Compass lubber line is +6 degrees

[Anonymous]

My steering compass lubber line is about 6 degrees starboard (it is mounted over the wheel which is on the port side and the lubber line points straight at the bow /forums/images/graemlins/shocked.gif). However, on a due W heading, it reads correctly. I have discovered that there are two sets of screw holes - the set that puts the lubber line over the bow and another that puts the lubber line parallel to the fore/aft line of the boat. When I put the compass into the 'correct' position it is 6 degrees out (not surprisingly). There is a very large cast steel hydraulic steering component within an inch of the compass.

When I take the compass out and rest it in a magnetically clear area on the boat's fore/aft line it behaves like the hand bearing compass - i.e. it does not appear to have been corrected by the internal compensators. It seems to me to be a fairly simple matter of putting the compass back into the 'correct' position and compensating for the deviation as far as possible then produce a deviation card.

However, I am puzzled. This boat, a 1999 Nauticat 42, is from a good stable and was sailed round the world. Would anyone likely to have worked on this boat have tried to correct the compass by turning it and drilling new holes? It seems incredible. I read on the compass manufacturer's website that they make three versions of this compass; one for the Northern hemisphere, one for the Tropics, and one for the Southern. Could it be that the previous owner had quite properly moved the compass while he was sailing in the Southern oceans?

 

[Ships_Cat]

Could it be that the previous owner had quite properly moved the compass while he was sailing in the Southern oceans?

I could not think of any way in which the desired alignment of the lubber line would be affected by the hemisphere correction. As you say, it should be aligned with the keel.

If the compass is made for one hemisphere or the other (is not always so) the correction is for magnetic dip which means if it is used in the opposite hemisphere the card may foul the innards of the compass through its exaggerated tilt on towards the nearest pole headings (sometimes so far as to make it unusable, depending on latitude - I have managed new builds going to the opposite hemisphere where it has been impossible to correct the compass before sea trials and shipping because of this).

Certainly seems strange and the extra holes makes it sound as if someone has had a fiddle.

John

 

[Robin]

[ QUOTE ]
There is a very large cast steel hydraulic steering component within an inch of the compass.

[/ QUOTE ]

You said the compass is over the wheel but on a Nauticat I suspect you mean on the top ahead of the wheel in the wheelhouse?

Firstly, the compass must be aligned correctly fore/aft parallel to the centre line as you rightly say.

Secondly you need to swing it for deviation. With the cast steel component you mentioned being so close, that might result in quite a lot of deviation on some headings.

Lateral thinking:-

Fit another (additional) compass, maybe even an electronic one in a better location. An electronic one can have it's sensor in a 'good' location and the display at the wheel. The existing one can be swung and adjusted as best you can, then a deviation card produced, but you will have the 'master' compass elsewhere (also swung).

You can swing the compass yourself with a little effort either the RYA way with transits or with a Pelorus/shadowpin and the sun. If you have a celestial nav program on your computer (free downloads are about) you can use this to give the sun's bearing at the time of the measurement, use the GPS clock for the accurate time. A pretty good swing can also be obtained just using the GPS (better still dGPS) in an area of NO tide by motoring steadily on each heading and comparing COG with ships head, you need to be doing maybe 4kts or so and have a helper so that the helm just has to keep right on course and 'call' when consitantly 'on'. I did a quick swing of ours this way when we bought the boat, intendind doing the Pelorus/sun swing later, but 3 seasons later and 'later' hasn't come, the original GPS one is still OK!

I don't think the 'dip' correction has anything to do with your situation, and the fact that the boat has already circumnavigated once OK is a tribute to modern electronics and GPS navigation!

 

[Anonymous]

Robin, thanks for your input. Taking your last point first, yes, if they had needed to rely on a compass then they wouldn't have made it! The Nauticat 42 has a deck saloon steering position below which, in ours, is laid out with an electronic console without a wheel. We do con from below, adjusting course by nudging the autopilot then going aloft (no wheelhouse) for all close quarters situations. The master compass and wheel are both there, in a moulded binnacle that was clearly purpose-designed. Interestingly, the hydraulic steering is, from the literature, standard and I wonder why they positioned a steering compass so close to a big lump of cast steel (circa 2 or 3 kgs). The problem I have in correcting the compass is that it has to be lifted out of the binnacle, adjusted then re-inserted before I can see the effect of the adjustment, so it might take an age. Since we are in Chichester, I might go across to Sandown Bay and anchor when it is very calm, with a bower and kedge, and carefully set the headings either with transits (there are a few), or using the hand bearing compass to measure a heading much as one would with a pelorus. Everywhere else around here is so tidal and busy with traffic that I fear that I will make a mess of it.

