NMEA Fluxgate recommendation?

[Roger Long]

I'd like to add a NMEA heading sensor so I can activate the MARPA functions on my Garmin radar. Hamilton suggests a KVH digital compass unit as the lowest cost alternative but it's pretty large for my challenging location alternatives and has pretty large power draw, more twice what my autopilot draws in standby mode. I also don't need the display functions or the battery energy expended in driving the card around.

Raymarine has a nice looking, compact unit but I then have to spend nearly as much for a Seatalk to NMEA converter.

Anyone else been through this or know of a good choice?

What would be really neat would be to figure out how to hack NMEA out of the gimballed fluxgate already in my Raymarine ST1000 tiller pilot. I already hacked to to provide limits switches

http://www.rogerlongboats.com/ST1000mods.htm

but the invisible data stuff is beyond me.

Another question: Does anyone know if a fluxgate can be mounted directly on top of an aluminum fuel tank? I know that it's not magnetic but large conductive things sometimes effect electric coils nearby even if non-ferrous.

 

[tsmwebb]

...
What would be really neat would be to figure out how to hack NMEA out of the gimballed fluxgate already in my Raymarine ST1000 tiller pilot. I already hacked to to provide limits switches
...

I suspect it can be done... But, you probably want one with a gyroscopic rate of change sensor to make you MARPA work. I use my AP's fluxgate for my non-MARPA RADAR (for course up True View and such) and find the response in waves is poor. YMMV.

--Tom.

 

[Maine Sail]

I suspect it can be done... But, you probably want one with a gyroscopic rate of change sensor to make you MARPA work. I use my AP's fluxgate for my non-MARPA RADAR (for course up True View and such) and find the response in waves is poor. YMMV.

--Tom.

Tom is right you'd want a rate gyro and a flux gate for the best MARPA tracking. Raymarine sells a Seatalk to NEMA converter box and it's about $135.00. SeaTalk to NEMA Box (LINK) If you have one of the newer autopilots they have a NEMA output right on the circuit board so perhaps this is a good cheater way to avoid the SeaTalk to NEMA box.

Having used MARPA I still find human interaction/tracking, and a helm mounted display, the best possible tracking solition. Marpa is nice in falt water but in rough seas the target can get lost. Once lost you lose a target, even breifly, MARPA tracking is also lost..

 

[Roger Long]

Marpa is nice in flat water but in rough seas the target can get lost. Once lost you lose a target, even breifly, MARPA tracking is also lost..

Hmm, considering how often I saw even fairly solid targets disappear in very moderate sea states, I think maybe I'll forget the whole thing. The overlay alignment is fine without a heading sensor and I don't see much point in relying on MARPA if it's going to fail me when workload and distraction are greatest. Better to get more practice doing mental tracking in calm conditions.

My autopilot is just a tiller pilot that tweaks the windvane linkage. It has a gimballed flux gate in it but you can really see the effects when there is a moderate roll. Keeps course closely enough though that I have zero desire to invest in the expense of anything more sophisticated.

 

[tsmwebb]

... Better to get more practice doing mental tracking in calm conditions.

My autopilot is just a tiller pilot that tweaks the windvane linkage. It has a gimballed flux gate in it but you can really see the effects when there is a moderate roll. Keeps course closely enough though that I have zero desire to invest in the expense of anything more sophisticated.

It is very, very hard to do good mental RADAR plotting. I use "china markers" -- colored wax pencils -- to plot interesting targets directly on my RADAR screen. I've tried dry erase markers but the ones that dry erase are too easy to erase and the ones that wet erase are too hard (for me). I use a tongue depressor marked with my range ring spacing, a stop watch and TSD calculator. I understand the workload thing. But, making a mark and noting the time can be done quickly and with the TSD calculator you don't really need to time things for easy mental math (eg. 6 min plots). For colision avoidance I find it much easier to do heading up plotting as it results directly in a relative motion plot.

Anyway, if you can, I suggest getting practice hopping down marking your target, your heading and the time (stop watches make time math easy) really quickly. On your second or third trip you should plot a CPA line and after that if it's ok just make sure the target stays on it... If the CPA looks problematic and you don't have a visual bearing, you really must plot.

The autopilot fluxgate thing would be a "project" I think. Unless it has NMEA onboard (unlikely) you would need to take the analogue output got to an ADC and then to NMEA RS232... There are microcontrollers that can be made to do all of those things (I like the AVR line) but you'll need to do hardware, software and testing... It might be a kind of fun thing to do on a slow winter fortnight but then you'd still not have a gyro...

--Tom.

 

[Roger Long]

It is very, very hard to do good mental RADAR plotting. I use "china markers" -- colored wax pencils -- to plot interesting targets directly on my RADAR screen.

It's kind of hard to imagine doing that on the little LCD Garmin screen with it's warnings to use only certain kinds of cleaners to avoid damaging the coating. Sounds like you've got a glass front cathode ray tube unit. If I end up spending a lot of time out in open water on long straight courses with heavy shipping traffic, I may wish I'd considered Mainesail's offer of his CRT unit more seriously.

Here in the complex topography of Maine, traffic is likely to change course at any time as am I. True plotting and probably even MARPA isn't fully practical shorthanded. The radar is an early warning and situational awareness tool. I still have to rely on taking early action and being nimble and quick to collision.

Now, this woud be cool. The Garmin does all it's signal processing up in the radome and connects with a network cable. Therefore, there ought to be nothing but software programing involved to plug it into a laptop computer with a bigger display and keyboard and mouse that would support much more sophisticated tracking tools and AIS intergration.

 

[tsmwebb]

It's kind of hard to imagine doing that on the little LCD Garmin screen with it's warnings to use only certain kinds of cleaners to avoid damaging the coating. Sounds like you've got a glass front cathode ray tube unit. ...

I've got a horrid old RL70 LCD PPI. It's actually easier to plot on than a CRT as it is perfectly flat. I remove the marks with a soft cloth. It hasn't been noticeably harmed by my scribbling on it pretty regularly over the last nine years... I can't say how the Garmin would hold up. But, any PPI you can't plot on is worthless, IMO. If you ever get ahold of Garmin you might ask them if you can plot directly on the screen. It is a common, even recommended, practice.

FWIW, and in the same category as USCG cert. nav lights, Rule 7(b) requires that you plot to determine if there is a risk of collision... You could do transfer plots, or "quick" plots (on the screen) or you can ignore the rule... I was told by the Captain who runs the USCG licensing school in Honolulu that (M)ARPA does not satisfy the plotting requirement...

--Tom.

 

[Roger Long]

FWIW, and in the same category as USCG cert. nav lights, Rule 7(b) requires that you plot to determine if there is a risk of collision..

That's precisely one of the reasons I put off getting radar so long. If you don't have it, you aren't violating any rule as long as you are using all available assets and equipment. Once you install it, you're going to be in violation of some rule sailing shorthanded in Maine 99% of the time. I also believe you are in violation if it's on board the boat and not turned on. I can't affort the power draw sailing on a long trip where I'm not running the engine much.

This is a sub set of a larger question. Can one person, or even two, responsibly take a boat anywhere and comply with all rules? In a world where only about 1 in 10 people on the water even know that there are rules, the signal kind of gets lost in the noise.