Using a chart plotted effectively

[C08]

I have had a Garmin 750 plotter for about 12 months now but I am not using it properly. When I just used paper charts I would plot the course to steer etc taking account of hourly tidal streams in the traditional fashion and mostly that was OK. I am having difficulty combining the plotter use of waypoints and the effect of tidal streams i.e. the track indicated on the plotter is OK but steering this track requires continuous adjustment of the boat heading to take account of the varying effects of tidal streams and is not an effective course to steer overall (I think). Reading the Garmin manual does not give any clue to this so how should this be done. How should I be doing this?

 

[Poecheng]

Well, one way is to link it to your autopilot and tell the pilot to go to the waypoint. It works it all out for you. Maybe not what you were asking though

 

[johnalison]

I would go back to basics. For a longish leg across the tide I would, and do, make a rough calculation of the offset by working from tidal diagrams or the plotter's own current info. This need only be quite rough, since we are not taking a YM exam. Steer the course plus the offset and just use the plotter to record your track, which will be a curve. Three quarters of the way across, look at where you are and just steer for the destination. If you steer the track in varying tides, you will arrive after the bars have shut.

 

[NealB]

Well, one way is to link it to your autopilot and tell the pilot to go to the waypoint. It works it all out for you. Maybe not what you were asking though

Does it?

Or does it make constant small course adjustment to keep you on the initial track?

If the latter, then, ok, it will get you there, but the passage will take longer than necessary.

 

[Poecheng]

Does it?

Or does it make constant small course adjustment to keep you on the initial track?

If the latter, then, ok, it will get you there, but the passage will take longer than necessary.

Think you are correct. If a destination is series of waypoints (say an hour apart) then probably as good as anything else. If it is a waypoint 12 hrs away then, as you say.

 

[C08]

Yes that is my point.

 

[FWB]

Or if you are in a mobo doing 25kts, just take the GC route to the next waypoint.

 

[FulmarJeddo]

Does it?

Or does it make constant small course adjustment to keep you on the initial track?

If the latter, then, ok, it will get you there, but the passage will take longer than necessary.

I think it probably depends on the autopilot. I have a Raymarine SPX5 with the separate course computer and Garmin plotter. If I set it to follow a track it seems to quite quickly settle down on a heading taking into account the tidal offset, you can see it quite clearly if the next waypoint is close to a buoy or fixed object.

It's not the plotter that works out the course to steer. It just supplies the pilot with bearing of next waypoint, distance and Cross track error. I suspect a more basic autopilot will keep correcting to follow the plotter's track.

 

[trapezeartist]

I use the rolling road display on the plotter to keep me on course. Just make sure the BTW and COG are the same. This works for hand-steering or using the autopilot to follow a heading. If there is going to be a lot of cross-tide, I just make a guessed adjustment to start and let it run from there.

 

[prv]

How should I be doing this?

Back to the paper charts for a long leg, I'm afraid

I think there may be high-end plotters that can do the course-to-steer calculations we're used to, and I know there is PC software, but the typical mid-range box just doesn't have that facility. For short legs I just put the green track line on the destination and keep it there, for longer ones I work it out traditionally and only use the plotter for the last bit.

Pete

 

[Tranona]

I have had a Garmin 750 plotter for about 12 months now but I am not using it properly. When I just used paper charts I would plot the course to steer etc taking account of hourly tidal streams in the traditional fashion and mostly that was OK. I am having difficulty combining the plotter use of waypoints and the effect of tidal streams i.e. the track indicated on the plotter is OK but steering this track requires continuous adjustment of the boat heading to take account of the varying effects of tidal streams and is not an effective course to steer overall (I think). Reading the Garmin manual does not give any clue to this so how should this be done. How should I be doing this?

Depends on what you mean by using it effectively. It is an aid, not something that tells you where you have to go or how you should sail your boat. In low speed craft and cross tides, steering to a waypoint is a waste of time. If you did not have a chart plotter, you would calculate the offsets over the duration of the passage and shape a course to arrive at your destination taking into account the net offset. Your course over the ground will not be the same as the course you are steering. If there are no obstructions then you can set a waypoint near your destination and use the XTE to monitor your position over the ground on a regular basis to see if the offset agrees with your estimate. No different from keeping a regular plot on a chart, which you would ideally do as well. You can then make your adjustments to course steered if required. No need for interim waypoints unless there are obstructions to avoid.

If you are going along the coast or in a non tidal area you can steer a direct line to a waypoint or a series of waypoints including telling the autopilot to do it. Again use the XTE to check you are not deviating too far because of tide, leeway etc.

With high speed craft tidal offsets can be largely ignored and you can "drive" using the plotter with a series of waypoints and steer to those - again really no different from navigating using conventional charts.

