Monitoring XTE

[[2574]]

I use Seapro to passage plan and use the "Course to Steer" (CTS) from the planning function extensively; I find it generally is spot on at the arrival destination. However as I sit and watch the XTE increase from the waypoint to waypoint straight line (the rhumb line?) I always wonder whether I'm at the predicted XTE point or have exceeded it and will end up down tide (or whatever) at the destination. Seapro does not seem to advise the lat/long of the expected positions at each hour of the passage - so it's always a guessing game. How do you monitor actual XTE as compared to expected XTE?

rob

 

[tillergirl]

Umm. Interesting. I think its a big question to ask of any programme to be able to predict XTE at any point on a passage between two points. When you are calculating a course to steer there is an assumption made that the tide will be constant for that whole hour. Which it won't of course. So if for example in hour -4, the tide may be quoted as 1.2kts at 237 degrees and 1.6 kts at 200 degrees in the next hour. At some point the tide speed is increasing and the direction changing and of course that is a gradual process rather than some switch taking place as the 'hour is struck'. And then there is the issue that as you progress on your track, you may be moving from one area of tide to another. For example, in doing the passage planning, you will use the nearest tidal diamond for your tide information. This might be close to your departure waypoint but some way from your destination waypoint which in turn might be closer to another tidal diamond that has different values for each hour. And then there is the variation between spring and neap tides to take into account to add to the equation. There is also the possibility that your boat average speed might alter (sea state, variations in wind). Whilst you would use an average for the basis of any calculations, contrasting with actual might mean that the expected XTE is a 'not that useful thing'. So the differences might not be large but all the variations mean either a vastly complex mathematical formula or a result that could be pretty inaccurate.

Everything in this is an estimate. It is reasonable to estimate a course to steer based upon a tidal prediction for a particular day at a particular time to go from A to B at a particular average speed. Yes, you could plot a theoretical curve of XTE based upon that estimation but the realities of the variations would mean that the actual XTE could differ. And what then would you conclude. Helming error - not necessarily; passage plan error - not necessarily; exceeding or failing to achive the expecting average speed - not necessarily and so on.

As you know, XTE is kind of handy to know to check that you are not being set down on any hazards (and if you are in a narrow swatchway, you want to keep the XTE very small) but I think that's about it.

Not sure I've explained my point of view that clearly. Interesting question.

 

[Robin]

I use Neptune C-Map version which will show the predicted groundtrack and the chart/track can be printed out and XTE measured if I'm that curious, but it is easy enough to estimate on screen if needs be. I don't use the laptop for on route navigation but the plotters display XTE and the ground track so the actual v expected can be compared.

It is even possible to do it for a simple x-channel type passage with a bit of effort with paper chart and pencil, but a bit time consuming! Generally the CTS will have been chosen to compensate for overall tide effects so if plotting with two opposing cross tides the max offset to one side should not be the total sum of the tide hours flow in that direction as you would have been steering a CTS to compensate for some of it - if that makes sense!

 

[tillergirl]

It makes sense but the CTS will be designed to compensate for the effect of both the hours. Are there any circumstances where the CTS (which is a compromise for the whole passage) might add to one or more of the sum of the tide hour's flow. I need to get a pen and paper out!

I think my only point to the first question is that a predicted XTE can never be more than a theoretical estimated prediction! And therefore of questionable value in comparing against the actual except where aviodance of a hazard is desireable. Does that make sense?

 

[[2574]]

Yes I think your reasoning sensible - I suppose that I had assumed that the software is more clever than it really is! The question arose out of seeing the image of the expected passage as compared to the rhumb line - see below:

Hopefully (!) you can see the blue dotted track (a plan from Helford to Scilly) going south of the solid blue rhumb line - this led me to think that the software could provide hourly expected plots of the expected track. If I could get these expected positions out of the software I could then compare these to actual.

rob

 

[Robin]

Yours makes sense to me too. I was thinking of a 60 mile X-Channel when I wrote my reply which has some easy mental arithmetic simply because at 60mls one deg of heading is one mile of offset, or half that half way and so on.

In the good old days pre-Decca/GPS I made a planning sheet to do it manually and the trip Poole to Cherbourg (our frequent fliers route) which is 60mls entrance to entrance and the tides are pretty well 90 degs on either beam. Thus we could do a simple prediction, flow rates corrected for springs v neaps, based on adding all the east going and west goings together, take the difference and compensate for CTS by converting the nms to degs 'left or right'! Once we had Decca we could check XTE versus the maximum East or West predicted and do a rough compensation in the head to allow that the CTS chose will have had some diminishing effect on the maximum off to the side. On steady sails at predicted we got very good at reaching the end points spot on without touching the autopilot, failure meant an inquisition from the 'other' navigator! We still use that planning sheet for CTS although nowadays it is on a spreadsheet printout and gets checked with the Neptune program.

XTE is a very useful tool in my book. We had a diabolical sail once from L'Aberwrac'h to St Peter Port, winds gusting at 55kts for most of the way and huge seas after over a week of successive westerly gales and then a secondary low nobody forecast until after we were out and commited. We were using the Decca repeater in the cockpit simply to stay offtrack between zero left and half a mile or so right of track because the compass was unreadable it was swinging so much. We knew we could err some to the right but not too much to the left if we didn't want to pile on the rocks of SW Guernsey.

 

[john_morris_uk]

You might also note that one (traditional?) way of monitoring XTE is to review your course to steer every time you half the distance to your destination. At the same time you also make sure that you don't end up down tide and down wind of your destination.

 

[snowleopard]

Here comes some heresy.

I select the waypoint, hit Goto on the GPS then Track on the autopilot. The XTE never gets above .01. I know it's marginally less than optimal but who cares? The great benefit is that you can draw a straight line on the chart between obstructions and know you will follow it precisely. On passage between Dover and the Humber this technique has proved invaluable several times. The only time it failed was when an unnoticed GPS datum discrepancy exactly cancelled out the offset I'd plotted from a buoy and we headed straight at it.

If the electrickery goes tits up I can still revert to paper and pencil or even dust off the sextant.

 

[[2574]]

Heresy indeed sir! But I expect that is exactly what I will do as well!

 

[tillergirl]

Oh no this post is going to rachet! Zero XTE indeed!!!! That's what comes of wandering off to finish off my neighbour's cabin windows.

On a long tracks with crosstide components 'marginally less than optimal' is arguable. There speaks someone with a fast boat!!!! It is, after all, a factor dependent upon boat speed relevant to speed of tide. If you were averaging 4 kts with say up to 2.5 knots cross tide, I think it would be more than marginal. But I can't argue with Snowleopard's point about obstructions. Allowing the cross track error to build when there are obstructions close to the track does score highly on the 'daft' scale. But isn't Dover to Humber largely a linear tide track unless you are cutting across the Thames Estuary, thus leading to pretty zero XTE anyway?

John's point is interesting. To do that you have to be able to compare estimated position against an expected one - in otherwords to have the expected XTE ready. But if we use the example of the chartlet - which is a good one - if approaching Wolf Rock if for some reason, the CTS had kept you on the rhumb line on the approach, and if you were going to continue to hold onto the CTS which presumably has some starboard component in it and the tide was not doing what was expected, then there's a choice between leaving Wolf Rock to port or mooring to it! Tee-hee.

I just think that trying to plot anything more precise/better defined than the example on the chartlet is kinda tricky. My first cross channel crossing was Poole to Cherbourg and I religiously worked out the tidal vectors to an infinite level of accuracy - it was a charted yacht and we had no Decca - I remember being deeply miffed at the surprise of the crew when Cherbourg appeared just where it was supposed to be!