We are on the same wavelength about fitting a second, possibly electronic, compass. I have been considering fitting a Sestrel Moore below and treat that as the steering compass. I posted a wanted ad in this forum a while ago but no joy yet and the Sestrel Moore is not inexpensive! I have also considered fitting an extra fluxgate. At present we have the ST7000's fluxgate which is mounted in an absurd position - in a wardrobe in the midships cabin. It is well off the centre line and I have to be careful to make sure that what is put into that wardrobe is non-magnetic. At present the fluxgate is reading around minus 11 degrees on our present heading in our berth. I need to 'calibrate' the fluxgate by following the Raymarine instructions of driving around in circles and see where that leaves us. Maybe I will move it. If I can get that set up right then I could probably buy a Raymarine display that would give a dedicated compass indication. But the Sestrel Moore would look nice (to me, at least), and is arguably one of the best compasses ever made. Thanks again, David

 

[Anonymous]

John, thanks - I had completely forgotten about the issue of dip. I still can't get over thinking about the individual who 'corrected' the compass by twisting it on its base. I'm wondering what other helpful contributions he might have made to my boat! David

 

[AndrewB]

As having the compass so close to a lump of steel is not a good idea, perhaps the previous owner had an alternative position for it, that might explain the second pair of screw holes?

One thing I quickly learnt with a yacht that had circumnavigated was that the previous owner had had plenty of time to work things out, there was nearly always a good reason for things that seemed odd. Many of my early 'corrections' had to be reversed. Keep looking for a logical explanation before making changes.

 

[Robin]

David

I quite like the Raymarine ST50 compass (I assume the ST60 may be the same) because if you install it with a separate fluxgate (which you could chose a better location for) it will give you the average error between a locked course and the course steered. By that I mean say you want to steer 225 degs, then you set up the autopilot to do this and lock the ST compass onto 225 too. Then after say 5 minutes you press the ST compass 'lock' button and it will tell you what the difference is, ie -3degs or + 2 degs whatever, then you can simply reset the pilot with the same number of clicks and check again a bit later. To do this though the ST compass must be separate from the rest of the system (not using the pilot fluxgate) because in that case the ST head will simply show the heading set on the pilot and the 'average error' doesn't work although the port/starboard off course pointer does show which way the heading is off. Also, when you do the circuits and bumps to calbrate the fluxgate(s), there will still be a residual error despite what the handbook says! So when you do a deviation card for the main compass it pays to then make a subsidiary one from it to cover the fluxgate one.

Another possibility would be to fit a bulkhead compass in the main bulkhead if that would be easily read from the wheel? There is no law that says wheel steered boats have to have pedestal compasses, especially if the pilot does the steering most of the time anyway!

I wouldn't write off the GPS COG comparison method to work a deviation card either, we did that with our current boat at slack water neaps in Studland bay (when it was quiet!) and the results were so good I haven't got round yet to doing the full pelorus swing!

It will keep you amused for a while anyway...

Robin

 

[Anonymous]

Robin,

The idea of fitting the extra fluxgate in the way you suggest does sound interesting and there is a spinoff in that I have an ST600R remote on a very long lead, which I found on board and have just reconnected. It will allow me to have an information display when I am off watch - it would be nice to have the actual compass heading available. (It will also allow us to steer the boat while sunbathing on the forward deck, but that is a different matter altogether and nothing to do with adjusting compasses!)

In considering the Sestrel Moore as a master compass below, it would be my plan to pronounce a compass heading specifically for the upper steering compass on every leg when we are steering a magnetic course (as opposed to wind vane mode or track mode). In this way it wouldn't matter much what the upper compass was reading - within reason. I would like between five and ten degrees worst case deviation but that should be possible.

David

 

[peterb]

Hard steel or soft iron?

[ QUOTE ]
There is a very large cast steel hydraulic steering component within an inch of the compass.

[/ QUOTE ]

Is your lump of steel giving a hard steel or a soft iron effect? Or a combination? If it's giving a hard steel effect, then it can be compensated by magnet correctors. If it's got a soft iron effect (i.e. is re-magnetised by the earth's field as the boat turns) then it can't be compensated by magnet correctors; it will need something like the big iron balls you see on old ship's binnacles. If you plot a deviation curve and find that it follows a nice sine wave then it is correctable; if it is asymmetric or has steps in it (i.e. it looks like the tidal curve for Poole) then it isn't, and your best solution is to get another compass and mount it away from the lump of iron.

In any case, a lump of iron of that size is quite likely to give you significant heeling errors, and may change its magnetic effect if left for a long time in one direction of the earth's field (as when laid up).

 

[Anonymous]

Re: Hard steel or soft iron?

Peter, thank you for that. I really don't know, of course, which effect I am observing and I will do as you suggest. David