Chartplotter - Course To Steer (CTS)

[Robin]

That's been the impression I get, and why I think the plotter manufacturers have little interest in providing this kind of function. Several books and blogs by Americans describe trepidation at visiting our tide-wracked waters and having to think about such things for the first time.

As you say, the US is a big place so I assume there are sailors in some parts of it who have to "do" tides - but they seem to be a small minority.

Pete

we met an American couple in a Cherbourg restaurant some years back. They had parked their boat inadvertently on the rocks east of the eastern entrance when crossing from the UK and getting the tides wrong, Pointing at the waypoint from an off track position is only OK if you are in a clear spot with a clear track from it to the wpt.:ambivalence:


They don't have tidal stream data for the ICW either as the various inlets from the ocean mean the currents switch back and forth depending which one you are nearest to. It would be nice to be able to calculate an optimum departure time for a trip but it seems you just go when you feel and deal with what you are dealt when you find it. But then most of them drive mobos. .

 

[AntarcticPilot]

I suspect the main reason most chart-plotters don't have a CTS capability is because of the storage overheads (you'd need detailed tidal flow charts for the whole of the chart area) and because of the computational requirements. It isn't a simple computational task; it requires the solution of a non-trivial minimization problem, which has to be done repeatedly as the voyage progresses. For a sailing boat, it would also ideally take into account the polar diagram of the boat and the wind-direction. A PC has the storage and processing power; a chart-plotter, in the interests of saving power, almost certainly doesn't. When a human does it, they are aided by being able to apply previous experience and local knowledge. An automated solution is almost certainly going to be like the curate's egg - good in parts! It will fail where detailed local knowledge of tidal streams will help, as (for example) I am told is the case round the Doruis Mhor near Crinan; in that area I am told that it is nearly always possible to make progress against the tide, but the Admiralty tidal charts do not have enough detail to show how. It will work well in open waters with consistent wind conditions.

Apps to do this do exist; I've even got one (trial version only) on my phone, but I've never tested it to find out how well it works. It uses GRIB data for wind direction and strength, and has a built-in tidal atlas.

 

[prv]

A PC has the storage and processing power; a chart-plotter, in the interests of saving power, almost certainly doesn't.

I'm quite sure that a modern plotter exceeds the processing power of a reasonable PC of not so very many years ago.

Pete

 

[flaming]

The thing that irks me about the plotter systems I've sailed with is that all they seem to do is correct courses based on a feedback loop, rather than predict based on recent history, even based on their inputs not predicted tidal streams.

So for example when crossing the thames estuary you'll have a series of waypoints as you dodge sandbanks, but often fairly minimal course changes at each one, and reach them much more quickly than the tide changes. So say you're doing 5 knots and you have a 1 knot crosstide. As you set off using the track function the pilot starts out by pointing straight at the mark, then corrects itself as it gets pushed off course. Only slightly annoying if you're motoring on a calm day, but more annoying if it has to do a significant course change when sailing.

But here's the thing. It knows what the tide is currently doing, because it's calculating it from water speed, heading and COGSOG and displaying it. So why haven't the software people taken the obvious next step of saying "I'm doing 5 knots and I've been asked to steer 180, but I know that the tide is currently 090 at 1 knot, therefore a good starting point would be to aim off x amount and then fine tune from there". Instead of steering 180 to start with, which is obviously not going to be the answer. It can make what is an excellent tool for fairly confined waters where you really want to be going down the line, frustrating as when you reach a waypoint that maybe marks a course change of a few degrees it then goes charging off in the wrong direction until it figures it out again.

 

[Jokani]

The Gamin GPSMAP displays data for for what it terms 'Set' and Drift'.

I had to look up what both these terms were. Wikipedia stated that they are bearing and speed of the tidal current. Knowing that the GPSMAP could display these I thought it would be possible to calculate the CTS, which is why I initially queried it with Garmin as I could not find the CTS as a display option.

After Garmin Support had confirmed a GPSMAP did/could not display a CTS, I queried out of interest how both 'Set' and Drift' were calculated, guessing it would either by tide table, of a calculation based upon data from the on-board transducers/equipment. I was keen to know if it was a broad estimate from a table or real live data.

The Garmin definition/calculation of 'Set' and Drift' is different to Wikipedia in that it includes Leeway. Garmin state the GPSMAP calculates 'Set' and Drift' by comparing the difference between, and your heading, and your course over the ground which is effected not just by tide but leeway as well.

With my lack of in-depth knowledge on the subject I am no doubt looking at it too simplistically, but I thought if you knew both the heading and the tide+leeway bearing and strength calculating the CTS would have been possible, and within the power constraints of a CP.

 

[AntarcticPilot]

I'm quite sure that a modern plotter exceeds the processing power of a reasonable PC of not so very many years ago.

