Checking the compass...

[WestWittering]

Whilst pottering the other day, I decided I would check the compass against the sat nab. Result = miles out

Whilst pottering yesterday, I thought I would put it in its mounting properly and check it again against my iPhone. Result = happiness

Good old iPhone - I must have been too close to the engine when I first checked it. Blonde moment!

Di

 

[johnalison]

The simplest way to check a compass is the compare its reading with one taken from a hand-bearing compass sighting along the hatch or anything parallel, from some way away, either well astern or from off the boat.

 

[emsworthy]

Don't forget your iPhone is not actually a compass, but a GPS masquerading as a compass and will therefore be reading True rather than Magnetic north.

 

[WestWittering]

Don't forget your iPhone is not actually a compass, but a GPS masquerading as a compass and will therefore be reading True rather than Magnetic north.

Actually, it has two settings - True or Magnetic

Di

 

[prv]

The simplest way to check a compass is the compare its reading with one taken from a hand-bearing compass sighting along the hatch or anything parallel, from some way away, either well astern or from off the boat.

I've never really understood why the hand compass is considered to be infallibly correct for checking the main compass. Surely it's just as susceptible to deviation.

Seems to me that a better approach is to line up on a charted transit or a known bearing (established from your GPS position) from a landmark.

Pete

 

[prv]

Don't forget your iPhone is not actually a compass, but a GPS masquerading as a compass

Nope, current models have a solid-state magnetic compass in them.

Wouldn't like to comment on its accuracy, but it's nothing to do with GPS.

Pete

 

[emsworthy]

Nope, current models have a solid-state magnetic compass in them.

Wouldn't like to comment on its accuracy, but it's nothing to do with GPS.

Pete

Well there you go! Every day's a schoolday!! Does it use the GPS to calculate the respective variation for the location?

 

[prv]

Does it use the GPS to calculate the respective variation for the location?

No idea. I assume it could, but I don't know if it does.

Pete

 

[VicS]

I've never really understood why the hand compass is considered to be infallibly correct for checking the main compass. Surely it's just as susceptible to deviation.

Seems to me that a better approach is to line up on a charted transit or a known bearing (established from your GPS position) from a landmark.

Pete

I think you will find that on a boat that does not have a steel hull it is considered that you can get far enough away from the magnetic influences of engines, ovens etc for the hand bearing compass to give an adequately deviation free reading.

 

[awol]

I spent about 3 very pleasant hours yesterday generating a deviation chart for my compass. Sun was shining, flat calm, warmest day of the year so far and I was circling around at the mouth of Loch Striven. Worst error was just under 5 degrees on a Plastimo (RIP) Contest compass - I reckon I would be lucky to steer a course better than that in anything but a flat calm.

 

[awol]

.... and something else I found was that one out of four lifejacket CO2 cylinders was magnetic. Add that to my magnetic spectacles and the hand bearing compass sights are looking a bit suspect.

 

[johnalison]

I spent about 3 very pleasant hours yesterday generating a deviation chart for my compass. Sun was shining, flat calm, warmest day of the year so far and I was circling around at the mouth of Loch Striven. Worst error was just under 5 degrees on a Plastimo (RIP) Contest compass - I reckon I would be lucky to steer a course better than that in anything but a flat calm.

Have you worked out what the error will be after 60 miles? We may like to depend on GPS but I would not be happy with 5 degrees. Although compass correction is quite an art, I have in the past done quite a good job, following instructions in a book on the subject. It might be better to move the beer cans.

 

[prv]

Add that to my magnetic spectacles and the hand bearing compass sights are looking a bit suspect.

I got new glasses the other week, and the bearing compass was moving a bit weirdly on the way back from Poole. Need to take them off and waft them near the compass to check if they really are affecting it.

Pete

 

[jbweston]

Have you worked out what the error will be after 60 miles? We may like to depend on GPS but I would not be happy with 5 degrees. Although compass correction is quite an art, I have in the past done quite a good job, following instructions in a book on the subject. It might be better to move the beer cans.

Unless I've forgotten my flying mental arithmetic, each degree off is one mile off in 60. Therefore 5 degrees = 5 miles off in 60. But do tell me if I'm wrong as I haven't the energy to look it up this evening!

