Question on tides & course to steer

[Conachair]

As far as I know, use of Traverse Tables has been dropped from the MN exam syllabus, as has the use of logarithms, and many of the other valuable tables.
Calculators seemed to be the preferred method.
I had two cadets onboard last trip, both were completely lost when it came to making calculations as neither had brought a scientific calculator, and we did not have on onboard.
Back in the days when I did my tickets, calculators were not permitted, but a slide rule could be used to check workings.

BTW, I was always taught, that a 2B pencil should be used for chartwork

Actually, when i said "no one uses them" I should have said "on yachts", don't know about commercial or navy.

But out of interest does any use traverse tables anymore??

Or would anyone ever have done so for coastal? Seems such a long way round. Offshore a click even on google earth gives you enough to go on, better sailing to the wind than to a course other than "sort of over there"

 

[Conachair]

Yes. HB is harder to erase and tends to leave permanent faint marks or creases. A propelling pencil with 2B leads is useful.

- W

+1

 

[snowleopard]

As a rough approximation you can take 12 hours 20 mins of the passage and the tides will cancel out, then at the end of the period you can either work out a course for the next pair of tides or plot a proper CTS to the destination.

If you're not in a hurry and you have a reasonably quick boat the difference between sailing the rhumb line and maintaining a constant bearing while letting the tide set you back and forth is only going to be the time for an extra cup of coffee after a channel crossing.

If you regard navigation as a hobby and are happy to spend a couple of hours at the chart table fine. If your only concern with navigation is to get you to your destination in safety there is another solution:

Input your landfall waymark to the GPS, hit 'goto', press 'track' on the autopilot and watch the DTW roll down to zero.

 

[webcraft]

Input your landfall waymark to the GPS, hit 'goto', press 'track' on the autopilot and watch the DTW roll down to zero.

I don't want to restart a conversation that has been done to death on here many times, but roughly . . .

On a 12 hour channel crossing, how much longer will it take if you roll down your GPS track as described as opposed to working out an accurate CTS on an hour by hour basis?

- W

 

[jimbaerselman]

On a 12 hour channel crossing, how much longer will it take if you roll down your GPS track as described as opposed to working out an accurate CTS on an hour by hour basis?

Depends on the ratio of boat speed to tidal current. A boat which makes only 4kts in a tidal stream of 4 kts will spend all its time going nowhere. A 20kt + mobo, on the other hand, offset by about 12 degrees, will only lose a few percent of its journey time.

 

[Simondjuk]

Webcraft,

When you say 'on an hour by hour basis', do you mean calculating the CTS hour by hour en-route and adjusting your course each hour to suit, or calculating it for all the expected hours of the passage before departure and steering to the result of that? The two give a different outcome.

 

[webcraft]

Webcraft,

When you say 'on an hour by hour basis', do you mean calculating the CTS hour by hour en-route and adjusting your course each hour to suit, or calculating it for all the expected hours of the passage before departure and steering to the result of that? The two give a different outcome.

I was originally taught that calculating CTS on an hour-by-hour basis and adjusting your course each hour to suit would give the most accurate result, but I can see why aggregating the tidal vector for the whole trip may result in a faster passage. I suspect it can depend on the passage in question, but we were talking about a 12 hour Channel crossing here which seems to be something a lot of people (though not me) are familiar with. Up here tidal vectors on most passages of any length tend to be 'with' or 'against' due to the topography.

In practice as Snowy said most people tend to either set the autopilot to 'track' or do the same thing manually by following the 'Highway' screen on their GPS or plotter. There have been long involved mathematical arguments on here as to how much less efficient that is than calculating CTS, generally with little or no agreement.

(And that's before we get into lee-bowing . . . )

- W

 

[Simondjuk]

On a passage of a duration which covers a something nearing a full tide cycle, if you calculate a new CTS each hour as you go, you're giving up more ground due to stemming the tide, since each time you calculate you turn up tide to compensate, purely for the folly of arriving an hour later at a position which you had no good reason to battle the tide to get to.

If you plot all the vectors before departure and sail the CTS for the entire passage, you avoid stemming the tide as much, simply allowing the ebb and flood to set you down then back up at no penalty.

The latter provides a superior VMG. I've not seen the discussions you mention, but find it hard to see how that could be disputed.

 

[webcraft]

The latter provides a superior VMG. I've not seen the discussions you mention, but find it hard to see how that could be disputed.