The big advantage of chart plotters is that you can better visualise where you are both in relation to your course and the general environment (other vessels, land, navigation marks, dangers etc) and for pilotage this is better than having to constantly refer to charts and other sources of pilotage information as you can call much of it up on your plotter.

One thing to remember is that not everybody uses such aids in the same way, just as there are different ways of conventional navigation. So use it in the way that you feel comfortable with.

 

[prv]

I think it probably depends on the autopilot. I have a Raymarine SPX5 with the separate course computer and Garmin plotter. If I set it to follow a track it seems to quite quickly settle down on a heading taking into account the tidal offset

Yes, it will put the predicted track onto the destination and keep it there regardless of tide as it changes. What it won't (can't) do is work out the net tide over your predicted passage time and only offset for that. Instead it'll have you fighting all of the ebb and then fight the other way against all of the flood. The classic example in my part of the world is a cross-Channel passage from the Solent to Cherbourg - if you do it right then your track over the ground looks like either an S or a C depending on timing. The autopilot would leave a straight track, having sailed though much more water.

Pete

 

[C08]

Yes, it will put the predicted track onto the destination and keep it there regardless of tide as it changes. What it won't (can't) do is work out the net tide over your predicted passage time and only offset for that. Instead it'll have you fighting all of the ebb and then fight the other way against all of the flood. The classic example in my part of the world is a cross-Channel passage from the Solent to Cherbourg - if you do it right then your track over the ground looks like either an S or a C depending on timing. The autopilot would leave a straight track, having sailed though much more water.

Pete

That is it in a nutshell and I am having difficulty combining the manual adjustments for tidal streams with the plotter track to waypoints. I am probably looking for something that does not exist, apart from expensive professional software. Thanks for thoughts and ideas.

 

[RichardS]

That is it in a nutshell and I am having difficulty combining the manual adjustments for tidal streams with the plotter track to waypoints. I am probably looking for something that does not exist, apart from expensive professional software. Thanks for thoughts and ideas.

if you know how to plot Pete's S or C track over the ground on a paper chart, can you not plot it on paper and then read the coordinates of a selection of points from the paper curve and transfer those to the chartplotter. You can then compile a route to tell the chartplotter to follow that sequence of waypoints.

I sail in the Med so this is not from personal experience.

Richard

 

[prv]

if you know how to plot Pete's S or C track over the ground on a paper chart, can you not plot it on paper and then read the coordinates of a selection of points from the paper curve and transfer those to the chartplotter. You can then compile a route to tell the chartplotter to follow that sequence of waypoints.

I suppose you could, but it seems an unnecessary faff. Work out the course to steer and then tell the autopilot to steer it. Don't involve the plotter in the steering at all (except perhaps for the last few miles).

Pete

 

[wooslehunter]

For a short trip, plot a single waypoint & head to it. The GPS will show you the bearing to the waypoint & your COG. You can adjust your course accordingly. i.e. If the COG is 053 & the bearing is 062, then change 9deg to stbd.

For a longer trip, constantly adjusting the course to take account of varying current is a pain as you say but also slower since you will always be heading slightly into the current to make a straight course. So, calculate the CTS as you did before, then using that, work out a series of hourly waypoints in the positions you'd expect to be in. Once you set your CTS, you'll soon see if the tide & leeway is as predicted & you can adjust accordingly. There's no real need to constantly adjust. You wouldn't do that if you were just taking hourly fixes or for that matter doing it the old way with paper & DR/EP. The only real difference between using a GPs, is they you can get accurate fixes along the passage & adjust to make sure you arrive spot on.

 

[dslittle]

Back to the paper charts for a long leg, I'm afraid

I think there may be high-end plotters that can do the course-to-steer calculations we're used to, and I know there is PC software, but the typical mid-range box just doesn't have that facility. For short legs I just put the green track line on the destination and keep it there, for longer ones I work it out traditionally and only use the plotter for the last bit.

Pete

Similarly, for longer passages I work out the CTS and just set the autopilot to that heading. During a Channel crossing the tides cancel themselves out. I still monitor the XTE on the chart plotter to see if it councides with my predictions. It does take a bit of faith when the XTE is in the miles though...

Edit - Aghh. Beaten to it!!!

 

[Tranona]

That is it in a nutshell and I am having difficulty combining the manual adjustments for tidal streams with the plotter track to waypoints. I am probably looking for something that does not exist, apart from expensive professional software. Thanks for thoughts and ideas.

Then follow my method of using the XTE. Don't continually adjust your course to head for a waypoint if you have calculated and allowed for your tidal offsets in your passage plan. The plotter will always show you where you are over the ground which will not be on a direct line to your waypoint. Keeping a plot of actual positions say hourly will tell you how accurate your original plan was and inform any adjustments to CTS to hit your waypoint. No different from plotting on a chart using DR except that your GPS derived position will be accurate.