Compass heading

Fr J Hackett

Fr J Hackett

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I have just installed a new course computer (Raymarine S3G) but in trying to speed up the process decided to use the allready installed fluxgate from an old ST4000 mk1 wheelpilot, which had worked perfectly well with both the 4000mk1 and a 4000mk2. On connecting the boats heading was 180 degrees out and refused to calibrate on a sea trial even when forced to accept the heading from the gps (COG) It appears that the deviation on a westerly heading is so great that the computer cannot handle it. I will next week try a new Fluxgate, but cannot understand why after several years the old one has suddenly packed in. My one thought is that I used an electric drill quite near the sender for several minutes is it likly that the magnetic field generated has affected the Fluxgate? One last question arrising from Homas post why is it not recommended to use SOG to supply information for MARPA calculation, I understand the ues of a fast heading info as opposed to COG for stabillity but I would have thought that SOG would be adequate for speed info for vector calculation.
 
Nope unless Raymarine have changed the colour code. I initially thought that and nearly opted for some experimentation but like you I don't know how they work and the thought of Bu**ering up the course computer put me off.
 
I can't imagine how you could permanently affect the flux gate, there are no permanent magnets in it, just a loop of wire (or equivalent).

180 degrees out. If looking for physical reasons, installing it upside down, or something equally silly but not obvious until you spot it? Electrical reasons have already been covered, although it would be worth double checking for big lumps of steel less than two feet from the compass head which would bugger up your calibrations.

Regarding SOG vs. speed through the water, surely both boats are affected by the tide which SOG takes out, so speed through the water (log) is going to be the better one?
 
MARPA - water speed and heading

In short..
For radar plotting you must always use course through the water and speed through the water. It is the other vessel's aspect that you must determine before you can interpret what course of action to take as per the Collision regulations. An assumption is made here that both vessels (own and target vessel) are experiencing the same current.

If you use SOG and CMG as suggested earlier, just think what the effect it would have to a slow moving target in a strong current - the aspect would be completely out.

Rgds
Homa
 
P&O Nedlloyd Vespucci had radar incorrectly set up to receive ground-based course & speed info (GPS) rather than water-based (log & compass) when she collided with the yacht Wahkuna in May 2003. The MAIB report, No 28/2003 is well worth a read. Lots of good lessons about radar work. The report is downloadable from the MAIB website.
 
Re: MARPA - water speed and heading

[ QUOTE ]
In short..
For radar plotting you must always use course through the water and speed through the water...

[/ QUOTE ]True - but "course through the water" is just about impossible to measure. The best approximation is compass heading which can be several degrees out due to leeway.
 
Re: MARPA - water speed and heading

Danny,

I stand corrected, typo on my part,

I should have written: own vessel's heading and speed through the water is required for MARPA.

Cheers
Homa
 
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