Question on tides & course to steer

[newsailorboy]

I have noticed that some skippers on a journey from A to B plot a new course every hour to take into account of the tidal stream.
However, whilst I can see that this has the advantage of staying nearer the original track it must take longer to sail.

On my theory course we added up all the tide going in one direction and then plotted the course. However, I seem to have forgotten what I need to do when the journey is over say 12 hours.

Do I plot both sets of east & west going tides before plotting my course?

Thanks

 

[binch]

aim-off

This depends on sea-room, wind reliability and a very good tidal stream atlas. Unlass you are in unrestricted sea-r00m, it is better to select a safe track and adjust course as necessary to stay in safe soundings. You cannot forecast your speed over 12 hours, at least I can't.
In theory, you are right but prudence often gives you no room for the ideal solution.

 

[VO5]

I have noticed that some skippers on a journey from A to B plot a new course every hour to take into account of the tidal stream.
However, whilst I can see that this has the advantage of staying nearer the original track it must take longer to sail.

On my theory course we added up all the tide going in one direction and then plotted the course. However, I seem to have forgotten what I need to do when the journey is over say 12 hours.

Do I plot both sets of east & west going tides before plotting my course?

Thanks

In your theory course (and just out of interest) did you plot all the tidal vectors on a chart and then take the aggregate for the run or did you use traverse tables ?

 

[Sans Bateau]

I have noticed that some skippers on a journey from A to B plot a new course every hour to take into account of the tidal stream.
However, whilst I can see that this has the advantage of staying nearer the original track it must take longer to sail.

On my theory course we added up all the tide going in one direction and then plotted the course. However, I seem to have forgotten what I need to do when the journey is over say 12 hours.

Do I plot both sets of east & west going tides before plotting my course?

Thanks

Lets say you are referring to a longer passage, like crossing the channel, which is a convenient example as it will take around 12 hours, only less if you are very lucky. It all depends on the average passage speed of your boat.

Before you set off you will work out using a tidal atlas all the east going and west going tide for the entire trip. This will let you work out a course to steer, as you will know the tide during the passage will take you first in one direction then in another direction. If your calculation are accurate, and there are no other influences you will finish up at your destination.

Whilst on the passage, it is good practice to put a plot on the chart every hour to record your position and check that the course you initially set is going to plan. What you may need to do, because dont forget the tidal chart is only a prediction that can be effected by the weather (high pressure for instance), you may need to make small course alterations as you close your destination. Also depending on the amount of lee way you make further alterations may be needed. Of course it is always better to arrive up tide of your destination rather than down tide.

So to sum up, work out the course to steer before you leave, but be prepared to make changes during the passage, to do that I always plot on a chart every hour.

 

[newsailorboy]

In your theory course (and just out of interest) did you plot all the tidal vectors on a chart and then take the aggregate for the run or did you use traverse tables ?

Hi Vos
We just used the aggregate..........What are Travers Tables?

 

[prv]

Do I plot both sets of east & west going tides before plotting my course?

Yes, that's right. On a 12-hour cross-channel trip they may even come close to cancelling out.

Other people's responses are right, but are answering a slightly wider question than what you asked.

Pete

 

[timbartlett]

Hi Vos
We just used the aggregate..........What are Travers Tables?

Cumbersome printed tables that essentially allow you to convert changes in longitude into nautical miles (and vice versa) so that you can use lots of arithmetic to achieve what you did in your class just by drawing lines on the chart.

The do not, in fact, date back to the days before the invention of the pencil, but they do go back to the days of black and white charts, printed from plates that were hand-engraved onto copper sheets, that were so valuable that no-one would dream of drawing on them.
Their continued existence is a mystery that can probably only be explained by those who devise the exam syllabusses for Merchant Navy cadets.

 

[Bosun Higgs]

I cant see what Tim Bartlett has written since I still have the miserable old scrote on "ignore". But since he is the author of one of the best text books I've read in a while, the RYA nav manual, I would suggest taking seriously what he has written.

Unless of course its another civil service / police diatribe>

 

[VO5]

Hi Vos
We just used the aggregate..........What are Travers Tables?

Traverse Tables are ready printed tables which allow you to work out a final destination from a zig zag course or a straight course or a course that contains alterations (for example) without first plotting on the chart. They are used to work out an Estimated Position ( in Lat. and Long.) which is then plotted on a chart.

 

[VO5]

Cumbersome printed tables that essentially allow you to convert changes in longitude into nautical miles (and vice versa) so that you can use lots of arithmetic to achieve what you did in your class just by drawing lines on the chart.

The do not, in fact, date back to the days before the invention of the pencil, but they do go back to the days of black and white charts, printed from plates that were hand-engraved onto copper sheets, that were so valuable that no-one would dream of drawing on them.
Their continued existence is a mystery that can probably only be explained by those who devise the exam syllabusses for Merchant Navy cadets.

Please try to refrain from dumbing down the reader.

Traverse tables are used to calculate distance (very useful when using Gnomonic Projections (where all courses are Great Circle courses whereas in Mercatorial Projections all courses are rhumb line) - (for example) in ocean sailing, that is the main reason why the MN continues to use them.

They present three arguments, Diff Long. Diff Lat. and Departure and thus allow you to work out from a distance travelled in Nautical Miles, to work out an Estimated Position (for further refinement by fix by Celestial Navigation) or to convert combined differences in Lat. and Long. to distance travelled, etc.,

I disagree with your notion of black and white charts being too valuable to write on.

All the workings on a Black and White were done in pencil, HB being the preferred hardness, because of its crispness and clarity and comparative ease of rubbing out.

