Course to steer software

[s/v Jedi]

Originally Posted by smac999

So your solution is What. Follow the tide. Heading 90 degrees off course? And then what. Tack back into it?... and this is a shorter distance and time then following xte?...

Check the message with the link above, learn how to do coastal navigation for tidal waters. It’s the part with the rule of 12ths.

You need to figure out things like the leeway for your particular boat, which is all explained. You come up with a time to leave and a CTS so use the autopilot in heading mode. On the chart plotter, in addition to your forward motion towards the heading, you also see the movements (vectors added) due to current, wind set etc. These confirm your math if done right and often enough. Then, when approaching your destination, you should arrive exactly at the calculated spot in an ideal world. In the real world, for a 12 hour trip, we often get as close as 0.5 - 2 nm from the destination. At 5 miles out you get a good idea where you’re gonna end up and can do a correction. That would be the point where you can change the autopilot to waypoint or no-drift mode. Up to that correction, you have done a straight line through the water, so shortest distance sailed.

We recently crossed the gulf stream from the Bahamas to Florida but we also had to sail way north to Port Canaveral. This is where I did the math again, which worked out great. Halfway you think you’re gonna end up in Georgia but then as the current start diminishing you see your COG line on the plotter start moving south.

By the way, the best software to do this anywhere in the world is called PredictWind. You do need to get the most expensive option to get the tide and current included in routing but it works pretty neat. It does complete passage planning (routing) based on weather grib files, tides and currents. Even does departure planning to find the best time to leave etc.
I combine that with Iridium Go to get the weather data over the satellite. I did not get the current and tides because for the Caribbean it’s not worth it... but for crossing the Caribbean Sea (we did Panama to Jamaica) the current is significant (2.4 kts) and very variable with eddies etc. which turn out to be in that current and tides info so next time I may order that package.

When you are in cellphone range, you can use windy.com which has current layers as well but doesn’t do the routing for you.

[stormalong]

Originally Posted by Hugh_Poore

Okey dokey. Out of interest do you use charts and tidal diamonds to calculate a course to steer in the area you sail in?

I sail Puget sound, San Juan Islands, the inside passage which is a spaghetti bowl of channels between Vancouver Island and the British Columbia mainland and have done SE Alaska and plan to do it again this year if possible. This entire area is intensely tidal and currents are often fierce.

Yes, I use electronic charts and tide prediction and current prediction programs for route planning and travel. I also use the Ports and Passes book that is published annually.

On the inside passage there are huge variations of channel widths and Bernoulli's law is in full force. There are rapids, and in one area there are three sets of rapids that need to be traversed at one go. They are just far enough apart that planning is very important if you want to get through them without drama. They also have very poor anchorages between them so getting through all three in one day is important.

[Paul Howard]

So, everyone thinks there should be an electronic gizmo to give them the answer to whatever question they ask. Being old enough to have relied on manual coastal navigation calculations, one of my favourite memories of calculating 'course to steer' for a strong tidal flow plus constant current was on passage from Holyhead, Wales, to Wicklow, Ireland, across the Irish Sea. This 100nm plus passage on our 39ft. catamaran nearly 20years ago had us searching for answers for tidal stream, currents, wind, leeway, etc. Being a former instructor for Canadian Yachting Association coastal navigation, I gathered all relevant data, added in guestimates for wind strength and boat speed based on forecast and our boat's leeway, hours of current going north, then south. I set a course to steer on the autopilot and watched the GPS showing us far north of our rumbline, then south, but we could have sailed straight into the entrance of Wicklow without touching the autopilot setting.

Don't rely on electronics exclusively, practice manual skills and you will know when the electronics are wrong, usually based on misinformation. Having the knowledge in your own head will give confidence to sail where ever you wish to point the bow(s).

[jmschmidt]

Oh, forgot there is a traffic separation zone around the west end of Cuba. Lots of traffic heading for the Canal. We got there at night so we went around to the outside of the zone. AIS, which we didn't have, would be helpful.

[Dockhead]

Originally Posted by stormalong

There is really no need for this calculation. Just set up your GPS or chart plotter to steer to a waypoint and correct for off track error as you go. The current changes so rapidly that any calculation would need redoing every few minutes. Even if you could get accurate tide data you would need a time and course spreadsheet.

Ah, you don't understand the principle of Course to Steer. This is an art which is pretty specific to the English Channel and North Sea -- hardly anyone else bothers to deal with this extremely complicated calculation.