Agreed - but anything can be disputed on here

I just wondered if anyone would care to or had worked out how much of a difference each of the three methods (hourly CTS, overall CTS or using GPS to follow track) would actually make on a 12-hour cross-channel passage? Snowy mentioned the time taken to drink a cup of coffee. If it is that little then the time taken to do the calculations almost cancels it out, and any unexpected course alterations (eg for traffic or windshifts) could perhaps negate the theoretical improvement in passage time gained.

- W

 

[timbartlett]

If you plot all the vectors before departure and sail the CTS for the entire passage, you avoid stemming the tide as much, simply allowing the ebb and flood to set you down then back up at no penalty.

The latter provides a superior VMG. I've not seen the discussions you mention, but find it hard to see how that could be disputed.

+1

It makes no difference how long or short the passage is: if you steer to stay on the "straight line" between two points, you must always be heading uptide of the direction you wish to travel. Whether a lot or a little depends in the relative speeds of the boat and tide -- but the point is that your speed over the ground will always be less than your speed though the water. In extremis, if your boat speed is the same as the tide speed, you will make no progress whatsoever.

...I just wondered if anyone would care to or had worked out how much of a difference each of the three methods (hourly CTS, overall CTS or using GPS to follow track) would actually make on a 12-hour cross-channel passage?

Depends on the relative speeds of boat and tide. For a fast motor boat across channel it's not much of a difference. For a sailing boat it could well run into hours.

 

[Simondjuk]

Calculating the difference involves too many variables to make it a simple exercise which yields a definitie answer. It depends on boat speed, whether it's springs or neaps, wind direction and how and when it changes in relation to the tide, pointing ability of the boat and the leeway she makes, among others.

You could assume common factors to achieve a theoreticsal answer, but it'd only be theoretical since there are some factors which are likely to change along the way on an actual passage. That said, if you start with the best theory, you should be best off.

 

[webcraft]

Calculating the difference involves too many variables to make it a simple exercise which yields a definitie answer. It depends on boat speed, whether it's springs or neaps, wind direction and how and when it changes in relation to the tide, pointing ability of the boat and the leeway she makes, among others.

You could assume common factors to achieve a theoreticsal answer, but it'd only be theoretical since there are some factors which are likely to change along the way on an actual passage. That said, if you start with the best theory, you should be best off.

I'm just interested because we generally steer to track using GPS but might calculate CTS for (eg) a passage from Portpatrick to Bangor where the tidal vectors are largely cross-track and there are no obstacles, assuming we weren't expecting a significant wind shift during the passage.

I guess the real question is - in the age of satellite navigation is CTS as important as it once was? It is essential to understand it, and reassuring to know that you can find your way anywhere if the system goes down - but in practice how many people use it and how much time does it save them?

- W

 

[Simondjuk]

Steering a track on the GPS is just a high resolution version of plotting an hourly CTS. You're always stemming the tide to stay on the track, only making the corrections for tide minute by minute rather than hour by hour.

I consider plotting a CTS very important to allow sailing the most efficient course. I also plot EPs, including a vector for leeway when beating, every hour or two, but then I quite often I sail with the plotter backlight off, only leaving it on at all to provide position data for the DSC VHF.

As for how much time it saves, I don't know. I do know that that it's some though, if a very variable 'some'.

 

[timbartlett]

I guess the real question is - in the age of satellite navigation is CTS as important as it once was? It is essential to understand it, and reassuring to know that you can find your way anywhere if the system goes down - but in practice how many people use it and how much time does it save them?

I don't think satellite navigation makes any difference, other than that it (perhaps) encourages us to use "steer to waypoint" rather than working it out!

But if you consider the fairly extreme but not entirely unrealistic situation of a yacht with a boat speed of 3kts setting off across a 36mile wide channel in which the tidal stream in the first hour is 1kt, then 2kt in the second hour, then 3kt in the third hour, then reduces in same 3-2-1 pattern before repeating the 1-2-3-3-2-1 pattern in the opposite direction.

The skipper who calculate his CTS for the whole trip would arrive after 12 hours, having been pushed first one way then the other.
The one who tries to stay on track would, I calculate, have covered about 20 miles in the same 12 hours, having spent four of those hours simply stemming the tide and making no forward progress whatsoever.

 

[Robin]

Agreed - but anything can be disputed on here

I just wondered if anyone would care to or had worked out how much of a difference each of the three methods (hourly CTS, overall CTS or using GPS to follow track) would actually make on a 12-hour cross-channel passage? Snowy mentioned the time taken to drink a cup of coffee. If it is that little then the time taken to do the calculations almost cancels it out, and any unexpected course alterations (eg for traffic or windshifts) could perhaps negate the theoretical improvement in passage time gained.

