Course to steer software

[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?

Originally Posted by stormalong

Of course adverse current of any kind will impede your progress. You didn't ask that question. You asked for software to calculate a course taking current into consideration.

I cruise in a very strong tidal area. The day before any run I look at the tide graph for the next day to decide when to leave. Sometimes that means an early morning or late afternoon departure and will affect the day's distance traveled. That is basic trip planning. Going from point to point still requires constant course adjustment to stay on the rhumb line and correcting cross track error does just that. These days most of my cruising is in channels of varying widths. No charting software is going to be able to predict the strength of the current in that channel or what position in that channel will have the most adverse current. GPS speed over ground and cross track error are the only practical methods of navigating these channels.

[Jim Cate]

Originally Posted by Hugh_Poore

Hi Jim, have a look at this link regarding regulations.
Regarding XTE course adjustment v Course to Steer I would say if you have a 2 knot tide pushing you sideways and you are on a passage of some hours the difference could be significant in terms of the overall distance travelled. https://www.google.com/url?sa=t&sour...yD1b0hVfT1BXZB

I refer to the bolded bits above: "Could be significant". That's pretty wishy washy to me, Hugh. I was looking for a number to chew on!

At any rate, those regs are couched in terms that seem to refer to RYA and the UK. While I have the greatest respect for RYA and UK yotties in general, the terms of those regs are pretty vague, and I have grave doubts that many or most UK recreational sailors generate a formal passage plan every time they set out on a weekend daysail.

And even if they did, would planning a CTS rather than following the rhumb line abrogate these regs? They do not seem to mandate maximun efficiency!

Anyhow, I wish you luck in finding some software that satisfies your requirements. I will continue to believe that such isn't needed or even relevant in most sailing venues where detailed current info is not available.

Cheers,

JIm

[muttnik]

Originally Posted by s/v Jedi

The correction is 0 degrees because you spend as much time being set south as being set North

Yes it's annoying when that happens, as it frequently does for me on a passage like Solent-Cherbourg in a boat cruising at 5-6kts. Having said that of course, not all passages cover even one tide. Moreover because you *really* never want to end up downtide of cherbourg in a sailing boat you'll probably re-calculate CTS during the trip if speed hasn't been what you based your initial calcs on.

Originally Posted by Jim Cate

Are we talking a few percent of time wasted or some significant amount?

I did a back-of-an-envelope calc for a boat doing 5 knots across a complete perpendicular tide which has a maximum rate of 2kts using the rule of twelfths and it looked like about 4%, which makes my tutting at pressing "go to waypoint" rather less valid than I thought it would be. I've not yet had second coffee of the day so my maths may be way off and I would encourage others to post what they think (or a reference to some learned work which states it), especially if it restores my moral high ground to the higher level that I thought it would be. Still, 4% is half an hour on a 12 hour passage which might make the difference between a pint at your destination and not.

[Hugh_Poore]

Hi Jim, the regulations are SOLAS published by the IMO. SOLAS chapter 5 refers to all vessels at sea. In particular all leisure vessels should have constructed a passage plan before leaving harbour. This is not something put together by the RYA. For example at the very basic level at least have constructed a plan using charts, considered the weather, tides, type of vessel, crew limitations, contingency harbours and told someone ashore.

Regarding the maths of what might be the additional mileage by not constructing a course to steer is something you could do at home with a chart by plotting a hypothetical course by plotting both types of course. It is a well documented point that a sailor will travel a greater distance by using corrections to XTE.

I hope this helps and I very happy to answer any further questions

[conachair]

Originally Posted by Jim Cate

Could you please elaborate on this statement? I was not aware of such a requirement for recreational sailing.

There are some requirements under solas V about passage planning and more which apply to all vessels-
https://www.rya.org.uk/knowledge-adv...gulations.aspx

Guidance on SOLAS Chapter V - Safety of Navigation

[s/v Jedi]

Originally Posted by stormalong

Of course adverse current of any kind will impede your progress. You didn't ask that question. You asked for software to calculate a course taking current into consideration.

I cruise in a very strong tidal area. The day before any run I look at the tide graph for the next day to decide when to leave. Sometimes that means an early morning or late afternoon departure and will affect the day's distance traveled. That is basic trip planning. Going from point to point still requires constant course adjustment to stay on the rhumb line and correcting cross track error does just that. These days most of my cruising is in channels of varying widths. No charting software is going to be able to predict the strength of the current in that channel or what position in that channel will have the most adverse current. GPS speed over ground and cross track error are the only practical methods of navigating these channels.

I don’t know where you are sailing but in Holland, Belgium, UK etc. we do have current maps that show exactly the data you think does not exist. We are not allowed to leave port without having passed exams that prove we can calculate drift for any hour of any day covered by the tide tables and tide maps.

About the significance: in my example of a cross current that reverses during the Lowestoft - IJmuiden passage, using GPS crosstrack steering to stay on the rhumb line means that one is fighting the current 100% of the time except during the short slack tide, while steering a constant heading means that there is no effect from current at all, except for a zig-zag like COG, which needs to be checked for shallows, obstructions etc.
Just steering rhumb line would mean many extra miles and hours passage time.