The principle of Course to Steer is that the shortest distance and thus fastest way from one waypoint to another across a moving body of water is a straight line -- but a straight line across the water, not over ground. A straight line through the water is represented by a constant compass heading, ignoring COG.


English Channel sailors are taught to take set and drift for every hour of the anticipated passage and do a vector sum of all of the tides -- and you thus get a single compass course to steer which will get you there fastest although you are being swept around and long S curve over ground. If you hold a constant compass heading you are sailing a straight line through the water, and that is what gets you there fastest. Don't pay attention to COG which will vary wildly in strongly tidal waters like the Channel.



There are different discussions on CF we've had about this.


And turning back to the OP's question -- the program I use is Neptune Planner +, which is only good for the Channel and North Sea, but is brilliantly effective and saves a great deal of effort.


Neptune Navigation Software


In other places I do the classical RYA hand calculation.



Or if it's a passage of more than 24 hours, I don't bother -- just steer a compass heading equal to the bearing to destination, knowing that it will all come out in the wash.

[Dockhead]

Originally Posted by Paul Howard

. . . Don't rely on electronics exclusively, practice manual skills and you will know when the electronics are wrong, usually based on misinformation. Having the knowledge in your own head will give confidence to sail where ever you wish to point the bow(s).

That's good advice, but when you get tired of filling out reams and reams of sheets of CTS calculations it is really sweet to just plug in the values into Neptune Planner + and get the result. Even better than that, you can keep re-running the numbers as your average passage speed becomes clearer as the passage develops. But I do agree with you that it is really good to know how to do it by hand. If you can't, then you don't really understand it.

[Dockhead]

Originally Posted by s/v Jedi

Sorry but it seems you are not at all familiar with tidal course calculations. Step back to understand this: your boat is not attached to the seabed. The only time that a straight COG is efficient is when there is no current at all. For an efficient course you want a straight course through the water, even when that whole water column is moving over the seabed... it is what your boat needs to do. Also, when you try to correct for a straight cog in a cross current, you steer into the current which is to be avoided as much as possible. It’s like walking escalators the wrong way.

Exactly. Crabbing along the rhumb line is NOT what you want to do, when making a passage across changing tidal waters. You will sail a lot of extra miles through the water that way.


We discussed it here:


https://www.cruisersforum.com/forums...ad-115361.html

[muttnik]

Originally Posted by Dockhead

the shortest distance and thus fastest way from one waypoint to another across a moving body of water is a straight line

...unless you look at it from a Sam Neil perspective and do some weird mashup of Dead Calm and Event Horizon...

(Edit to clarify because not everyone has seen the movies I assume everyone must have seen...):

https://www.youtube.com/watch?v=n6lDG-bP3zg

[Sail Away 3]

Originally Posted by Hugh_Poore

Does anyone know of a charting software that enables construction of a Course to Steer. The software wild need to have tidal information to do this. Eg cant do a course to steer with Navionics.

I would think most modern plotters with associated software will have current and tide info, I know Navionics on my phone does.
Are you hand steering or want the auto to steer?
I thought I saw a comment on FTL to Panama, was that you?
Need more details of your needs

[goeasy123]

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/


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

https://www.youtube.com/watch?v=nHMwbKzWyXk

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.

[seandepagnier]

Originally Posted by Jim Cate

There was a lengthy thread about means of doing the calculations here on CF a few years back, and some innovative solutions were developed, but not software methods.

Jim

I developed the "best" software for this as it is a free software: the weather routing plugin of opencpn. You can verify the calculations are correct. Other software may not actually do a full search and lead to less optimal routes. My software also has many more configuration options.

You can only do the calculations with a computer if you have accurate tide, wind and polars for your boat. A small change in weather predictions sometimes gives completely different roues.

For all those who think they can "manually" compute it. You can get close with relatively predictable conditions, but you will not actually find the optimal route because the amount of calculations is very large.

Originally Posted by rslifkin

I wonder how a good autopilot would compare to that theoretical calculation. In theory, if the autopilot knows course over ground, the software can steer to a target COG rather than target heading and then correcting for XTE. Using COG to steer would seem to solve most of the inefficien cy unless conditions are changing too rapidly for that to be effective.

XTE logic is from airplanes and not actually what is desirable for boats, but somehow they read the same control theory books. It's basically a sub-optimal method for route following.

The autopilot route plugin of opencpn offers an alternative to XTE which gives improved route following performance. It does not suffer from over/under correcting and poor handling at each waypoint.

