Course to steer software

[Yellowtulip]

Originally Posted by Idlegreg

I don't know for sure but I very much doubt Neptune is calculating the Tide streams directly using an algorithm. I expect that, like other software, it uses a table of data in just the same way you do when plotting the vectors manually. As far as I know some software uses Tidal Diamond data (which I believe is empirical) and others use data calculated using mathematical models but in those cases the modelling is done on much fancier machines than your humble PC and the output predictions are stored in the software in the same way as diamond data would be. Having said that, Neptune probably does use an algorithm to calculate a HW time for the day in question, which enables it to pull the correct stream data out of the table. If it does that using a simplified harmonic method then the HW time and hence the times of the streams may be slightly less well predicted than you could achieve if you have almanac data, but this difference is unlikely to be very significant.

But all this does not make the output a “Guess” any more than someone painstakingly plotting the tidal and vessel vectors on paper is a Guess. Just like our traditional plotting the tide stream in the real World will not exactly match the predictions but in most cases doing the calculation will give a better result than just a Guess - whether using computer plotting software or plotting by hand on a chart. The time of HW on which you base your plan might be slightly better if done manually but the computer will not only do the calculations much faster but it is less likely to make a mistake and it will almost certainly be more accurate in deciding where your boat is at each point in the passage which could well mean a more accurate output.

Ok, I'm very happy to change 'guess' to 'estimate' or 'calculation'. I look forward to trying the software IRL.

[Yellowtulip]

Originally Posted by Dockhead

Well, your method IS a CTS method, just a crude one (or rather a good and handy one) based on a unique coincidence of factors.


I agree that ALL CTS calculations require guesswork, but I do not agree about guessing the actual tidal currents -- the models are extremely accurate, to a tenth or a couple of tenths of a knot. The currents may vary somewhat if a strong wind up- or down-Channel is moving the surface of the water, but otherwise they are bang on. What's less accurate is using a tidal atlas which just gives hourly tides, and doing the tidal vectors only on an hourly basis. Neptune uses the UKHO digital models which are on 6 minute increments and does a 6-minutely vector calculation so a big leap forward in accuracy.


Where we can't be so accurate is with our passage speed, which is the real guess, so updating the calculations based on plan vs actual passage speed is sensible.


The way I do it is to make a series of calculations based on different passage speeds from the low end of possible to the high end of possible, and that gives a range of CTS. That way I can see the sensitivity of CTS to different average passage speeds so I'll know how to respond as the actual average passage speed starts to be clear. It's usually not more than a few degrees, but can be a lot in some cases. So as the passage develops and I see what the average speed is looking like, then I can change the autopilot to the appropriate heading, then about mid-Channel re-run all the numbers.


This is much more accurate than the RYA hand method, which I used for my first 50 Channel crossings, but the RYA hand method while inherently inaccurate, is still extremely useful, and using that I was rarely more than a mile off at the end, especially if I did a mid-Channel correction. When you're one hour out then you can pretty much just steer by COG the rest of the way in. Works great.


The main point is that even a pretty rough and dirty CTS works better than crabbing along the rhumb line. If you are sailing at 6 knots such that you will get an equal amount of ebb and flood between Needles and C-bourg, then CTS will be very close to 180. I'm usually quite a bit faster than that, so the CTS calculation is more critical. At 9 knots I will have almost double one tide vs the other.


Neptune gives a different calculation which is particularly useful if you're not crossing perpendicular. If you're sailing say Alderny -- Brighton or something like that. It will tell you what departure time is favored by the tides, and tell you exactly what the passage times will be at different departure times, based on a given average speed through water. The differences can be huge and the favored departure time is often non-obvious.

I don't think I was contradicting the concept of a CTS. I certainly didn't intend to. My secondary point was that running Neptune for today, there is only a half-hour's difference or so in the list of optimised courses which could very easily be lost anyway. But that's in the simplest situation.

As to its tidal predictions, I look forward to trying them out.

[Yellowtulip]

Originally Posted by Idlegreg

Yes you can plans often get hijacked by life. With a PC it is easy to calculate a new best route to the destination if you find you have been taken well off plan. I still remember learning from Mary Blewitt’s lovely little book that once I am off my plan I must rub out all the lines I so painstakingly drew and create a new plan - from where I am NOW to the finish, rather than trying to get back onto my original route.





