Compass: digital or conventional ?

Nick Reid · 12 February 2002, 14:43

Any thoughts on compass choice. I've used a digital on my sailboat for a few years but don't know how they'd perform on a galloping rib. Any reports? Which compass?

Nick

John Kennett · 12 February 2002, 14:55

They work, and they have good write ups. The two that spring to mind are KVH and Ritchie.

I personally prefer a good old fashioned magnetic compass. I've got a Ritchie electronic compass on Phoenix -- Alan Priddy used it a lot when he owned the boat but I've never really got on with it.

Of course, even if you have an electronic compass you will still ned a magnetic as backup . . .

My choice is a nice big Plastimo offshore compass. Easy to read, and about as steady as you could reasonably expect.

John

Keith Hart · 12 February 2002, 16:46

One of my Admiralty charts, West Coast of Scotland near to Gruinard Island (remember the Anthrax), shows an area of 'local magnetic anomaly'. I understand that these rather odd anomolies are caused by local iron deposits in the rock (I could of course be wrong, I often am). Do these anomolies affect electronic compasses in the same way as magnetic ones?

Keith Hart

Alan Priddy · 12 February 2002, 22:26

Some do and some dont. On Spirit we use compasses that get their information from the DGPS signal and they are very accurate.
I have a huge one which I keep my base ball cap on just in case!
Over to you Allen C

Allen · 14 February 2002, 21:01

How can I resist -

Judging by the smile on the helmsman's face (and the look of concentration on yours), I guess you're letting him have a turn of binnacle ..

wavelength · 14 February 2002, 22:44

after the piccie I hardly dare ask - but whereabouts have people found the best place to stick the sensor(for the fluxgate compass thingy of course!)

GraemeCooper · 15 February 2002, 14:06

Hi Nick

Thought I would add my recent experiences , hope it helps.

On my 6.2 metre RIB I have a GPS system and a conventional compass. I tend to use the GPS for Navigation, and the compass for general headings. When in any rough sea the conventional does jump around a bit, and isnt really practical for exacting navigation - wouldnt want to rely on it to get me to within 10 degrees or so over any distance However, forgeneral bearings - o in power failure situations - I wouldnt be without it.

On my small 4 metre RIB I only have a digital cmpass, which nicely gives out readings, but doesnt tell you general drections.

I guess given the choice of one or the other depend what you want to do - how far and in what sort of conditions.

Guess thats not really a lot of help but perhaps something to think about.

Nick Reid · 20 February 2002, 01:47

Thanks to all for good info. I'm inclined at this point to put in a flush mount Ritchie conventional compass. So much of my navigation in the past few years has been chasing down GPS waypoints that the compass in some ways has become secondary, that is until there is a power or other kind of failure when the good old hand bearing compass, steering compass, chart, and deviders prove their value once again.

batfalcon · 20 February 2002, 17:08

Right decision. You can always rely on a GOOD STEADY compass.

Jono Garton · 12 March 2002, 06:36

Again i plan to use GPS for navigation however in a chop its hard to see the screen so clearly, so i'm looking at getting a flush mount compass, but..... I have a chrome bar running over my console (see pic). Will this affect the compass? Also GPS & VHF are 12" away....

Anyones thoughts / advice...

GraemeCooper · 12 March 2002, 07:07

Jono

You will proabably be left with the same problem I have - which is apparently easily solved - and that is that the compass needs to be adjusted once in situ. This takes account of any deviations caused by surrounding equipment.

Mine is currently some 20 degrees out - will hopefully be able to sort this when we get to Cardiff and have a nice flat area inside the breakwater where we can do this.

Jono Garton · 12 March 2002, 07:11

Cheers Graeme

Is it a case of just sticking some metal bits around the compass?

GraemeCooper · 12 March 2002, 07:36

Jono

From what I can see there are 2 metal bars under the compass. These are both connected to a screw head fitting. The instructions tell you to face N or S, then adjust the compass one way, then face E or W and do the same until you match either your GPS reading, or another hand held compass kept wll away from the instruments.

Apparently this has to be done on all compasses when fitted to boats - and apparently should only take a few minutes

Splishsplash · 12 March 2002, 09:37

As the compass will always deviate from the actual magnetic bearings (compensated or not) the only proper way to go about it is to make a deviation chart to go with the (fixed) compass.
The chart should give you the deviation for every 22,5 degrees.

If used for general headings only, this is not so important, then just compensating will do, but mind you that the deviation after compensation can, for some bearings, still be up to 10 degrees!

Succes!

Eddy

John Kennett · 12 March 2002, 11:23

That bar is likely to be stainless steel, not chrome. As decent stainless steel tends to be non-magnetic you may find that it's not actually a problem.

Before you start fiddling with anything in the compass, do check to make sure it's necessary.

Another point to bear in mind, is if you are only using the compass to keep on a bearing provided by the GPS it doesn't actually matter if it points in the correct direction.

Consider this:

The GPS course reads 020, you get on course via the GPS and want to use the compass to stay on course, but it reads 045. What to do? Just keep it on 045 and you won't go far wrong!

John

GraemeCooper · 12 March 2002, 14:12

.... until the power fails, the GPS goes off and you have to guess the deviation from then on in ......

jools · 12 March 2002, 15:08

On the PoB we have the ship's compass adjusted every year or so.

This involves getting a "Compass Adjuster" on the ship and then taking the ship out into the Bristol Channel (to avoid interferance from other ships / building in the harbour)

Once in the channel we advise Avonmouth Radio that we are having our compass adjusted, then raise 2 flags (can't remember which) to notify other ships that our compass is being adjusted.

The compass adjuster then uses a Sun Compass on the deck - it has to be a sunny day - to find our true heading. He then places small magnets of various strengths arround our compass in special "pods".

This is repeated at different headings untill he gets the compass as near as possible.

He then draws up a compass deviation chart showing any differences.

eg

N - 0 degrees
E - +1 degree
S - -1 degree
W - +2 degrees

etc. - you get the idea
The Maximum deviation on our compass was 2 degrees - and only for a very limited heading.

The deviation differs depending on the heading - so you can't say its always +5 degrees for example.

The compass has to be done regularly (especialy in a steel ship) because all sorts of things arround the compass change there magnetic strength over time.

Things magnitise as they are moved through the earths magnetic force - a bit like stroking a nail with a magnet will turn the nail into a magnet. - Have you ever noticed that cutlery on planes (before they were replaced with plastic) were quite powerful magnets!!