GPS & Tides

chappy

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How do i allow for the effects of tides on gps.
Do you punch what the tides are for that area in the gps system or do you add it on your self to your own bearing if so can someone give us an example if possible /forums/images/graemlins/confused.gif
 

Joe_Cole

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On my GPS's there is no provision for tidal data. You draw up a tidal vector on your chart. I'ld post an example only I havn't got the faintest idea how to post a drawing!
 

FWB

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Re: GPS & Tides

Not quite sure what you are asking.
The line between 2 waypoints is your desired track. Your heading {the direction in which the boat is pointing} will constantly have to vary to maintain the track. This is because of the effect of the wind and tide. If you have an autopilot which will talk to the GPS you can select TRACK on the autopilot and provided you keep the sails balanced the pilot will steer the boat along the track. If you steer yourself you will see that you will be steering different headings at different times in order to maintain the track.
On a long leg you can let the boat go off track for a while --hazards permitting-- then steer a new heading to regain the track.
I hope this is clear enough to help. It becomes obvious when you are actually doing it.
 

MarkJohnson12345

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The traditional way of countering a tide, is to work out the vector's and make your course the sum of the vectors.......

Nothing wrong with that method.

With a GPS you can just watch the cross track error. Start out by heading directly for your waypoint, and head off a few degrees to counter the tide. The cross track error will tell you which way to change course to maintain the original course.

To maintain this track you have to keep changing course slighly.

Pro's are that you are sailing the shortest distance, con's are that you are punching the tide, thus might be sailing slower. Not quite sure which is the most economical method for time.

If you are close to the wind then you just aim as close to the course as you can get maintaining best speed.

Hope that helps
 

Birdseye

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[ QUOTE ]
How do i allow for the effects of tides on gps.
Do you punch what the tides are for that area in the gps system or do you add it on your self to your own bearing if so can someone give us an example if possible /forums/images/graemlins/confused.gif

[/ QUOTE ]

I know of no GPS where you can punch in tide details. Maybe on a plotter, but not on a GPS.

The simple solution is to compare the bearing (BRG) of the waypoint you are aiming at with the track the boat is making (TRK0 and altering course accordingly. So if the BRG is 250 and the TRK is 245 then punch the autohelm buttons that add 5 degrees to the course. Doing this takes care of leeway as well.

It is not the pukka correct by-the-book RYA method. Its simply a practical "bodge" that generally works as well as doing a "course to steer" since the latter depends on the accuracy of tidal flow data which is not good..
 
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For the benefit of those who are new to GPS or not very at home with navigation, if, taking your example, they do end up on a new track of 250 it is vital to check that an approach to the next waypoint/destination on that radial of 250 is safe and does not lead the vessel into danger. The easiest way to check this is to put a protractor on the waypoint (on the chart) and draw the line 250 - then look along it and check there are no dangers to either side. Otherwise the danger is that people will keep plugging in new 'bearing to waypoint' values and end up off their orignally planned track and into shallows, rocks, or whatever.
 

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Once you have input the waypoint you wish to go to you will just follow that track. If you use the highway mode and stay on the centre line you will exactly follow that track and sail the shortest distance between A and B. The GPS will also show you the bearing to the waypoint, the differance between this and the Track displayed will be the error detected by leeway caused by tide/wind etc. It really is simple, just follow the track and forget the tide, even better conect to autopilot sit back and enjoy.

Good luck,

Paul.
 
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Anonymous

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[ QUOTE ]
...even better conect to autopilot sit back and enjoy.

[/ QUOTE ]Even better take the Day Skipper course (or self study - it's not too difficult with the books and videos available) as DS covers all this stuff and in the context of basic nav rather than just GPS usage.
 

chappy

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[ QUOTE ]
[ QUOTE ]
...even better conect to autopilot sit back and enjoy.

[/ QUOTE ]Even better take the Day Skipper course (or self study - it's not too difficult with the books and videos available) as DS covers all this stuff and in the context of basic nav rather than just GPS usage.

[/ QUOTE ]
I have done the day skipper course but we did not cover the allowing for tides on gps.
 

duncan

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no they don't - as you point out. However they keep saying they will...........
big issue as sort of highlighted above is the difference between theory and practive. For many the decision tree around steering the vector or using XTE will be almost automatic but it's actually quite complex to draw up.
One key comes out over and over again and that is that by plotting to a paper chart potential hazards that just don't seem to be there on electronic screens just scream at you!
 

starboard

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Yes maybe but after a lifetime at sea I will now sit back put on the pilot and enjoy it.......not only that but fully understand what I am enjoying without a piece of paper to say so!!!!!!!!!
 

jimi

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Re: GPS & Tides

Reluctantly I feel I must corect the statement.
If you follow the direct line to the waypoint tou will cover the shortest distance over ground to the waypoint. However what you are actually interested in is the shortest distance through the water and you have to plot a course to steer to do that allowing for tidal offset. In an English Channel crossing of 12 hours the difference between a direct course over the ground or a direct course through the water can be as much as 30% (or 3 to 4 hours)!!!
 

