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Analysis of data from SA Nationals

Postby Slow Hand on Tue Jan 22, 2008 10:42 am


I read with interest the comment from Richard regarding the movement of marks at theNats due to tide.

During the Nationals the tide movement (betwen high and low) was quite small, and recorded as 0.1m on a couple of days - reflecting the local phenomenon of a dodge tide. I have copied the following text from website http://www.mesa.edu.au/friends/seashore ... _zone.html

"Occasionally on South Australian shores there may be a "dodge tide", where the tide may remain fairly stationary for a couple of days. If this occurs during the hot summer months and there is a strong searing wind blowing from the arid interior, all creatures of the high shore will perish."

So my question/s are:
- what extent of movement does the GPS data show
- how do we know it is due to tide, compared to movement due to wind (if there was any on the days of recording)
- am I misunderstanding the interpretation of a dodge tide

NOTE - this is not a discussion of compass, Tactik, GPS technologies

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Postby Richard Jackson 1660 on Tue Jan 22, 2008 7:19 pm

I have been trying to work out now to upload the jpeg file photos of my tracks but can't figure it out. I haven't put all the races on my computer as I lost the lead in the camping ground somewhere. I have another one somewhere, am still to find it though. The race I was referring to re mark movement was race two on the 30th. The wing mark moved about 115m and the bottom mark about 92 metres. This thing has a scale ruler. The top mark didn't move at all. I don't think it was that windy that day and can't remember what the tide was doing. The first race and invitation race the marks didn't move much at all. I haven't looked at any of the other races yet. Mark movement does seem to happen alot be it wind, tide or anchors getting thrown in tangled up with the rope. Anyway, it was all good fun out there, just wish it blew a bit more..., all creatures of the high shore nearly did perish, lucky there is beer!
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Creatures perishing

Postby Slow Hand on Wed Jan 23, 2008 10:25 am

I am glad you picked up on the creatures bit!! I thought it a nice twist, without distorting the facts of describing the dodge tide.

How do you know the mark moved? Is the position based on where you changed direction?? Could it be that you rounded the mark 115m away from where you rounded it the first time? For example, having to give bouy room to a lot of boats. But I guess that wouldn't be 115m would it. Maybe 10m in a bad situation.

If you get the data loaded and learn/discover anything else I would be interested to know.

Good sailing,

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movement of marks

Postby fitzwarryne on Sun Jan 27, 2008 2:59 pm

Having run numerous regional and national sailing events round the world I was very interested in the discussion on movement of marks. This has become important with the use of GPS for both course laying and navigation.

Both tidal flow and wave/wind action move the surface location relative to the anchor. Even minor variations of these will move the buoy to the maximum distance away from the anchor vertical location. Tidal range is not a direct factor. The critical question is how much slack to give after the anchor is lowered to the bottom.

If for example the depth is 100m, allowing an extra 5m of line will result in a variation of the buoy position by 60m. Allow 10m of slack and the variation is 90m. In comparison, Australian tidal ranges will have a relatively minimal impact on the variation of the surface position.

Thus, there could have been different variations in the amount the buoys moved during the race depending on both the depth and the slack to depth ratio, not on the tidal range. Remember tidal flow can be strong even when there is no change in tide height. In the Gulf, tidal flow can continue up to an hour after high/low tide [race strategy hint!].

If you think there are problems at Adelaide, think of the problem at places like Hawaaii when depths are in thousands of metres. Apart from problems of an accurate GPS set, cutting the anchor line is a viable option after the race due to the time to retrieve the anchor from the depths.

A buoy is only a mark when it's on the leg you are racing!

Last edited by fitzwarryne on Sun Jan 27, 2008 5:02 pm, edited 1 time in total.
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dodge tide

Postby fitzwarryne on Sun Jan 27, 2008 4:39 pm


The term 'dodge tide' was invented by Flinders to describe a Gulf condition which is found in only about five places in the world where there is a day of minimal tide.

In simple terms there is a sun tide cycle of 12 hours and a moon tide cycle of 12 hours 25 minutes. Thus, every 14.77 days they tend to cancel each other out resulting in the neap tide with minimal difference between high and low tide. Technically a semi-diurnal tide cycle.

In the case of the Gulf, the land is so shaped as to trap water flowing with the west- east earth rotation ocean tide. The result is that every 14.77 days there is a chance of a 'dodge tide' of around .5m rather than a normal 1.5 m at Port Adelaide.

The impact on sailing is that there is limited cold water flowing into the Gulf which vastly reduces sea breeze. The result is lake conditions; flat water and little breeze. Typical ACT afternoon. Very rarely after an Antarctic storm you get long heavy swells and no breeze. The hot air weather condition is a seperate issue which can lead to dust storms in Melbourne and an increase in beer consumption!

So you are right in your understanding of a 'dodge tide'. However there is still a little bit of current which will float the buoys out to the maximum length of the anchor line.

The only other place in Australia to get 'dodge tides' is the Torres Strait. In 1944 an American mathematician was given the job of predicting tides for the US Navy. It so frustrated him he committed suicide. These days with computers it is slightly easier and someone at Adelaide Uni has just finished a PhD thesis on predicting tides in the Gulf. Very interesting mathematically but unfortunately not useful when sailing a Sabre.

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