Question re- compasses

boatmike

boatmike

Well-Known Member
Joined
30 Jun 2002
Messages
7,049
Location
Solent
Visit site
Without re-visiting all the previous stuff regarding MARPA I am considering upgrading the compass input to my C70Plus Raymarine radar.
Raymarine have a smart heading whotsit which appears to be a fluxgate with what is called a rate-gyro attached . Looking at the specs it is no better than plus or minus 2 degrees accuracy. It's about £565. The cheapest proper Gyro is afaiks the KVH gyrotrac at about £2600. Thats a bit outside my budget although previous big ship experience always led me to believe a proper gyro was essential for ARPA to work properly.
The SIMRAD fluxgate I have at the moment can only achieve an NMEA delivery rate of 5 times per second. Raymarine suggest a minimum of 8Hz for MARPA with their smart whotsit delivering 10.
The question is this.
Any solid experience out there of using MARPA with the smart heading thing?
Any experts on compasses that can suggest an alternative?
I have seen a Maretron at £459 that is a rate gyro fluxgate with claimed accuracy of +/- 1 degree at 10Hz. Anyone got one of those?
With AIS on my nice shiny new Horizon 500 should I really care about MARPA at all? The most risk is crossing traffic lanes in fog. Perhaps I will concentrate on AIS and just use my Radar on "head up display" with EBL as I am used to?
 
AIS is only a legal requirement on things that float that are heavier than 300 Tons (IIRC), so any smaller vessel will not show up on AIS, unless they are posh enough to have splashed out on one. Unlikely on small fishing boats, which should show up on your MARPA thingie.
 
What you should have said is will show up on my RADAR thingy. They may or may not appear on MARPA dependent on the sea state and the quality of heading data which is what I am trying to improve. Have you not used MARPA in anger and found that the "lock" on an acquired target keeps getting lost? It's largely due to bad or erratic heading data. I also would like them to show up on my RADAR thingy in the right place. 2 degree compass error equates to nearly 1/4 mile at 7 miles distant as the skipper of Whispa found to his cost.... I know the limitations of AIS, thats why I want MARPA working properly too to identify things not on AIS.
 
Yes I know that too, in theory you are absolutely right but do you know of a proper Gyro under £2500? Even that's a budget one. Most are over £7,000 What I am trying to acertain is what performance people get with the Raymarine in actuality and indeed what other alternatives are available at an affordable price.
I really don't think most yachties will have a gyrocompass on board. Do you?
 
I have a Raymarine Smart Heading System connected to my C-series radar/plotter. As you say, it's a fluxgate compass whose output is modified by a rate gyro sensor. I find it works quite well with MARPA, and gives reasonably stable results. Where small boat MARPA does have problems is when targets get close together and the MARPA system sometimes gets confused and swaps one target for another. In fairness, this is probably also a function of the beamwidth of a small radome - the narrower beamwidth of a bigger scanner would possibly discriminate better between targets.
 
At last! Someone who has one. Thanks for that pvb. What sort of boat have you got it fitted on and have you actually used it with a good sea running? Do you get believable CPA and TCPA?
 
Yes, I reckon CPA/TCPA are believable. The boat's a 35ft long-keeler, so motion is reasonably steady. I only have an 18" radome, so I do think beamwidth may be an issue.
 
You are right. It's just a hobby horse with me since so many appear to put and almost blind faith in MARPA when the limitations of almost all leisure sets render MARPA a (very) rough guide.

Just be aware that heading change requires MARPA to do a huge amount of maths. Given how boats yaw, especially in anything but smooth seas, updates will be unreliable.

If you can live with this, and MARPA is a must, choose Raymarine's gyro for your set-up.
 
The other choice is to upgrade my autopilot to a Simrad RC25 or 36 these appear to be 10 Hz and Gyro rate stabilised too. Seems like a lot of techie questions for the boat show!
Yes of course Piers as we have both said elsewhere, know the limitations. It's not magic. My thinking however is that better heading info will make everything more accurate and when on "course up" will give a better bearing by EBL even if I don't believe Mrs MARPA. Oh and 37 ft cats are pretty good at holding a course too with very limited roll /forums/images/graemlins/wink.gif /forums/images/graemlins/wink.gif
 
Hi boatmike. I have used Simrad for years and really rate it. However, I'd be a tad cautious about mixing manufacturers unless you are sure it will all talk well. My preference is Simrad. Ask those qns - I'd be interested to know the result.