Pete

In processing power, maybe, but certainly not in storage capacity. The other problem, of course, is that Chart-plotters are "closed" systems, so no-one except the manufacturers can produce software for them. I recall a post from a Raymarine person on a post a year or so ago who said a) that their software was memory limited (the problem was display issues at 180 degrees) and b) that until recently, all their plotters had different software environments.

The general problem of routing a sailing boat to take best advantage of wind and tide during a voyage is still a very difficult one, which a human can carry out using previous experience and "intuition", but which is algorithmically very difficult. For example, beyond the optimization problem, how are you going to handle errors in forecast conditions (wind or tide or speed over the water (which might be affected by sea-state)), so that the solution provided can be adjusted appropriately if conditions go outside the envelope of predictions?

Of course, the algorithm also has to cope with problems like helmspersons such as myself, who will always attempt to edge up to windward if at all possible!

Basically, I think there is scope for automating CTS for long-distance passages in open waters, but I'd imagine that the lower bound would be a cross-channel passage, and that even longer passages in confined waters would be difficult to the point of impossibility. And a package that can take everything, like TSS, dangers, shallows etc. into account is a very long way off.

 

[RichardS]

The Gamin GPSMAP displays data for for what it terms 'Set' and Drift'.

I had to look up what both these terms were. Wikipedia stated that they are bearing and speed of the tidal current. Knowing that the GPSMAP could display these I thought it would be possible to calculate the CTS, which is why I initially queried it with Garmin as I could not find the CTS as a display option.

After Garmin Support had confirmed a GPSMAP did/could not display a CTS, I queried out of interest how both 'Set' and Drift' were calculated, guessing it would either by tide table, of a calculation based upon data from the on-board transducers/equipment. I was keen to know if it was a broad estimate from a table or real live data.

The Garmin definition/calculation of 'Set' and Drift' is different to Wikipedia in that it includes Leeway. Garmin state the GPSMAP calculates 'Set' and Drift' by comparing the difference between, and your heading, and your course over the ground which is effected not just by tide but leeway as well.

With my lack of in-depth knowledge on the subject I am no doubt looking at it too simplistically, but I thought if you knew both the heading and the tide+leeway bearing and strength calculating the CTS would have been possible, and within the power constraints of a CP.

But presumably the problem is that if the waypoint you are heading for is several hours away the chartplotter needs to know what the tidal offset will be at each point of the journey during the total time period up to the point of arrival. If the chartplotter software only allows for the drift at the present moment it might plot a course which would prove to be incorrect i.e. too long, for the full journey.

Richard

 

[Birdseye]

I am disappointed to find out that a Garmin GPSMAP 7408xsv cannot display a Course To Steer (CTS).

I found this surprising as it can display Bearing To Waypoint (BTW), Set and Drift.

Were my expectations unrealistic, are other Garmin models or makes able to display a CTS, or is is simply not possible to calculate a CTS?

I'm not sure what you are getting at here. Sure the BRG doesnt usually take account of tide or leeway but if you put the target into the autopilot then it will steer a course that effectively takes this into account. And whilst its nice to be able to play around with charts and tabl;es and work out a properCTS its only really worth doing on a longer trip. Even then the tide data and leeway simply arent reliable enough to do more than give an initial CTS. Tide data is extrapolated from individual point data and leeway alters every time the wind angle or strength alters.

 

[Daydream believer]

With a Yeoman one can look up the tide speed & direction from info such as reeds etc. Enter it on the chart then obtain the CTS.
All done in a few seconds once you know the tide direction & rate

 

[prv]

I recall a post from a Raymarine person on a post a year or so ago who said a) that their software was memory limited (the problem was display issues at 180 degrees) and b) that until recently, all their plotters had different software environments.

I vaguely remember that issue on the old C-series. But they're a decade out of date; their new plotters are effectively a Linux PC with a custom UI.

Read some of the Panbo posts about the signal processing involved in B&G's Halo radar - the idea that a CTS calculation is more complicated than what they are doing is laughable.

The general problem of routing a sailing boat to take best advantage of wind and tide during a voyage is still a very difficult one, which a human can carry out using previous experience and "intuition", but which is algorithmically very difficult. For example, beyond the optimization problem, how are you going to handle errors in forecast conditions (wind or tide or speed over the water (which might be affected by sea-state)), so that the solution provided can be adjusted appropriately if conditions go outside the envelope of predictions?

Nobody asked for all that (though it doesn't seem implausible compared to the radar and image-processing going on in modern kit). Just an automated version of what I do with a notebook and pencil as we pass the Needles Fairway buoy. Even a ZX Spectrum, competently programmed, could work out that sum quicker than me, and I have no more data available than a fairly low-resolution tidal-stream atlas and a rough guess of our likely speed.

Pete

 

[KellysEye]

>So for example when crossing the thames estuary you'll have a series of waypoints as you dodge sandbanks, but often fairly minimal course changes at each one, and reach them much more quickly than the tide changes. So say you're doing 5 knots and you have a 1 knot crosstide. As you set off using the track function the pilot starts out by pointing straight at the mark, then corrects itself as it gets pushed off course. Only slightly annoying if you're motoring on a calm day, but more annoying if it has to do a significant course change when sailing.