Marsupial

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Re: GPS & Tides

Spot on Jimi,

Often the tide will take you 15 miles off track, (according to the GPS) fight if if you want to but its often quicker not to. Plot a series of waypoints that mimic the tidal arc across a tide and you can then use the tide to your advantage. Or you canb sit there on auto and enjoy a longer ride!
 

john_morris_uk

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Re: GPS & Tides

Agree Jimi! You are correct, and one of my favourite questions to candidates is to discuss how they decide on their 'course to steer' across the channel. I had one candidate who claimed to be a successful racer who said they always followed the track on the GPS on cross channel races. I suggested that I would like to race against him for a small stake!

To reiterate, quite often the quickest route is to allow the boat to be taken off track by the tide and then back on track by the subsequent tide. It means that the XTE will rise and fall as you cross the channel. You will sail the shortest distance through the water which is more important than sailing the shortest distanc across the ground.

By the way I think a good rule of thumb is to re-evaluate how things ar going every time you halve the distance to your destination. Actually, by the time you've done this a couple of times (at half way and three quarters or thereabouts) its pretty obvious whats is going on, and you can make the appropriate adjustments to ensure that you are not downwind and down tide as you you approach your landfall.

The GPS is a wonderful instrument, but on pasages over more than one tide, you need to do some paperwork to work out the best course to steer. On short passages you can use the GPS to keep yourself on track and that is what I have done in round the can's races when you are on a downwind or reaching leg across the tide and can't see the next mark. (Of course as soon as you can see the mark and a transit with it, its obvious what is happening...)

Whoops Short post ended up in a slight rant...
 

chappy

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Re: GPS & Tides

I'm a bit confused now because some people have said if you follow the XTE that will be the shortest route & others are saying work out your tidel differances, can someone put me out of my missery & confused state /forums/images/graemlins/confused.gif
 

jfm

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Re: GPS & Tides

Chappy, there's a difference bewtween shortest distance over ground and thru water.

Imagine a trip from somewhere near Needles, to Cherbourg, say 60miles due south over the ground. Imagine for the first half of the trip the tide is taking you westwards, then for the second half of the trip it takes you back an equal amount eastwards

What Jimi and others are saying (100% correct) is that you should put the boat on a heading 180degrees and hold that heading. The tide will take you in a course over the ground that looks like a big "C" shape if you plot it on the chart. You will indeed travel in a "C" shape over the ground. Say, 80miles over the ground. BUT you will have only travelled 60miles thru water, and that's what matters if you want to get to cherbourg asap.

In contrast if you use autopilot it can only follow a ground track so it would take you in a straight line over the ground but for the whole trip you would have your bow nosed slightly into the tidal current and you would in this example travel 80miles thru water so would take 4/3rds as much time to get to cherbourg.

The above is a made up example, just to illustrate. The point is, on longer multitide/cross tide trips, using a/pilot "track" function is often not smart because the machine will force you over a vertain ground track. Doing the pencil paper chartwork and using autopilot "Heading" or "Auto" function is often smarter. Also. you have to plot the "C" shape to figure out if there are any obstacles under it
 

starboard

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Re: GPS & Tides

Sorry Jimi still disagree. Lets take example you want to go from A to B
The track from A to B is 360 and with no tide the Course would be 360. Now lets assume a tide from W to E of 5 kts. Manually to compensate for that tide you would calculate tidal vectors based on maybe inacurate tidal information and probably lay off a course of maybe 330. Steering this over the period of time you have calculated all being well i.e your planned speed and the estimated tide you should end up at B.

Now if you decide to follow the highway mode on the GPS or get the pilot to track to the waypoint initially as you leave A both track and course will be 360, as soon as the gps calculates the cross track error that it is beeing influanced by the tide/windage etc the course to steer will keep changing to keep you on that exact track over the ground of 360. Therefore if you follow the centre line the course may read 330 compensating for a cross tide but you will remain at all times on the track line of 360. To prove this point try it next time you cross the channel if you have done as advised the track you have followed will be shown as a straight line on your track plotter. The advantage of this method is that the unknown tide will always be compensated for by the GPS detecting any latteral off track movement and continually update new course to steer to either the pilot or the visual display for the helm to steer. I do this many times crossing the North Channel to Ireland and in doing so save many miles OTG.

Paul.
 
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