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Auto Helm / chart plotter?

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Steve Blackburn
Master Marine Consultant

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Canada
1091 Posts

Initially Posted - 06/21/2007 : 19:06:13

Can someone give me a basic explanation as to how an auto helm works in conjunction with a GPS or GPS chart plotter? I'm planning on purchasing a GPS this winter and would like to have some direction on what to look for.

I'm thinking that with a GPS plotter you can plot your course and the auto helm will follow it? Am I correct in assuming that a GPS plotter will show you tides and current marking it somehow when your course goes over a certain current preset speed? In other words I could tell my chart plotter to not let me navigate in more than 3 knot head on currents and by calculating my speed etc, it could predict where I'm going to be at a given time and what the currents will be like then?

I also carry a laptop, and my understanding is that you can plan your course on PC software then upload this to your GPS? Maybe on the laptop you could get the current warnings, but not on the GPS?

Steve Blackburn, Calgary, AB
C250WB - 1999 - Hull 396

Edited by - Steve Blackburn on 06/21/2007 19:09:06

Bay Tripper
1st Mate

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USA
56 Posts

Response Posted - 06/21/2007 : 19:28:30

Go to a West Marine store and let them show you a set up that will do this for you

Edited by - on

Steve Blackburn
Master Marine Consultant

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Canada
1091 Posts

Response Posted - 06/21/2007 : 23:48:36

None of those here in Canada. All we have are moose and beer.

Edited by - on

Nautiduck
Master Marine Consultant

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USA
3704 Posts

Response Posted - 06/22/2007 : 00:04:46

Have a few beers and then ask a moose.

Edited by - on

CaptRon400
1st Mate

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USA
90 Posts

Response Posted - 06/22/2007 : 08:08:00

Steve -

Very simplified, the autopilot steers and the gps tells it where to go. Sort of like a backseat driver. A NMEA data stream containing certain data (varies based on models) is transmitted from the gps or chart plotter to the pilot which will apply the data as a course correction. This happens constantly to keep the boat on track and could be 1 degree or less of turn every few seconds. Arriving at a waypoint could produce a request for more of a turn, and many pilots will wait for the helmsman to respond with a keystroke before it does the major course change. Someone has to move the sails, right?

The pilot can be programmed to follow it's own compaass, the data stream from the gps, and even the wind if a transducer is present.

A real chartplotter has advantages over a gps with routing capabilities. The most important is that you can graphically see exactly whats happening as opposed to a series of numeric coordinates. The other features like tide, current and port data are nice too. Because they plot and compute based on ground speed and position, they will take into account current (and wind). But you have to go to the top end models (expensive) to get the device to talk with you about it. I've used some of these high end toys. Columbus didn't know what he was missing, but it probably costs more than all 3 of his boats together.

A laptop can do all of the above (and more) with the right software. Depending on which chart plotter or gps you get, there are different ways of moving the data over (make sure the one you get can do it!). You also have the option of connecting the laptop directly to the pilot and instruments and letting it run the show.

Edited by - on

Arlyn Stewart
Master Marine Consultant

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USA
2980 Posts

Response Posted - 06/22/2007 : 09:24:53

An autohelm has two basic modes, auto and track. Under auto, the course is established by the autohelm when the auto button is pressed or one of the course adjustment buttons. That course will be maintained without any windage or current allowances.

A few years ago a large Catalina was lost in the Sea of Cortez when the Captain set auto on a course to clear a headland ahead and went below to work on his income taxes leaving the watch to his wife who was reading a book and perched on the blind side of the jib to the headland. While the course was maintained, windage crept them in and they foundered on the rocks.

Track mode differs greatly from auto in several ways. 1st, the gps becomes involved and instead of a course being held, a track between two points is held with the great advantage that corrections are made for windage and tidal or other currents. The saftey of track mode should be obvious, when two points producing a safe rhumb line are selected, the gps working with the autopilot will keep the boat on that safe rhumb line or warn you if it cannot do so.