 

[Seajet]

I spent about 3 very pleasant hours yesterday generating a deviation chart for my compass. Sun was shining, flat calm, warmest day of the year so far and I was circling around at the mouth of Loch Striven. Worst error was just under 5 degrees on a Plastimo (RIP) Contest compass - I reckon I would be lucky to steer a course better than that in anything but a flat calm.

Hear hear !

To calibrate my compass - also a Plastimo Contest - I towed a dinghy a good length behind with self or scum crew sighting a hand bearing compass ( which he later stole along with other things ) to the line of mast and forestay then did steady runs at all the angles, producing a variation ( deviation ? ) chart.

In fact the errors were so small they could virtually be ignored, but we keep them in mind.

On any calm journey when motoring with autohelm, I have a good look at the various compass readings, it can be quite educational.

The worst I've known is when my Dad - great engineer but never a great navigator - fitted mountings for a 1970's big stereo radio cassette job in the cabin just below the compass; I'm lucky I'm not typing this from Canada or the South Pole !

 

[grumpy_o_g]

I got new glasses the other week, and the bearing compass was moving a bit weirdly on the way back from Poole. Need to take them off and waft them near the compass to check if they really are affecting it.

Pete

Stop drinking from the new glasses and you might find the compass stops moving around too

 

[grumpy_o_g]

I spent about 3 very pleasant hours yesterday generating a deviation chart for my compass. Sun was shining, flat calm, warmest day of the year so far and I was circling around at the mouth of Loch Striven. Worst error was just under 5 degrees on a Plastimo (RIP) Contest compass - I reckon I would be lucky to steer a course better than that in anything but a flat calm.

Swinging the E2B emergency compass on fast jets we would expect to get deviation below 5 degrees at the very most and often 2 or 3 degrees. That was basically an upmarket car compass and the it was in the cockpit surounded by electronics and with all sorts going on around it - the airframe was non-ferrous in fairness though. On the boat swings I've done I've usually managed to get deviation pretty low on bulkhead compasses unless there's something wierd (such as a bizarrely magnetic old echo-sounder the other side of the bulkhead). If it was a full swing with adjustments I'd have expected a little better than that from a compass like a Contest in a fairly benign surrounding. Was the deviation card a reasonable sinewave with maximum errors roughly the same but on opposite headings?

 

[Seajet]

Swinging the E2B emergency compass on fast jets we would expect to get deviation below 5 degrees at the very most and often 2 or 3 degrees. That was basically an upmarket car compass and the it was in the cockpit surounded by electronics and with all sorts going on around it - the airframe was non-ferrous in fairness though. On the boat swings I've done I've usually managed to get deviation pretty low on bulkhead compasses unless there's something wierd (such as a bizarrely magnetic old echo-sounder the other side of the bulkhead). If it was a full swing with adjustments I'd have expected a little better than that from a compass like a Contest in a fairly benign surrounding. Was the deviation card a reasonable sinewave with maximum errors roughly the same but on opposite headings?

The first compass on my boat was an aircraft E-2 donated by Hawkers.

Every UK Harrier ( and original Hawk ) built had its' E-2 compass swung before flight by being towed around a circular pad at Dunsfold, if watching ' Top Gear ' nowadays it's off to the left of the finish line.

The pilot had a little deviation card next to the Head Up Display in case all the electronics went blank, though I have serious doubts he'd have time to study it and think " Oh port 3 degrees then !

 

[VicS]

To calibrate my compass I towed a dinghy a good length behind with scum crew sighting a hand bearing compass ( which he later stole along with other things ) to the line of mast and forestay then did steady runs at all the angles, producing a variation ( deviation ? ) chart.

I'm lucky I'm not typing this from Canada or the South Pole !

Who could blame him after that sort of treatment.


Lucky in deed not to be somewhere you did not intend to be if you are unsure of the difference between variation and deviation.

 

[Seajet]

Who could blame him after that sort of treatment.


Lucky in deed not to be somewhere you did not intend to be if you are unsure of the difference between variation and deviation.

VicS,

you are beginning to bore me with half-arsed comments;

who anyway would own a SeaWych whose main feature is vertical keels for easy moulding ? Not a sailor that's for sure...

I happen to know the difference between variation and deviation, am a YM offshore and have sailed cross- Channel as skipper since I was 16 - how about you ?