That some clowns insist on doing chartwork in ball pen is altogether another topic. You occasionally observe this travesty on second hand charts.

The exception to this rule was the practice in the MN to so mark charts in folios where predetermined voyages of a repetitive nature were the norm, as "courses required" = "courses expected to be made good".

 

[Sans Bateau]

newsailorboy, welcome to the forum. You asked a simple valid question, you got what I hope was an answer to that question.

You will now see an example of what makes the forum so entertaining, just sit back and watch whilst forumites slug it out try to prove who is the smartest.

 

[jimbaerselman]

Of course it is always better to arrive up tide of your destination rather than down tide.

Or better still, plan to arrive a little up tide . . . especially if that's up wind as well

 

[dt4134]

Do I plot both sets of east & west going tides before plotting my course?

That's it. You should start off diligently doing that and getting practised at it. Cross check against the GPS as you go when you're at sea.

Later on, you can learn when it is ok to guesstimate and when you have to put the effort in to be precise. And by then you'll have learnt the difference between precision & accuracy in these calculations so you'll learn to gauge what fiddle factor to put in to err on the side you want to err on.

It really does work. After one race I studied the tracker on the web afterward to see which tactics had worked and which hadn't. At one point we go off on a big curve whilst the other boats went straight down the rhumb line. It was no cunning plan, we simply let the tide carry us off to one side for six hours in order to let it bring us back (although it did fit in well with the wind strategy too). In those twelve hours we went from about 7th to 1st.

 

[prv]

It really does work.

Yep!

The Scuttlebutt Cherbourg outing was the first long tidal trip I've had the navigator's hat on for (previously someone else navigated, we were in the Med or Baltic, or legs were short enough to go by eye). On the return leg, it was almost magical to watch our hourly plots fall on and around the predicted track as it S-curved across the rhumb-line.

Pete

 

[Conachair]

Hi Vos
We just used the aggregate..........What are Travers Tables?

Traverse tables are albert being a wind up merchant. No one uses them anymore. In days long gone by they were used to convert a course and distance into a change of lat and long, or vice versa. Unlikely you could even buy them today, maybe in some second hand shop.

As to the question, it probably depends where you are going, across the Thames estuary then hourly would make a bit of sense, lots of sandbanks to hit, crosschannel then plotting all the tidal vectors at once might be fine if there's plenty of open water to get pushed around in by the tide.

 

[guernseyman]

I have noticed that some skippers on a journey from A to B plot a new course every hour to take into account of the tidal stream.
However, whilst I can see that this has the advantage of staying nearer the original track it must take longer to sail.

On my theory course we added up all the tide going in one direction and then plotted the course. However, I seem to have forgotten what I need to do when the journey is over say 12 hours.

Do I plot both sets of east & west going tides before plotting my course?

Thanks

You are absolutely right to think that hourly corrections to stay on course take longer.

I don't see what the problem is with passages taking over 12 hours. The objective is to calculate the nett effect of the tidal current over the estimated duration. 6, 12, 18 hours, or anything in between: the same process works.

 

[grumpy_o_g]

I have noticed that some skippers on a journey from A to B plot a new course every hour to take into account of the tidal stream.
However, whilst I can see that this has the advantage of staying nearer the original track it must take longer to sail.

On my theory course we added up all the tide going in one direction and then plotted the course. However, I seem to have forgotten what I need to do when the journey is over say 12 hours.

Do I plot both sets of east & west going tides before plotting my course?

Thanks

There's a key bit of your question that sort of contradicts itself. Plotting the course every hour won't make any difference to the passage time - altering course every hour probably will though. You should definitely make sure you're approximately aware of your position as much of the time as possible - increase the certainty of your fix as you get nearer to either danger or your landfall.

On any passage, even short ones, I split it into legs - each leg ends when I expect conditions to change, be it weather, tide, sea-state, course change, etc. In practice most changes are gradual so you have to make an arbitrary point where to start and end each leg and then just interpolate (i.e. guess) the overall effect on the whole passage. I then try and play it through in my mind and on paper -not just the track of the boat but what beat you'll be on, what the sea state will be, visibility, etc. so you know what to expect - half the fun of navigating for me is seeing exactly where I got it wrong.

If you're sailing a longish passage though many things will decide your track and heading. If you're sailing on a beat then you should always try lee-bow - i.e take the tack that let's the tide push you down towards your target. If you're heading from say Poole to Cherbourg then the tides are a lot stronger off the French coast. That can be important for a number of reasons - first never get downtide of Cherbourg entrance as it's a very hard slog back up to it!

Secondly taking a passage of say 12 hours you will typically start of early in light winds in the morning, the breeze will get up the day progresses and you get further offshore and then may die down again as the day ends and you near your destination. Unless you use your engine you could be doing 3 knots at each end of the passage but 8 knots in the middle. There's loads of others things as well, some of which I've probably forgotten and some of which I never knew as well no doubt.

This is purely the way I do it though and was actually learnt from flying more than sailing. Like all the good things in life practice makes you better and better and you'll have great fun trying new ways and finding your own methods that work for you (that's what I tell the Missus anyway).

 

[newsailorboy]

Tides & course to steer

(that's what I tell the Missus anyway).

Thanks Grumpy, that's what I do anyway. If she asks where we are, just a confident point to the chart and she is happy, very happy. If I start thinking and plotting and taking more than a few seconds whilst she is watching then you see the confidence and panic in her poor little face. lol

 

[nigel1]

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

 

[webcraft]

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

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

- W