Course to steer software

[s/v Jedi]

Originally Posted by Kolchac

PredictWind is looking OK.

PredictWind is great but very expensive when you want tides and currents. It works differently than the math by hand: it creates a route among many waypoints which optimizes in regards to polar graph, wind angles, currents, waves etc. Next you transfer that list to your autopilot and steer to waypoints instead of heading. This works because it anticipates on drift etc. while it’s sheer impossible to do that by hand.

[Dockhead]

Originally Posted by Andy Todd

You don't need software. Here's a spreadsheet for it.... http://pzsc.org.uk/wp-content/upload...calculator.xls

and the explanation.... https://pzsc.org.uk/shorebased/coursetosteer/

That spreadsheet is handy, but the advantage of Neptune Plus and no doubt the OpenCPN router is that it uses the 6 minute increment tide data.

Originally Posted by Andy Todd

Here's a interesting vid with a real example.... see at 13:08...

The CTS is pretty extreme.

If you go from the Needles to Cherbourg without calculating CTS there's a chance you'll not get there.

Indeed!


We discuss CTS navigation on here once every couple of years, and there is always someone who can't quite imagine the problem and can't accept that you shouldn't just steer the rhumb line. This kind of example leads to a lot of "aha!" moments.

[Idlegreg]

Hugh

Commercial PC chart plotting software has offered this facility for many years. As well as Neptune you can certainly do this in seaPro, also with Maxsea software and I assume others.

seaPro is nice in that it shows the COG to be expected if you hold the CTS that the program suggests will give fastest passage between 2 waypoints (again I expect this works in some other packages). Having the expected COG displayed is important because it might take you into danger (or into a shipping lane). So while for a typical UK to France trip in a 5 knot boat the most efficient course time wise would allow the boat to make a big 'S' shape over the ground, if you are sailing in an area with multiple dangers using the Autopilots XTE steering might be better to follow tightly a planned COG past the dangers. The thing with technology is that it can make life easier but you need to understand what is happening and make smart decisions according to the situation.

To those who say you don’t need the plotter to do this – of course not we did it for years with no electronics but it is a job done best by an iterative calculation and computers do those much faster and with fewer errors than human beings as long as the humans give the computers the right inputs and it pays to understand the limitations of the output.

Oh and saying you don’t need “software” then posting an Excel spreadsheet is just funny 😊


As is clear from the answers this kind of planning is not really relevant in many regions but those crossing N-S between England and France or those heading E-W across the North Sea were brought up doing these calcs by hand, as described by others and it is no accident that software designed in the UK and France includes these calcs.


Most of this discussion has related to tides running across the desired course (and most relevant for passages over around 6 hours where the tide will reverse therfore making the minimum XTE method kind of dumb becasue you will fight a tide going one way then late fight the tide going the other way when you can just let them cancel out). But


The software will also help with the question of a tide along the course line. In some ways I think that is easier by hand than a cross tide calc and that may explain why some who sail with strong tidal streams still don't see the need for software help - especially if you are going along a narrow channel either with or against the tide but again using seaPro for example you can give a time of departure and route then ask the software which will graph the expected passage time for each of the following 12 hours showing easily when the best time of departure is.


Of course many passages along a coast and past headlands will incorporate a bit of both situations and the software just gets on and deals with it.

[Idlegreg]

This image of my track a few years ago gives an idea of the type of curve seen if crossing what we Brits like to call the English Channel using single CTS for best efficiency - there might of been a bit of Shipping lane dodging in here as well I cannot remember but it is a pretty typical..

[agwmarpil]

Working it out is quite simple. True course to Compass course initially then apply the set and leeway. Set comes from the currents on the chart and leeway from the wind.
Theres many numonics around to remember how to get from True to Compass course - Timid Virgins Make Dull Companions! True/Variation/Magnetic/Deviation/Compass. Error East Compass Least or Error West Compass Best.
Chartwork courses are the best way to learn this stuff and once you understand the principals its straightforward and very rewarding when you get it right and end up where you expected to be!
Good Luck.

[Idlegreg]

Well, some at least, did ask…. I have run the calcs in seaPro for the 12th of March this year (because that was a day of high spring tides and so highlights the issue – on many days the effect would be much less).

If I ask the software to calculate a single CTS for a passage from the Needles Fairway to the Cherbourg East Ent and set out at 00:00 (which looks like it will lead to the fastest passage, taking 11h 41m) I get the first pic..
A bearing of 179 degrees and 59 Miles


The software says the CTS is also 179 degrees suggesting the East and West going tide streams will almost exactly cancel out. The dotted line shows the predicted track if I maintain 5 Knots and steer 179 for the whole passage.




If I leave later (at 07:00), which is predicted as the slowest passage (12h 10m), then the recommended CTS is 184 degrees and the predicted track looks like the 2nd pic..
So leaving after some breakfast (which is always the best time 😊 ) would only add half an hour to the journey.



But what if I wanted to follow the direct ground track, perhaps using the Autopilots ability to steer so as to minimize XTE?
To persuade the plotting software to do this I insert a bunch of waypoints along the direct course (foolish in this case but might be sensible if I am trying to avoid hazards).

I get something like the 3rd pic...
With a fastest predicted passage of 14h 02m and a slowest journey of 14h 48m




So if I believe these calcs and choose the fastest time for each method I get to choose between 11 ¾ hours or 14 hours at sea – more than 2 hours saving, which is enough time for a nice French meal with a good bottle of wine. This might help explain why it is the Brits who care 😊

[marcvilleneuve]

WE very often race with lateral currents on the St-Laurence River and I have to compensate my heading because of a lateral current. Knowing that the fastest route between two points is a staight line I make sure I don’t create XTE by constantly adjusting my heading so:
Bearing to next Mark = COG (Cap Over Ground)
The result is a straight trace on my GPS.
I configured my GPS with these two values side by side to facilitate that.

