What is this Compass thing?

[Danf]

The other day whilst driving a mates boat, I noticed the compass was not reading correctly for that particular course I was steering. It was in error (deviation)by at least 15 degrees compared to the magnetic heading on the GPS. I did not bother to check the readings across the rose, but I said that we should swing this compass, by adjusting the N-S and E-W compensators. We swung it through its cardinal points using GPS and the I Phone compass app, I did use a hand bearing compass back in the 80’s, but that is long gone.

Now this is the thing! Here in Queensland, a compass is only recommended for recreational boating offshore, while in Victoria it is mandatory requirement, at least that is what I Googled. Apparently, recreational boating rules are state regulated, while commercial are National.
Any thoughts?

[Volvo]

If the compass was wandering offcourse with excess error it was getting interference from somewhere..
Try holding your mobile phone or even a screwdriver next to your compass and see how it goes hayware to extent..
Maybe some live wiring behind or adjascent to it etc ??..

[Mopheus]

IMO in a small boat you want to set your compass up with all electronics off and just the motor running. That represents reproducible conditions for an oh-shit moment at sea when you lose your GPS/plotter.

Course compensating a compass while your electronics are working largely defeats the purpose of having it.

[Volvo]

And.......................there are Compasses and there are Compasses if you are going to fit one incase you will need it for navigation purposes in case of emergancy then do your homework and go the bigger the better for vissabillity , Steadiness and save going crosseyed trying to keep and eye on it. If you place one in the boat just for legalities then " Who cares ".
Yonks back prior to GPS's we used to navigate back n forth out wide by Compass only and used to own a Sunto or something where you turnd the dial to the heading you wanted and kept that course in between the red parralel Lines,
Had the Compass fitted at chest height when sitted making for easy viewing. and it was a steady Compass which wouldnt wander with every wave your Boat went over ..Do your homework and read reviews in this arena as they are a Very Important Piece of equipment..

[Danf]

If the compass was wandering offcourse with excess error it was getting interference from somewhere..
Try holding your mobile phone or even a screwdriver next to your compass and see how it goes hayware to extent..
Maybe some live wiring behind or adjascent to it etc ??..

A compass in a boat will suffer from deviation, the biggest error will be when the boat's magnetic field is perpendicular to the earths magnetic field. There should be 2 points where the error is zero.

Also, I believe a compass should be error adjusted with all electronics running!

[Mopheus]

Also, I believe a compass should be error adjusted with all electronics running!

If you regularly use the compass for navigation, sure.

Most small boats don't though - the compass is just there for emergency usage. Given your emergency probably involves electronics NOT running, it makes sense to adjust them for that condition. You can always switch off the house battery or turn electronics off to reproduce those conditions and have confidence that your compass is reading true.

[toecutter]

With even the most expensive compass, you still need to allow for magnetic variation and the deviation errors of the compass itself. Toss in trying to get an accurate reading while bouncing along and the best you can really expect is for the compass to get you somewhere in the proximity of the direction in which you want to go.

On a small planing boat with an ECS installed, the best use for a compass is as an aid to steering a straight(ish) course or for getting you at least heading in the general direction of safety if the SHTF. Neither purpose requires a super accuracy, imo.

If you're into traditional navigation though, you'll probably do better with a hand held compass as this also allows plotting "top hats" (triangulated positions) on the chart which is usefull for verifying your position when land marks or structures are in sight.

Sent from my SM-T575 using Tapatalk

[Danf]

If you regularly use the compass for navigation, sure.

Most small boats don't though - the compass is just there for emergency usage. Given your emergency probably involves electronics NOT running, it makes sense to adjust them for that condition. You can always switch off the house battery or turn electronics off to reproduce those conditions and have confidence that your compass is reading true.

In the days before GPS, a compass along with a speedometer in knots (pitot) and a watch (time) was all we had to get to a reef some 20 to 30 NM in distance. When the time was getting close, we kept an eye on the sounder until the reef appeared. That is why we swung the compass with the electronics running. I suppose now, one should make a deviation chart with electronics on and one for off, to see if there is any noticeable difference.

[Danf]

With even the most expensive compass, you still need to allow for magnetic variation and the deviation errors of the compass itself. Toss in trying to get an accurate reading while bouncing along and the best you can really expect is for the compass to get you somewhere in the proximity of the direction in which you want to go.

On a small planing boat with an ECS installed, the best use for a compass is as an aid to steering a straight(ish) course or for getting you at least heading in the general direction of safety if the SHTF. Neither purpose requires a super accuracy, imo.

If you're into traditional navigation though, you'll probably do better with a hand held compass as this also allows plotting "top hats" (triangulated positions) on the chart which is usefull for verifying your position when land marks or structures are in sight.

Sent from my SM-T575 using Tapatalk

We carried a hand bearing compass along with a chart of the area. Triangulating position (cocked hat) was sometimes performed to verify position within a certain margin.

[toecutter]

We carried a hand bearing compass along with a chart of the area. Triangulating position (cocked hat) was sometimes performed to verify position within a certain margin.

Ahh, the perils of posting late at night. Hand bearing compass and cocked hat is what I should have written myself.

Thinking about compass accuracy in the modern era, a compass by itself is more or less a last line of defense to get you back in a known direction. Even with a calibrated compass and deviation card, you will still experience drift and set, i.e. the boat may not actually be pointed in the direction it's heading and this can introduce a significant margin of error. In the old days this is a reason why it was important to do regular "cocked hat" bearings when coastal. We usually did these every thirty minutes and used them to calculate course deviation and to make corrections. Of course tides are always interesting, because they will invariably turn your straight line course into a "S" shape if you travel through a change of tide.

I guess the summary is, that a compass by itself is pretty much useless as a navigation device if used alone, but it is a good emergency device to at least get you pointed back somewhere to a place of refuge if need be.Having said that, it's more the norm now that even a small boat will have two or more sources of electronic navigation available if one includes smart phone capability into the mix.

Coincidentally, I recently removed a compass installed by a PO from my tinny after I changed the old chartplotter out for a new one. First trip out, it was a bit rough and it was getting dark and I had to navigate around some sand bars and reefs to get back into the river and went to switch the new chartplotter to "highway" display like my old unit so I could follow my breadcrumbs back in. It was then I discovered the new device has no highway mode. At that point I kind of wished I not removed the compass, as it would have mad following the track a lot easier!

[BigE]

And you didn't actually turn the sounder on until you were nearly at the reef ...... wouldn't want to waste all that paper on the way out.

BigE

[Danf]

And you didn't actually turn the sounder on until you were nearly at the reef ...... wouldn't want to waste all that paper on the way out.

BigE

That's right. Still got the paper rolls with the soundings.Paper Sounder.jpg

[Dignity]

That's right. Still got the paper rolls with the soundings.Paper Sounder.jpg

And you turned them upside down to save $$$'s

[bobfromblacky]

Bloody heck that takes me back

[shortthenlong]

And you turned them upside down to save $$$'s

The story of my youth. Trying to learn to read the sounder when my dad had turned out upside-down to re-use