Two compasses, why?

[Gitane]

This maybe a stupid question, but here it goes.

I have noticed that some sailboats have two compasses, one on the port side and one on the starboard side.

Why is this?

Does this have to do with visibility, or avoiding misreading a course? Or, is this a backup compass in case one fails? Not sure how a compass fails though.

 

[stevebrassett]

Usually because you sit to windward, and so you can get a straight view of the compass.

 

[Praxinoscope]

One boat I used to sail on had two compasses, it did overcome the parallax that you get when helming and the compass is on the otherside of the cockpit, but I don’t think I would go to the expense of fitting a second compass.

 

[fisherman]

I knew a fisherman who was terminally paranoid, and took a second compass to sea as he didn't trust the first....this was in the days of watch and compass, nothing else, no GPS.

 

[lw395]

Only two?
We had one in each side deck, one on the steering binnacle and an electronic one going to the displays.
On a fully crewed boat, there is usually someone in the way!
It's more about spotting windshifts on the beat than steering a course, generally.

 

[zoidberg]

Or, is this a backup compass in case one fails? Not sure how a compass fails though.

"No such t'ing as stupid questions - only stupid answers"

Compasses, even balanced card jobs on sailboats, can and do fail. Most cards are mounted on a pin-bearing. That can break. The card can stick. There are other modes of failure.

Aircraft have more than one compass. Usually, a little one as a 'get you home' instrument....

 

[CCJ]

I knew a fisherman who was terminally paranoid, and took a second compass to sea as he didn't trust the first....this was in the days of watch and compass, nothing else, no GPS.

A man with one watch, knows the time, a man with 2 watches doesnt

 

[Gitane]

A man with one watch, knows the time, a man with 2 watches doesnt

On commercial aircraft, you need to triplicate key instruments. This is because if you only have two of a key instrument and one shows a different reading from another, then which one is correct and which one is incorrect?

 

[dslittle]

On commercial aircraft, you need to triplicate key instruments. This is because if you only have two of a key instrument and one shows a different reading from another, then which one is correct and which one is incorrect?

The question made me think what I have on board.

Two compasses at the helms, a fluxgate compass for the auto helm, a handheld, a compass in the binoculars. one on the handheld GPS. one in each of the three iPads and two iPhones.

I don’t use them all at once but so far we have managed not to get lost...

 

[Neeves]

In addition to the 2 'magnetic compass we have 2 x fluxgate (we have 2 independent nav systems (one old and one newer) and another magnetic in the berth. The magnetic in the berth is to check for wind shifts, fronts coming through, without the need to get out of bed, (it has unobtrusive lighting). We have one hand bearing compass.

But the answer to the question, as said, to give the helm a clear view of the compass.

Today sailing to bearing would be conducted using an electronic display (using the autopilot) and I'm pretty sure most yachts won't have correction cards. I'm sure some will have correction cards and some will steer by hand - but the majority???

As Zoidberg says - no questions are ever stupid not naive.

Jonathan

 

[zoidberg]

On commercial aircraft, you need to triplicate key instruments.

The majority of commercial a/c now operate with two engines.... and lots of military a/c, two. Are you suggesting those should fit a third set of engine instruments?

It's fundamental principle of professional navigation that one should be regularly checking one source of important information - such as direction, or altitude - against another. It's not a question of 'which one should I believe', but to alert the pro navigator to the existence of a discrepancy so that s/he can investigate and resolve using knowledge and skills.

It's also a fundamental principle not to hang one's hat/career/prospects of making it home on a blind belief that 'something is usually reliable so it's probably all right'. The pro nav is there to spot - systematically - the once-in-a-blue-moon occasion when the probability turns out to be seriously misleading. "Past performance is no guarantee of future results"......

 

[escapism]

Or, is this a backup compass in case one fails? Not sure how a compass fails though

Left Calais, encountered thick fog, turned to return, compass stuck pointing NW. Thumped it, but no response. In our case the only backup was an elderly cockpit mounted GPS128, but enough to find our way back while avoiding the ferry routes. No idea why it suddenly decided to stick, but it was good to have a backup. Not sure that I'd want 2 cockpit mounted magnetic compasses, but they can and do fail. Just remembered, at a pinch, the handbearing compass could have been Plan C if needed.

 

[Daydream believer]

In the 70's we were in thick fog & had to get from Margate to Burnham. The bolts holding the Stuart Turner P4MC down on to the bed ( no rubber mounts) came loose such that the vibration made the fixed compass go round & round. Fortunately we had a Sestral hand bearing compass otherwise it would have been somewhat difficult. On my current boat I have 3 magnetic compasses ( all pretty much in alignment) plus one electric (unreliable) plus chart plotter & backup GPS. So effectively 6 sets of compass

 

[Praxinoscope]

I have just thought about this and worked out that I have the following on board
1x Cockpit mounted bulkhead compass
1x Small compass fitted beside chart table
1x Hand bearing Compass
1x Pair 7x50 binoculars with compass
Then the fixed gps and a hand held gps in the nav’ locker, which give a course being steered, the autohelm which has a fluxgate compass in it and finally there’s the Sun, Moon and Stars if all else fails.
Strangely enough in 50 years of mucking about in boats I have never had a compass failure. (bet I get one having just said that).

 

[fisherman]

wife bought a led camping lantern, it has a compass in the top...and a steel carrying handle, the compass only works if I remove the handle.

 

[[163233]]

I guess on some bigger boats if there were one compass it would have to be on the bulkhead which is both a long way from the helm and also easily blocked by other crew.

There's an argument that you're just doubling the chances of having to replace a compass.

 

[colind3782]

My Furia has the bulkhead compass visible from inside the heads so, when you're sitting on the bog, you can keep an eye on it using the mirror on the door. Great if you're S/H.

 

[zoidberg]

I guess on some bigger boats if there were one compass it would have to be on the bulkhead which is both a long way from the helm and also easily blocked by other crew.

There's an argument that you're just doubling the chances of having to replace a compass.

The logical extension to that argument is to have NO compass. That's cheaper, and no call to replace..... ( smilie thingy )

As for 'blocked by crew', either leave 'em ashore or, alternately, ask them to move.... "and 2 sugars, please!"

 

[mjcoon]

But note that multiple compasses must be mounted far enough apart that their magnetic fields do not interfere.

There is a story about a skipper with novice crew who kept seeing random errors in his compass reading. He asked the crew if anyone had anything magnetic; no-one did. On effectively frisking them a pocket brass compass was discovered; "but it is brass, not magnetic!".

Mike.

 

[[163233]]

The logical extension to that argument is to have NO compass. That's cheaper, and no call to replace..... ( smilie thingy )

Hard to correct a handheld compass, but it'd be sufficient most of the time.

I'm pretty sure they're not worrying about redundancy when they install two. It'll be a mix of aesthetics and ergonomics.