Waypoints, cross track errors and tides!

[woody001]

Hi,

Quick question: -

Whilst sailing x channel, taking into account of a way point how do we deal with cross track error?
Ie. the tide will push the boat out of the "motorway"

I think you can reset cross track error? Is this the best way?

Simon

 

[fireball]

GPS xtrack error

Why don't you do a bit of classic Nav and plot a midway point (or more) on where you expect to be taken by the tides ?? Not sure there is any other way of doing it as the GPS will always try and take you the shortest over-the-land route to a waypoint ...

Are you working on a basic GPS or Chartplotter?

 

[tcm]

good windup!

heehee!

 

[Ifraser]

if your passage across the channel is anywhere near 12 hours then the tide will push you as far one way up the channel as it does back hence cancelling out the tide. If you try and stay on the motoreay you will waste valuable time fighting the tide when you don't need to. The best thing to do would be to calculate your estimated position every so often ( 30 minutes or an hour ) and plot it on a chart. You can calculate what your cross track error should be and where you should be then cross reference that to a fix from the G.P.S. If using an autopilot you should use it only to keep your ships head not to try and stay on track fighting the tide. You will end up making a sine wave shaped passage across the channel. You don't mention what navigation level you have attained and, without being rude, if the above is double dutch you might consider some training....if it needs a bit of clarification p.m me...cheers Iain

 

[Anonymous]

Do exactly as Iain says - it is the 'right' answer. In addition, it can be useful to calculate the maximum XTE that you expect to see in each direction, at the time you expect it to happen. It gives a good bit of confidence to see that the tide has turned on schedule (eat your heart out, Canute) and that you are 9nm to the right of the centre of the motorway, for example. Then watch the XTE come back to zero again all by itself.

 

[ashanta]

Simon,
A lot of sailors want to have a waypoint mid channel but in reality there is no need. A previous response has mentioned the tide equalisation and therefore in that respect you want to plan to arriving at the appropriate time up tide of your destination.
If there's fog in the channel, Which many of us that know is a common occurence your waypoint becames meaningless, as physically you cannot see it and you accept that it's there.
To help you, you can set channel bouys as waypoints for bearing purposes in daylight if you feel you need them but you are likely to be slowing your progress at the mercy of the tide.
If your using a plotter, set up the chart rose as waypoint, or a position as waypoint only to take a bearing and postion line on the chart (not to actually be there) Take your vector for your landfall once approaching or in sight of land assuming you are arriving on the correct side of your destination.
I can honestly say that after a 12 hr sail from the Exe I did not alter my ST1000 until I arrived at the Little Russell in Guernsey. That was nearly 10 years ago and my 2 little ones(then) were on board.
Don't get hung up on the Nav. Learn it properly and then you won't over do it all. But be careful nontheless.
Kind regard.

petre

Regards.

peter.

 

[Gunfleet]

Don't forget to lee bow for the shortest course. Or is it wind bow?

 

[ashanta]

Simon,

Sorry but I just had to check your profile because I was making an assumption that you had a Yacht?
Thankfully you do!
Please Simon do not follow the GPS road across the Channel.

Draw a line from your open sea departure to your destination. use your tide almanac to see what effects there are and you will see that over passage you will be moved each way along the channel by the tide. If you know how to plot the tidal movements on your charts you will create the vector required to bring you to your destination, excluding leeway and deviation.
You need to be competent to do these things as basic and if you don't you should not be skippering a sailing boat. The power boat goes at speeds that often negates the the tide and therefore they can in many circumstances, follow the GPS road.
Please PM me if you require any help.

Kind regards.

Peter.

 

[woody001]

Ok guys - thankyou for the replys.
Don't get me wrong i understand basic nav work fine..
I am fully aware of the effects of the tide, ie. tide canceling each other out. ie the "s"
It might be that i expect to much from GPS, but wouldnt it be neat if they could handle the effects of the tide as well.
I have a few point's that have been answered from various people, (ie using xte to work out the turn in the tide) however guys, can we not asume iam a total deepstick!
I will using "old fashioned" nav, and plotting my pos. on the hour. I was just hoping that someone knew of a better way using GPS, maybe in 5 years or so.
thanks for your help thro

si

 

[woody001]

also, a special thanks to ashanta for your help.