When we were sailng in the UK we kept our boat in Burnham-on-Crouch and crossed the Thames estuary many times with a GPS and no chartplotter just waypoints, it's very straight forward as long as you know the tide times and flow at points along the way. When I helped the new owner deliver my boat we sailed from Lymington to Scarborough and used no waypoints at all and just marked chart positions. It strikes me that from all the chartplotter threads and posts that a chartplotter seems to make people think that what is actually simple navigation with GPS and tide tables becomes something complicated which it isn't. The only things needed are basic navigation skills and experience

 

[Pagetslady]

Yep - pain in the bum

I often find myself wanting to set a zoom level where the boat is currently just off the edge of the screen but the destination is on-screen, and I want to see the green line sitting over it. But I have to zoom out so that both are visible, which usually gives less detail at the destination than I would like.

Pete

Hi I agree it's a shame but the same charts on iPad the COG line doesn't disappear when the boat goes off screen.

 

[LadyInBed]

I often find myself wanting to set a zoom level where the boat is currently just off the edge of the screen but the destination is on-screen, and I want to see the green line sitting over it. But I have to zoom out so that both are visible, which usually gives less detail at the destination than I would like.

Pete

I don't know what brand you are using but with SH you can set the projected COG line to several distances, including infinite, so you can zoom to a destination many umpty miles away and, if your on course for it you will see your projected COG line.
PS - Same with Open CPN

 

[Pagetslady]

I don't know what brand you are using but with SH you can set the projected COG line to several distances, including infinite, so you can zoom to a destination many umpty miles away and, if your on course for it you will see your projected COG line.
PS - Same with Open CPN

v
On my C120 as long as the boat is shown on the screen you can se the COG projected across the screen but to keep it in view you have to zoom out if you want to see it any distance, then you lose detail

 

[LadyInBed]

On my C120 as long as the boat is shown on the screen you can se the COG projected across the screen but to keep it in view you have to zoom out if you want to see it any distance, then you lose detail

RTM ! P104 at the bottom
http://www.manualslib.com/manual/138190/Raymarine-C-Series.html?page=104#manual
Infinite can be selected.

 

[dnickj]

Which PC programmes will do this?

try http://www.neptune-navigation.com/

 

[whiteshythe]

Together with my C120, I use a dedicated ST60 Graphic Display showing COG (Course over Ground) and BTW (Bearing to Waypoint). Provided I steer to maintain these values the same I will arrive at my waypoint, regardless of what I am viewing on the C120. I also like to display VMG (Velocity made Good) which I find helpfull when deciding which tack to be on when I cannot steer the direct course to the waypoint.

 

[RichardS]

RTM ! P104 at the bottom
http://www.manualslib.com/manual/138190/Raymarine-C-Series.html?page=104#manual
Infinite can be selected.

RTT ..... Read the Thread

Raymarine C Classic technical manual:

The length of these lines is determined by the distance your boat will travel in the time you have specified (3 mins, 6 mins or infinite) in the Chart Setup Menu (see page 90) at
the current speed. Any times that you specify will apply to all chart views and if INFINITE is selected the vector will extend to the edge of the chart window.

Richard

 

[LadyInBed]

Raymarine C Classic technical manual:

The length of these lines is determined by the distance your boat will travel in the time you have specified (3 mins, 6 mins or infinite) in the Chart Setup Menu (see page 90) at
the current speed. Any times that you specify will apply to all chart views and if INFINITE is selected the vector will extend to the edge of the chart window.

Richard

P90 on my link is about Radar and MARPA.
Am I looking at a manual that is N/A to your equipment?
PS - I did actually reply to Pagetlady!

 

[flaming]

When we were sailng in the UK we kept our boat in Burnham-on-Crouch and crossed the Thames estuary many times with a GPS and no chartplotter just waypoints, it's very straight forward as long as you know the tide times and flow at points along the way. When I helped the new owner deliver my boat we sailed from Lymington to Scarborough and used no waypoints at all and just marked chart positions. It strikes me that from all the chartplotter threads and posts that a chartplotter seems to make people think that what is actually simple navigation with GPS and tide tables becomes something complicated which it isn't. The only things needed are basic navigation skills and experience

I totally agree with that, but you seem to have missed the thrust of my post, which is that as the integrated systems people (because of course ability to make an autopilot track to a waypoint predates chartplotters) seemed to have ended development of their product at the "reacts to COG not matching BTW then corrects heading" stage. Rather than the next stage, which is "measure current tide using instruments, and use this to predict best heading as a starting point given current STW - then compare actual COG to BTW"

A minor thing really, and to be honest I suspect a lot of sailors are like me and only really use the track function on light wind days because we're feeling lazy, not because we cannot navigate properly.