I know the ex-Service Manager of Simrad who now has his own business in case you want to ask the real teckie qn. If so, PM me.
 
Autopilot ?

Hi Boatmike,

Just a thought.....

I have the same C70Plus radar as you - I wanted MARPA at a budget price, so I bought the radar secondhand.

I went through the same thought process as you as regards finding a suitable heading input.
I needed a new Autopilot last year & in the end bought the Raymarine S1G autopilot (with rate gyro compass). The great thing here is it is designed for the autopilot and is significantly better than the previous older Autohelm autopilot without the rate gyro. The Pilot almost senses the boat move before it yaws and - and hence steers a much much better course, particularly in quarterly and following seas - which IMO is generally a good test for any autopilot.
It's a lot of money for that little bit of improvement in heading info. But now I have it I wouldn't want to be without it. And like you I am familiar with proper gyros and ARPA etc.
I've only just fitted the radar so haven't had the oportinity to test the MARPA in anger yet.
 
Re: Autopilot ?

Yes indeed homa. You don't need a Raymarine autopilot to do that though. I have a Simrad which will do the same thing and output NMEA to the C70Plus. In fact thats how it is set up now. Its just that the compass is not rate stabilised and only outputs NMEA at 5Hz. Updating the compass to RS and 10Hz is currently my leading plan but I will not finally decide until Exel. I think the Raymarine compass is not as good at +/- 2deg as the Simrad which I think is +/- 1 degree, but we will see.. Thanks for the thought though, I think in general terms it might be the way to go.
 
Re: Autopilot ?

I have a C70 system with a S2 autopilot and separate raymarine gyroplus unit. Raymarine and their agents have totally different views on how to connect them together. Both claim the others way doesn't work. Either way the performance is mediocre. If going the raymarine route get the gyro that fits inside the autopilot course computer casing.
 
Re: Autopilot ?

Thats interesting.... When you say the performance is mediocre what do you mean? What are you dissatisfied with?
 
Re: Autopilot ?

The marpa tails and CPA calculations jump around until the target is between one and two miles away. By this time it does not take marpa to work out whether a risk of collision exists. Whilst the radar makes navigating in fog less stressful the marpa does not add to stress reduction because I do not trust it.
 
Re: Autopilot ?

That sounds like the heading data is not getting through properly. Even with a geriatric fluxgate at 5 Hz mine is better than that. Is it wired according to the installation diagram that came with it? Have you gone through the calibration sequence of sailing around in circles etc? Sorry if I am asking obvious questions here but what was the difference between the local installers wiring and Raymarines recommendations?
 
Re: Autopilot ?

The installation diagram did not cover fitting to C series. Both opal who fitted the unit and Nautiradar - Portugese main agent who fitted the new course computer after the first one died at 2.5 years insist that the method described by raymarine wont work. If anyone can describe how this set up can be made to work well I would be overjoyed.
 
Re: Autopilot ?

To connect to the C-series, you need to take NMEA fast heading output from the NMEA1 port of your course computer and feed it in to the C-series NMEA input. You need to connect SeaTalk to the course computer and to the C-series. The C-series should then be able to adjust/linearise the compass via SeaTalk, using the Compass Setup menu on the C-series.

I wouldn't have expected there to be any real difference in performance between a GyroPlus fitted internally in the course computer or externally in its own case - the system is essentially the same. It is, of course, important to follow the mounting instructions to the letter, with the unit vertical.
 
Re: Autopilot ?

Any doubt go to the Raymarine website, Click on customer support, owners manuals, radar, Pathfinder heading systems to download a complete installation manual. If your local man says Raymarines own instructions dont work I would check it is as per this manual. They are probably wrong. All the kit is theirs they should know how to wire it and it's not rocket science.
 
You must log in or register to reply here.

Share this page

Top