ Really? Shouldn’t they mind us?

So how do we cross a bar?

Firstly, we always observe the bar’s behavior for a few minutes before picking our way through the waves. We are always prepared to abort the entry if it looks too wild, regardless of any advice given. Try to observe from the side if you can, as it can be deceptive seen from the ocean side of the wave.

If it looks pretty wild  and your water tanks are in the front of your boat as on a Lagoon, consider emptying them as to lighten the bow. Similarly is you are storing diesel in containers up front, move them to the back. Store all loose lines and take down sails.

If another vessel is about to enter or exit, we wait and see how that vessel behaves in the waves. Don’t be too eager.

If you see crowds of people with cameras and 7 News, abort!

We study carefully, where the shallow and deep water is, helped by noticing the colour of the water if the sun is overhead.

We engage both engines and start to pick a path through the least active waves, at the same time observing the navigational aids.

We wear our safety gear, are both up on the bridge and keep an eye on the waves both in front and behind us.

Ideally, a boat should stay on the back of an incoming wave, but that may be a challenge for sailing vessels. Sometimes one will have to let a wave run under the boat and then one needs to slow the boat down to maintain steerage. Use warps or sea anchor to help you.

A strong outgoing tide will affect the boat’s speed and you may need to add power.

Accelerate gently over the wave peaks, which are sometimes formed at the mouth of an inlet on an outgoing tide.

Watch our movie to see this in practice Sailing to Tasmania

 

 

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