Setting waypoints with a safe rhumb line is at the heart of navigation using track mode. This can be done from a historical data base of waypoints, from charts, chartplotting software or chartplotting device. If only a few waypoints are needed, they can be read into the gps directly off the chart but manually entering is both tedious and allows for possible errors. If many waypoints are needed, far better is to use chartplotting software or hardware so the points are loaded electronically.

As to the lap top running chartplotting software, it needs only a basic gps to provide full featured track navigation. In fact, a lap top is not even needed...as the home computer preplanning provides most of the same features, with the exception of moving map display in real time as does the Chartplotting gps.

Here is something interesting to consider... while many may believe that moving map on a chartplotting gps is the only way to note where one is safely at... it may not be as safe as one might imagine or even the safest way. What do I mean?

Constant attention to the chartplotting display can be a distraction from other important task. The helmsperson is navigating in a feedback mode, making adjustments for wind and current to keep the boat positioned where desired on the moving map display. In adverse conditions this can become a consuming issue with most other sailing chores set aside or ignored.

I think the better way is to allow the gps and autohelm to do the navigation and allow the skipper the time and energy to maintain overall watch. The dynamic that allows this is the preplanning of the waypoints. Remember, if a series of waypoints are programmed into the gps and we know for a fact that between any given set of consecutive numbers the rhumb line is safe... then as we allow the gps to track us on that rhumb line we know we are safe and that when we reach the next point and pick up the next track, we will be safe on that rhumb line as well.

My argument is this... if a chartplotter owner continues to do the preplanning and has safe waypoints established well before any rough conditions so that the helm responsibility can be given to the autopilot, then a chartplotter does provide additional safety... but if the chartplotting moving map ability lulls the skipper into thinking he doesn't need safe waypoints any more cuz he has moving map... then he is in fact less safe.

Given the choice between having known safe waypoints or a chartplotter... give me the safe waypoints and a basic gps and I believe I'm safer.

More than you asked... but maybe helpful

Edited by - on

Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 06/22/2007 : 12:08:53

High-end chartplotters do indeed store current data, but since that data is time-dependent, and time under sail is hard to predict, even with all that hardware and software, advance planning for currents is best done by the "warm-ware" (you and me) before setting out. Once under way, as Arlyn explains, the GPS will direct the autopilot to stay on a track to a waypoint <i>regardless</i> of wind or current--all it knows is where you are and in what direction you are actually moving over the bottom. It will cause helm corrections to maintain that track. That's it.

Around here, most sailors have an Eldridge Tide and Pilot Book on hand, and use it to plan, for example, whether to sail inside or outside of Long Sand Shoal at a given time, and how to get through The Race (or avoid it)--often resulting in decisions on what days they will sail from one place to another. The same data is in the newer chartplotters, but it's best used for advanced planning--again, by the warm-ware.

Edited by - Dave Bristle on 06/22/2007 12:09:48

Steve Blackburn
Master Marine Consultant

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Canada
1091 Posts

Response Posted - 06/22/2007 : 12:28:01

Great explanations guys. Thank you very much.

Much better than the explanation the moose gave me after a few beers. I shot the moose, and we are having moose burgers tonight.

Would you have suggestions as to a good GPS and PC software (pre planning)?

Edited by - on

atgep
Master Marine Consultant

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1009 Posts

Response Posted - 06/24/2007 : 10:10:49

I have been using Lowrance products for a few years an really like their stuff. I have a small chartplotter/sounder combo and really like the features it offers. I always keep the charts in the cockpit and use the plotter to back up my fix. The charts Lowrance offer are fantastic. One chip for less than $100 that covers North America and Bahamas. Sorry Canada is extra! I like being able to look at the plotter to see charted depths, the sounder to verify, and the paper to see what NOAA says it should be. Very good Situational awareness

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