[rslifkin]

Originally Posted by muttnik

I did a back-of-an-envelope calc for a boat doing 5 knots across a complete perpendicular tide which has a maximum rate of 2kts using the rule of twelfths and it looked like about 4%, which makes my tutting at pressing "go to waypoint" rather less valid than I thought it would be. I've not yet had second coffee of the day so my maths may be way off and I would encourage others to post what they think (or a reference to some learned work which states it), especially if it restores my moral high ground to the higher level that I thought it would be. Still, 4% is half an hour on a 12 hour passage which might make the difference between a pint at your destination and not.

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 inefficiency unless conditions are changing too rapidly for that to be effective.

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.

[muttnik]

Originally Posted by rslifkin

Using COG to steer would seem to solve most of the inefficiency unless conditions are changing too rapidly for that to be effective.

Either I'm misinterpreting you or you misinterpreted what I was doing which was to try to very roughly calculate how much *less* efficient using COG/XTE was vs calculating a course to steer for an example which used speed akin to that of a modest cruising yacht on a long passage across a perpendicular tidal current. With the former you travel a straight line over the ground but a (roughly) sinusoidal shape through the water which will take you longer than a correctly calculated CTS which is a sinusoid over the ground, but a straight line through the water. We all know it's longer but Jim_Cate was effectively asking (I think) "yes but is it sufficiently longer to really care about?". My super-rough-and-possibly-just-wrong calculations indicate a difference which is enough for *me* to bother with (because my sail trim is rubbish but navigation passable, so I play to my strengths :-) but it's probably not enough for me to fault anyone who says "I know what I'm doing isn't optimal, but it's all an approximation anyway, I'm in no hurry and I can't be bothered".

It was very very rough and for an average-ish channel crossing. In a small slow yacht at springs it would be much more. Those in minimally tidal areas probably don't care.

(and apologies if it was me misinterpreting)

[rslifkin]

Using XTE as the primary correction will lead to extra distance, as you pointed out. I'm thinking that if you ignore heading and use COG as the primary steering target, then once you make the initial XTE correction after coming into the current, you should be able to track pretty accurately based on COG. Basically making small heading adjustments to maintain COG, so never allowing any meaningful XTE to happen.

As an example, you're being pushed south by the current and heading to a waypoint due East, so a bearing of 90*. Steering at a heading of 90*, then correcting for XTE is inefficient. However, if you steer to a COG of 90* and ignore that you're actually facing, say, 85*, that should keep you on a pretty much straight line to your waypoint with minimal XTE correction needed. Basically, instead of thinking of the boat as moving in a straight line forward, you're accounting for the fact that the boat is actually moving diagonally. So you make that diagonal movement line up with your destination, rather than actually pointing towards your destination.

[nigel1]

Screen shot from the Neptune Planner software. User enters boats speed through water, departure date and time,and the software calculates a course to steer.
Will also calculate optimal departure time to take advantage of favourable tidal streams

[Adrian Chen]

I have used pcplotter and Neptune's to uk. Pc plotter can use navionics charts and Neptune uses admiralty. Both give cuts, and can give a range of results based on different start times. Interesting to show less experienced people why using the tides can be important

[s/v Jedi]

Originally Posted by rslifkin

Using XTE as the primary correction will lead to extra distance, as you pointed out. I'm thinking that if you ignore heading and use COG as the primary steering target, then once you make the initial XTE correction after coming into the current, you should be able to track pretty accurately based on COG. Basically making small heading adjustments to maintain COG, so never allowing any meaningful XTE to happen.

As an example, you're being pushed south by the current and heading to a waypoint due East, so a bearing of 90*. Steering at a heading of 90*, then correcting for XTE is inefficient. However, if you steer to a COG of 90* and ignore that you're actually facing, say, 85*, that should keep you on a pretty much straight line to your waypoint with minimal XTE correction needed. Basically, instead of thinking of the boat as moving in a straight line forward, you're accounting for the fact that the boat is actually moving diagonally. So you make that diagonal movement line up with your destination, rather than actually pointing towards your destination.

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.

[muttnik]

Originally Posted by rslifkin

Basically, instead of thinking of the boat as moving in a straight line forward, you're accounting for the fact that the boat is actually moving diagonally.

Like s/v Jedi I think there may be some misinterpretation of how we're using the term "Course to Steer".

And if I may add to s/v Jedi's explanation (aka Jedi beat me to it but I'm going to post anyway :-)...

"CTS" is *not* bearing to waypoint, ie it's not necessarily your heading if you were to point your nose at your destination. It is also not keeping your heading pointing at your destination (which is usually inefficient and requires continual heading changes).

CTS is the bearing to your destination corrected to account for the cumulative effects of currents along your route and any leeway, then corrected for variation (and deviation for a compass course)

Sailtrain: Navigation and Chartwork,

Get it right and theoretically you steer a single heading all the way and arrive at your destination even though you weren't pointing at it most of the time. In practice there's a lot of guesstimation involved :-)

If the current and your speed and leeway are constant then a correct CTS will make your COG point right at your destination (even though your heading may be different). However in tidal areas current generally increases, decreases, reverses, increases, decreases and so on. To keep a constant COG you have to keep changing your heading. On a multi-hour passage perpendicular to a tidal stream this will result in your course *through the water* being a series of 'S's. If instead you use your well-calculated CTS, your course *over the ground* will be a series of "S"s (your heading is constant but the surface of the water is moving back and forth over the ground as the tide ebbs and flows) but you'll be doing a straight line *through the water*, which should take you less time.

[bobgarrett]

Yes, Neptune is good but for limited areas.

[smac999]

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.

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?...