 

[LadyInBed]

My ploys
1. Draw a straight line on chart from start to destination
2. When you are out into open water (at start) set the wp of the destination in the gps and put your head on that course.
3. Look at Dover tides, work out time to slack water (tide change)
4. Put line(s) on chart (East to West) where you estimate how far across you will be at these times.
5. Calculate cumulative current for these periods of time and mark North South lines on chart away from the line drawn in (1). This will give you your approximate maximum CTE in each tide.
6. Put chart away in chart table, get outside, lookout for ships, enjoy the trip and amend your course when you are two to three hours away from destination.

 

[ashanta]

Eh pal i just hoped you would't take it the wrong way because we get some reactions on here!
You'll do OK and maybe well see each other on the water.,
Take care.

peter

 

[ashanta]

Ian,

Very well said an presented.

Regards.

Peter.

 

[duncan]

here's a a cross channel track prepared earlier by tome

illustrates things nice and clearly

 

[lockwood]

Good post - and well explained by a lot of you. Everyday is a schoolday!

 

[tcm]

the gps with "goto" and a waypoint assumes no tide. As if you were on land, for example. If you reset, it will again set a new route, but again asume no tide. This means that provided you have done the nav and determined a course to steer the usefulness of a gps/plotter/autopilot setup on a sailing boat x-channel is, in order of importance and usefulness

1) you can helm handsfree: check heading against compasses, set to auto, it holds that heading - this is the best thing bout apilots. I reckon autpilots would be worth about as much if they had no readoutts at all, just black box that said "auto" and "manual" and a button to toggle.

2) the gps gives you a lat long of exactly where you are for check against a route if you have plotted the exact track, and to restart nav if all goes to rats or lecky goes, so write this down as often as you wd be happy not knowing exactly where you were. Also, praps have another backup gps, and another. Before (or without) autopilots, this is the most important feature.

3) the little picture with chart and blob answers the "how far to go" question for crew

4) During the last mile/last few miles, any correction you need to make to reach the final destination/entrance waypoint shows the error in your CTS calculation - but nice excuse is some slowing don en route for shipping lanes so not your fault ahem...

5) if you press a "go to" function at the start, yep it will give you a "motorway" and might makes bleeps when you go off track. This motorway fuction is only useful at final approach to destination wpt, and is otherwise only "interesting" - you can point at and say "look at the effect of the tide, hm, see?"

6) if you do no nav at all, you can set a direct course, and fight the changing tidal streams the whole way. This is almost a disadvantage, and it wd be best if gps/autpilots made this a lot clearer.

 

[snowleopard]

Enter the devil\'s advocate...

There is another way of doing it for the lazy navigator.

1. Draw a straight line from departure point to arrival waypoint on the chart and check it doesn't go over anything nasty.

2. Enter the destination waypoint on the GPS and press Goto

3. Press Track on the autopilot (or whatever it is on a non-Raymarine autopilot).

4. Trim sails, adjust throttles etc. and dodge any shipping before resuming track then wait for a beep to tell you that you've arrived.

More complicated routes are equally easily done using the GPS route facility.

This will of course cause outrage among the purists but it does work and it is safe though it may be a little slower. If your boat speed is low in relation to the tidal speeds or if there is a headwind and you can't lay the course you will have to think again of course. It isn't a substitute for seamanship and navigational skills.

Another benefit of this approach is that you can draw a straight line through a complicated series of shoals and know that the boat will follow it to within 1/100 of a mile.

 

[LeonF]

Re: Enter the devil\'s advocate...

On my first crossing.. and only one so far..Ramsgate to Ostend I did exactly this...but it WAS slower! When I go across next I shall adopt the approaches detailed above....why fight the tide... learn to use it ...

 

[Oldhand]

If you want it to be as easy as you hope, then you need a reasonably sophisticated chartplotting PC software package with good tidal flow algorithms. For instance in MaxSea, assuming one has obtained the appropriate tidal packs, you enter your route, enter your start time and expected boat speed on each route leg. Then in tabular form you can see predicted course to steer, predicted average SOG over each leg, predicted log readings at each waypoint and predicted ETA's and even fuel used if you have entered speed/fuel consumtion data into the software and you are motoring. Its great for printing out a passage plan too. It is also easier and quicker to get updated passage information when achievable speeds are different to those planned than having to re-do the whole manual process. I suspect most people would alter there manually produced plan by informed guess work rather than do the whole process again while en-route.

So if you want more than what a plain GPS or chartplotter can provide, you need to invest in a PC (if you don't already have one) expensive software, electronic charts and tidal packs. That said, I still go through the tidal stream atlas and jot down some notes on current change, expected X-track deviations to check the predicted track is safe and that the PC derived information is in the right ballpark.

 

[joss]

What is the name of this package? I googled 'velocity passage planner' but nothing...