- W

You are making the assumption that a sailing boat can actually make all of the course alterations required hour by hour to maintain the ground track. On a typical cross Channel trip for example from the Needles or from Poole to Cherbourg, the course can usually be laid on one tack in a typical SW wind, even allowing for any total offset east/west used in calculating an overall CTS. Do that trip on the same day by staying right on track and at some point in the east going flood tide you will be aimed off so much as to be head to wind.

The Channel tides are strong and especially so in the central part between IOW - Weymouth and Cherbourg peninsular to Alderney, cross tides of up to 5kts. Farther west, our frequent flyer trip Dartmouth to Chenal Du Four is wholly different and the total tide offsets over maybe 18 hours exposure amounts to very little because the streams in the crossing (albeit not weak off the Brittany coast at the end) are not that strong in the western Channel itself.

In our new to us cruising ground going from Florida to the Bahamas is another different case because that entails crossing the Gulf Stream which runs at up to 4kts, but always in one direction only, south to north. In this case having a mobo now, at least we can point it to stay on track regardless of wind direction and in fact must do so because there is no give and take in flow directions.

Going back to your main question, one of the Channel Tide Atlases (Reeves Foulkes I think) did give an explanation and calculation of both method's times in the preface. I no longer have a copy so can't look it up.

 

[jimbaerselman]

Calculating the difference involves too many variables to make it a simple exercise which yields a definitie answer. It depends on boat speed, whether it's springs or neaps, wind direction and how and when it changes in relation to the tide, pointing ability of the boat and the leeway she makes, among others.

Crossing speed would be speed multiplied by the cosine of the drift angle integrated over time. The drift angle being the difference between the bearing of the destination, and the boat's heading (corrected for leeway if a sail boat).

For a 12:20 cycle with constant streams cancelling out during the crossing, using a pre-calculated constant heading, drift angle could be zero, cosine 1, crossing speed=boat speed.

For a 4kt boat in a 4kt cross stream, keeping on track creates a 90 degrees drift angle, cosine 0, crossing speed=zero for the periods the stream is 4 kts.

Of course when the tide is turning, progress is made for an hour or two.

Keeping track in such a slow boat when it's not necessary would allow you progress for about 3 hours in 12, one knot. I think I prefer a crossing speed of 4kts, obtainable from a constant heading - errrm - if I can find a passage measuring 48nm. Then 12 hours vs 48 hours is a no brainer.

More realistic is a 120 mile passage in a 10kt boat with 3kts of tide. Keeping on track you'd lose about 7% of your crossing speed for two thirds of the passage. Say 5% over all (give or take an inch) - 36 minutes.

30 degrees of drift (6kt boat in 3 kt stream) you'd be losing 10% of your crossing speed over 12 hours

Big stuff if you're racing. Just a cup of tea for cruisers.

 

[snowleopard]

The latter provides a superior VMG. I've not seen the discussions you mention, but find it hard to see how that could be disputed.

No, not a better VMG but a shorter water track. The two aren't necessarily the same in a sailing boat.

If your autopilot or short-term plotting has you luffing up when the tide is with the wind and bearing away when it is against the wind your boat speed will vary. To take extreme example, imagine your destination bears due South and the wind is in the North. Steering a constant heading will have you on a dead run whereas my rhumb line track will bring the wind onto one quarter or the other for a good part of the passage. For most boats that will be a faster and more comfortable sail.

 

[Simondjuk]

Snowleopard,

Agreed. I realised that the VMG thing wasn't strictly accurate after I posted it. When you're being carried across track by the strongest tide, the VMG could appear worse than if you used some boat speed to stem the tide at that time. Overall though, the former will be faster.

 

[Simondjuk]

jimbaerselman,

Thanks for that. I tend to assume a worst case average of 5 knots rather than 10 though. So, I reckon that's enough time for a pint rather than a cup of tea, which makes it well worthwhile.

 

[prv]

The Channel Pilot has a page describing this method, using an example passage, if I remember rightly, of the Needles to Cherbourg. According to the author, a "typical" boat (possibly slightly smaller/slower than is really typical nowadays) would sail an extra 15 miles on the 60 mile passage if she just followed the GPS track. If that's right (and I'd be grateful if someone with the book to hand could check, I may have misremembered), then it's another three and a half hours for me, which I could well do without.

Pete