I guess there's also no reason you couldn't do that manually and steer your course based on the COG reported on the plotter (while monitoring XTE) rather than steering by the compass.

pypilot (also the "best" autopilot) steers course over ground by steering a compass course. This compass course is adjusted continuously to give the desired course over ground since compass and gyros react very fast and gps lags by a few seconds in most cases.

There are many modes and several different pilots. For example, pypilot can steer a course over ground compensated by the wind sensors instead of the compass in this way to follow wind shifts closely while following a route.

[Scotty Kiwi]

Am looking forward to have a look at Neptune and seeing exactly what it does and if it can be adapted.

Predict Wind and Iridium ... so expensive even before you get to the Pro version, so I've never tried it in my home area. Our normal peak is 5 kts tidal current in Cook Strait between North and South Islands, 7 kts at springs, and there are other tidal flows in and of various departure and arrival points on each side of the straight.

At 6 kts boat speed, it you get the tide right, it can be a four hour trip, get it wrong, it could be 12.

So there's three different (but related) tidal current patterns to cope with.

Then there's some wind current, and a notice-able but unknown (to me) amount of variation in the predicted time of high tide depending on god knows what.

Seen sailors who are unaware sitting out there going nowhere for 4 hours, probably wondering wtf is wrong with the boat.

The planning is critical for cruising ... and then there's the racing.

When cruising and can pick a departure time, and only when the gale is from the right direction (avoid going against it) the best I could ever do was local knowledge and the local cruising guide, which had the approx hourly tidal flows about every three hours over the 2 x 12 hour 15 minute tides, judge according to boat speed for departure timing (typical 2 hrs before high at average 6 kts), avoid specific areas of known high flow (off reefs and "peninsulars"), steer about 5d true south of destination until the 15 minute before predicted high tide, head straight for it until 15 minutes after predicted high tide, and then aim about 3nm north of destination.

Check all this (is it a kind of dead reckoning?) with the XTE and the difference between log speed (through water) and SOG on GPS.

And this is what the oldies in the yacht club bars will tell anyone who asks and listens.

I'd be very impressed if Neptune (route planner, not the guy that gets the whiskey at the equator) are could do this as well as or better the manual system. hmmm. Can I finally see a win in the annual Cook Strait race (with the eventual new boat)?

[Hugh_Poore]

Hi Paul, you are right about practicing the manual skills. I teach YM and DS theory and I know that as soon as my students complete the course they will soon forget their skills when offered a shiny chart plotter. Hence the reason why I'm trying to find a gizmo that when students understand paper charts, tides, tidal streams and navigation including course to steer I can demonstrate the same electronically. This way they have best of both worlds - traditional and electronic skills.

We all know the weaknesses of technology and that the law says that you must have paper charts and a magnetic card compass however, chart plotters or multi-functional devices lure us into a cosy easy world. People don't care about the disclaimer button you have to press each time you turn a chart plotter on.

Originally Posted by Paul Howard

So, everyone thinks there should be an electronic gizmo to give them the answer to whatever question they ask. Being old enough to have relied on manual coastal navigation calculations, one of my favourite memories of calculating 'course to steer' for a strong tidal flow plus constant current was on passage from Holyhead, Wales, to Wicklow, Ireland, across the Irish Sea. This 100nm plus passage on our 39ft. catamaran nearly 20years ago had us searching for answers for tidal stream, currents, wind, leeway, etc. Being a former instructor for Canadian Yachting Association coastal navigation, I gathered all relevant data, added in guestimates for wind strength and boat speed based on forecast and our boat's leeway, hours of current going north, then south. I set a course to steer on the autopilot and watched the GPS showing us far north of our rumbline, then south, but we could have sailed straight into the entrance of Wicklow without touching the autopilot setting.

Don't rely on electronics exclusively, practice manual skills and you will know when the electronics are wrong, usually based on misinformation. Having the knowledge in your own head will give confidence to sail where ever you wish to point the bow(s).

[MarkN]

If you have a Navionics chart card, you can purchase their Navigation Planner software which enables you to load your navionics chart onto your PC to do passage planning which includes tidal planning and CTS etc. You can also export the routes via an SD card to load into your chart plotter.

There is also some free software qtVlm ( https://www.meltemus.com/index.php/en/) that enable you to load charts, you can also load tidal grib files and wind files and it will do weather routing for you as well.

[Kolchac]

PredictWind is looking OK.

https://www.youtube.com/watch?v=j3op3EX7IMo&t=18s