It is not clear to me if you are actually plotting the cumulative vectors and correcting for that or not, but of course you are making a passage that you have done before, at a predefined time (with respect to the tide) and it so happens that the crossing works out conveniently to 12 hours, so the tides almost cancel out, meaning the course will be similar whether tides are strong or weak that day. You are clearly doing the right thing and not fighting the tide in an attempt to maintain a straight ground track, so your system works well. But if you were making a passage new to you then software might have more benefit (or where you have been forced some way off track it could make recalculating much quicker).

Despite all this if you like doing it the paper way then I am not trying to suggest you stop – we choose to go sailing and it really is not the fastest, easiest or cheapest way to get about, if you are happy or even prefer calculating tide vectors on paper charts I would not want to discourage you and if a young sailor on a vey tight budget has to choose between not going or going without any electronics I would encourage them to get out and sail.

Yes, I do plot the vectors usually for three boat speeds/ wind strengths. I just think the concept of optimising for such a simple voyage is a little unnecessary, but I accept that I might be pleasantly surprised by it in other cases. St. Peter Port to Treguier might be interesting, for example.

By the way, apologies to the rest of the world for being so location specific!

[Idlegreg]

Originally Posted by Yellowtulip

By the way, apologies to the rest of the world for being so location specific!

Indeed - although we have a bit of an excuse becasue the OP (who appears to have got bored of our ramblings) did ask about the existence of software he could show students after he had taught them how to do these calcs by hand. Hopefully he now knows that there is a choice of such software.

[marcvilleneuve]

I think it works well when the distance is short (1-2 miles) and the current is strong and unpredictable like in a short race. There is no time nor accurate information to support any calculation anyway.

It does not work at all for long legs like going through the North Channel. I would definitively prefer a routing software for that.

[Paul Jaques]

Hallo,
Calculating the course to steer is easy with the Excel worksheet y have made.
The excel file is too big to post on the forum, so I made a picture of the worksheet "COURSE TO STEER".
If anyone is interested in the excel worksheet please contact me and I can send the files to your email address.
You can choose from three sizes, according to the size of your screen. (There is also a possibility to adjust the columns and rows, if your screen size differs from the three created sizes in the attachment. Choose a size that is closest to fits your screen size).
This Excel worksheet should simplify navigation and can also be used to check your own calculations.
For the people who know excel, this worksheet should not cause any problems.
If you have looked up the tidal data on the nautical chart or almanac and entered in the green boxes, the tidal vector, which otherwise has to be drawn on the nautical chart, can be calculated for you in this worksheet and with input from all other data available. , the course to steer is then also calculated.
The course to steer can be easily recalculated every hour.
The helmsman can then sail with the calculated course to steer on the steering compass.
It took several hours to realize this worksheet in excel.
Additional worksheets were also made to support the main worksheet “COURSE TO STEER”
Unfortunately, I have not been able to test the excel worksheet “COURSE to STEER” sufficiently in practice and I ask the people who read this to test this excel worksheet in practice.
Hopefully I will get a lot of comments about how user-friendly the excel worksheet works and also where improvements can be made. Also whether the calculations are correct and certainly the calculations that are made when there is a crossing, so if the course can not be sailed.
When you open the excel, the program starts on the excel worksheet “COURSE to STEER”. There you can enter all data in the green boxes. It may be that the sheet does not fit completely on your screen, but you can adjust that. Please read the Excel worksheet ”HELP” (click on the light blue box “HELP” to do so.
All worksheets are protected with a code so that the formulas cannot be changed.
To display the worksheet completely on the screen, press the “ctrl” key and the “a” key at the same time.
Any change to a formula in the worksheets can be detrimental to the calculations.
Good luck with the testing and be sure to let us know.

[rgleason]

See https://opencpn.org/wiki/dokuwiki/do...ns:otidalroute


Don't see my previous post here. This calcs ETA could be augmented to calc CTS too using and referring to the spreadsheet solution.

[rgleason]

That post is gone too. See this thread.

https://www.cruisersforum.com/forums...in-199079.html