Your compass and New electronics

[Dennis W.]

removehtml]An observation and follow-up question. I would guess that many of you will be doing upgrades for the coming season. It is a given that all changes and additions to your equipment and particularly your electronics affects your compass and it's deviation. Has the compass on your boat ever been "swung"? Do you have a deviation table for your boat? Do you rely only on electronic navigation? If so what would you do in the case of total failure of your electronics, as in the case in So. Cal. boat "Blew by You". Does anyone still swing their own compass? What method do you use? Can you explain to others a proper or acceptable method to accomplish this very important safety measure?Dennis W.S/V FullSailedError: Error: expected [/URL], but found [/removehtml] instead[/removehtml]

 

[Patrick S/V Shangra-La]

swinging the compass

is usually done by a professional. It is done to place magnets around the compass to off set metal on the boat. Electronics placed on our boats effect the compass not only by placement but by the magnetic field created while the instrument is operating. The most economical procedure you can do is set your self in a position where you know the true heading from point A to point B both nort and south and east and west. determin what your diviation is with the equipment installed. Then turn the equipment on and observe the deviation change. On my boat I have a 5 degree diviation with equipment installed, and a 8 degree with equipment on. As you have asked with a total failure knowing my deviation of 5 degree I can D R navigate as needed. the instruments will be there but no magnetic distortion from current in the wires.If I want to conserve battery power, in deep water I can shut off the GPS, using a 8 degree deviation I can D R and check my position every 12 hours with the GPS. Its a great way to keep your navagation skills sharp

 

[Maine Sail]

I have..

I have mine professionally "swung" every two years. The price is about $90.00 and it takes about 1 hour to complete in calm water. One can try to swing a compass and spend many, many hours but with out the right equipment read ($$$$$$) you'll be running in circles and will wind up being way off on many points of sail. Swinging the compass is NOT pulling along side a dock you "know" the orientation of and adjusting you compass to it!To do it properly it is done and checked at all major points on the compass and then those four, N,S,E & W are each spit in half and it's checked again.. The $90.00 is well worth it to me!!For those of you located in Maine you'll want to use Maine Compass Service. Charlie Cook is the owner/operator and he's also a marine surveyor'Maine Compass Service:(207) 865-6645

 

[tcbro]

Patrick

Your deviation will vary depending on your direction. I doubt it's 5* in every direction. The equipment will effect the compass differently when it's oriented to the north of the compass as opposed to south of the compass. The same for east and west. Think about it like this; if you have a huge chunk of metal (or an instrument creating a magnetic field) directly forward of your compass it will draw the "north" point of the compass to it. When going north, no deviation, no problem. Now, turn to the east. This puts that chunk of metal to the east of the compass and draws the north point of the compass east X number of degrees. Now head west and the north point of the compass is drawn to the west X number of degrees. The amount of your turn will determine how much that chunk of metal effects the compass thus a different deviation value for each compass point. Also, as you turn, different things around your boat will have varying effects on the compass.I was taught that you make up a "deviation card" showing the amount of deviation (+ or -) for each of the 16 named points on a compass.Have I done that?? No. The distances that I would have to sail without electronics and in zero visibility would be relatively short here in the Chesapeake and a few degrees of deviation would get me close enough to recognize something. (Put that on my tombstone)Toms/v Orion's Child

 

[Ross]

I check my compass against my gps heading on every trip and find

that because of the currents in the bay often times I steer as much as 20 degrees off course magnetic in order to maintain the course I need. Knowing the course from Red 2 to red 4 from the chart and then steering to compensate for set because the current is at 90 degrees to the course makes the compass accuracy relative.

 

[Mike B]

Know issues in the upper bay

Ross some of your offset may be from know issues in the upper bay. Per the chart, "Differences as much as 5 degrees from normal variation have been observed in the channel from Pooles Island to Howell Pt and 3-8 degrees in Elk River Channel from Grove Pt to Courthouse Pt". No matter the cause, as Tom said, you're not going to travel very far in the northern part of the bay that a couple of degrees will cause you to miss your mark.Mike

 

[David10790]

Ross Makes a Good Point

When steering by the compass (ie without GPS or other aids) the accuracy of the compass is only one of a number of things which will determine where you end up. Set and drift of the local currents, leeway, steering error and compass deviation all will play a part. On a 10 nm run, an error of 1 degree in any of those four factors will put you 345 yards off your destination at the end. It is pretty easy to make a total error of 6 degrees which will put you one nm from where you want to be. This being the case, it makes sense to control what you can, which means making your compass as accurate as possible.

 

[Malcolm Young]

Deviation compensation and a link on how

tcbro is correct about deviation - it varies with your heading as it depends on how your boat distorts the earth's magnetic field. His explanation is good. I compensate my compass myself. Ritchie has a good outline for two methods on how to compensate a compass - see link http://www.ritchienavigation.com/service/compensation.htmlEdited to add: A further point along the lines that the previous post was indicating - compensating your compass directly using the reading from your GPS will not work well - the GPS reading is based on what your course over ground was a moment ago. It is not based on your current heading. This brings two potentially significant errors into compensating your compass. One error is moment to moment steering errors which are not accurately indicated by a GPS which simply calculates the direction between two positions. The second error is the effect of crabbing due to wind and current ie. your vessel is not going exactly in the direction it is heading. Never try to compensate your compass if there is wind or current.

 

[Warren Milberg]

I used to think

my aging Danforth binacle compass could not be properly swung. I followed all the directions for swinging it, but yet often found 5-8 deg deviations from known courses. I accepted this for a while. Then, one day, I went sailing and did not place my air horn in the pedestal-mounted foam holder where I usually put it. Bingo. The compass was correct! I now put the air horn, with its metal canister, in a cup holder as far from the compass as I can get it....

 

[Mike B]

Air horm and other things

Warren, I have cup holders built into the binnacle on either side of the compass. Of course it's where I keep my air horn and remote VHF mic. Like you I can watch it swing towards the air canister. In the upper bay I don't worry about the accuracy of the compass. I only use it to take a reference when on a tack. For those times when I want more accuracy I make sure I remove anything from around the compass, take a reading, then replace the items remembering what the deviation was. I also do a quick comaparison against my GPS heading. If I were to do any blue water sailing I'd be sure to keep the compass area clear but for the bay line of sight will do.Mike

 

[Tom Harris]

I have never swung the compass. It always seemed close enough to get me home.

Owners should be aware that some of the compasses had bad seals. The fluid leaks out and unless you look closely,it was was not noticible. If you have this problem, the manufacturer will correct the problem at no cost.

 

[Capt. Kermie]

Only the Fluxgate

When I got my Autohelm, it came with a fluxgate compass which I mounted in the most perfect spot with one caveat, I mounted it facing aft so that it would be out of the way from damage which put it out by 180 degrees. I had to take the boat out and go through the swinging process a couple of times while correcting for 180 degrees, I used my plastimo hand held and GPS for reference not to mention I easily knew by the landmarks which way was North. I have a trailerable and while in my east west facing driveway I can check all compasses any time for accuracy as my driveway is only off by about 5 degrees as is the street out front, it is only off by about 5 degrees from true north and the variation in these parts is about 19-20 degrees east. I use my compass while out daysailing to get an accurate idea as to how close hauled I can sail.