[s/v Jedi]

Originally Posted by marcvilleneuve

WE very often race with lateral currents on the St-Laurence River and I have to compensate my heading because of a lateral current. Knowing that the fastest route between two points is a staight line I make sure I don’t create XTE by constantly adjusting my heading so:
Bearing to next Mark = COG (Cap Over Ground)
The result is a straight trace on my GPS.
I configured my GPS with these two values side by side to facilitate that.

The reason you see a straight trace on the gps is that the tide didn’t turn and the current didn’t change much. When they do, you see curved or even sinusoid traces on the gps. Shortest distance through a moving column of water is the game.

[Dockhead]

Originally Posted by marcvilleneuve

WE very often race with lateral currents on the St-Laurence River and I have to compensate my heading because of a lateral current. Knowing that the fastest route between two points is a staight line I make sure I don’t create XTE by constantly adjusting my heading so:
Bearing to next Mark = COG (Cap Over Ground)
The result is a straight trace on my GPS.
I configured my GPS with these two values side by side to facilitate that.

This method gives you a straight line over ground -- constant COG. Does your boat have legs, which reach down to the bottom, or does she sail through the water?


If she doesn't have legs, then the fastest route between two points will be a straight line -- through the water. That is a constant heading, not constant COG.

Originally Posted by s/v Jedi

Shortest distance through a moving column of water is the game.

This.

[marcvilleneuve]

Originally Posted by Dockhead

If she doesn't have legs, then the fastest route between two points will be a straight line -- through the water. That is a constant heading, not constant COG.

This.

A straight line is a straight line; is it over the ground, the water or the air. My approach works well between two marks that are a few miles from each other with the assumption that the direction of the current will not change. You right it means that to maintain a constant COG I CONTANTLY have to adapt my heading according to the strength of the lateral current so COG=Bearing to next mark.

By the way, there a very easy way to achieve that same straight line when a visual on the mark is available. Just keep a constant alignment between the mark and a background feature by modifying the heading. Works great!

Many people discussed crossing a channel here and I think that it is a different and more complex issue better managed by a routing software that will take wind as well as current into account.

Good racing!

[Idlegreg]

Originally Posted by marcvilleneuve

.....with the assumption that the direction of the current will not change.

I am with those who say you want shortest distance through the water/constant heading - not necessarily the shortest distance on your GPS screen.

Both current direction AND speed need to remain constant for the straight ground track to make sense.

Imagine an extreme example of a race with 2 boats sailing at 5 knots crossing a river which has a 5 knot flow in the middle and a slower flow at each side.


1 Guy decides to stick to his COG between start and finish altering his heading as required - he gets to the middle of the river and then he is stuck going nowhere, heading directly into the current until it changes, or he lets his boat move off the chosen COG.


The other guy chooses a heading that takes him across the river most quickly, even though that means the boat gets set downstream, then he makes up to the finish once he has crossed the middle and is in the slower flow on the destination side. This guy wins, even though he has not sailed the most efficient course.



Better (If you have information allowing you to predict the flow at different stages of the leg) is to calculate the cumulative effect of the stream and choose a single heading that will mean you reach your destination without altering course – unless the direction and flow rate are constant this will mean your COG is not a straight line but it will be fastest.

[s/v Jedi]

It’s amazing how people don’t understand this... I may take up racing again

[Fbfisher]

How can you plot a single “course to steer” with constantly changing variables like wind, waves and current? My ST60 Raymarine (circa 2005) autopilot has a handy track button. Activate that after you’ve entered a waypoint and the XTE reads 0.00 (occasionally 0.01) for the 4.5-5 hours it takes me to reach my waypoint. Or am I just not understanding what the question is?

[Adelie]

Originally Posted by Fbfisher

How can you plot a single “course to steer” with constantly changing variables like wind, waves and current? My ST60 Raymarine (circa 2005) autopilot has a handy track button. Activate that after you’ve entered a waypoint and the XTE reads 0.00 (occasionally 0.01) for the 4.5-5 hours it takes me to reach my waypoint. Or am I just not understanding what the question is?

You can’t do it for waves, I don’t know of any really good prediction software for that.

For wind there are decent predictions for most coastal area and if you have your polars a properly programmed computer could pick a course.

Current predictions in tidal areas are also pretty good and a computer could do this.

Given the computer would have to deal with both wind and current Thera probably not a straightforward analytical solution so the computer would have to simulate a bunch of courses to find the quickest.
Given varying wind and current the computer may find a multiple leg heading that’s the fastest provided the forecasts are accurate.

I bet somebody is working on this but it’s not a trivial task.

[Tillsbury]

Originally Posted by Idlegreg

So if I believe these calcs and choose the fastest time for each method I get to choose between 11 ¾ hours or 14 hours at sea – more than 2 hours saving, which is enough time for a nice French meal with a good bottle of wine. This might help explain why it is the Brits who care 😊

Thanks very much for taking the time to show this on the plotter. I’ve certainly experienced this crossing the channel, and agree with your results that it saves a couple of hours — more than the 4% mentioned above and well worth the trouble.

You don’t need to be bang on, of course. Estimate the number of hours, add the tides in each direction, draw the vector sum. Just like the YouTube video above, which is a clear description of how to do it simply. You’ll save most of the two hours with a decent estimate.