Say what?
. I don't have one. Not going to happen in Rhode Island Sound/Buzzards Bay. Certainly not in the Westport River, or outside in front of Horseneck Beach.
Compass, AP, GPS conflict
- Thread starter KennyBene
- Start date
. I don't have one. 
In days of old when many coastal seaports had degaussing buoys, they were used by the compass adjusters to swing a compass. I should think range marks, if there are any in your area, would do nicely if you wanted to check your compass on at least two courses. Obviously, multiple range marks would be better.
Even down here there are functioning range marks into the ports, so there should be some in most areas in the States and Canada with commercial ports. I remember them all the way up the St Lawrence Seaway, for instance.
Even down here there are functioning range marks into the ports, so there should be some in most areas in the States and Canada with commercial ports. I remember them all the way up the St Lawrence Seaway, for instance.
There are none in my area that I know of. Maybe I am not looking well enough. But even so, how would you account for the current, if it wasn't aligned with the range?
Math. Simple Vector addition can be performed on a chart by connecting two arrows that represent your heading and the wind/current tip to end to get your COG. If you are traveling at an average speed velocity of 6 knots and the current is an average of 2 knots then draw a line for your COG from your location to your destination, then draw an arrow parallel to the current moving away from that COG line, in the direction of the current, with the tail of the arrow touching the COG line. This represents your current and needs to be two units long, to represent the velocity of the current (2 knots). Next, take a compass and set it at 6 units long (This represents your vessel's through the water velocity, 6 knots) and place the pivot pin at the tip of your current vector, then swing the compass until it touches the COG line in the direction of your intended travel. (there will be two points that cross the COG, you want the one that advances you towards your destination). Draw that line from tip of current vector to the intersection with COG line you just found. Now, measure the angle of that line. This is your heading to point while traveling from your start to your destination. The COG isn't what you are looking for on the compass, your heading is. You will make your destination without ever pointing straight at it.
Fun stuff.
However, I don't see why you need to actually be moving while swinging your compass. If you can know your mooring position and have accurate information to directions of landmarks, just swinging around your mooring can give you enough information to create your correction tables.
The problem I see is the moving magnetic pole. Be sure to get up to date charts and GPS maps that know where magnetic North actually is in relationship to you. Otherwise, it doesn't matter if your compass is accurate or not when your course data is in error.
-Will (Dragonfly)
@jviss with out a formal rangemark you can improvise. Find two navigational buoys. Identify their locations on the chart. Identify the magnetic course between them. Ideally they are a nautical mile a part. If not no worries. Identify the bearing and then go and sail the bearing and the reciprocal. Did th encompass match the bearing you expected? Adjust the compass to the map bearing. Try again. Until it is consistent. Bear in mind there is a magnetic deviations printed on your chart. You can use it to identify the divination of the compass.
As was sad by @dlochner the precision of the compasses on most boats is +or- 5 degrees. A problem if your crossing the Atlantic. Might hit France rather than England. But in local waters you should be able to get close enough to home to recognize something even in a New England fog. Or there is always Captain Ron’s suggestion. “Just pull over at the next Island and ask directions”
As was sad by @dlochner the precision of the compasses on most boats is +or- 5 degrees. A problem if your crossing the Atlantic. Might hit France rather than England. But in local waters you should be able to get close enough to home to recognize something even in a New England fog. Or there is always Captain Ron’s suggestion. “Just pull over at the next Island and ask directions”
If there are no formal range marks near by, use a fixed point on land and a buoy or best 2 fixed points on land. Most charts show objects such as radio antennas, church steeples, towers, etc on the chart. Use the chart to determine the range bearing in degrees magnetic. On the compass rose there will be both a magnetic rose and true rose with a note about the declination and the annual change. Calculate the annual change and apply it.
Given the lack of precision in our steering compasses, using the GPS and COG bearing is probably adequate. Even with current, heading directly downstream or upstream shouldn't induce leeway and again be reasonably accurate. Even in tidal waters, there is always a short period of slack water during which the deviation can be determined.
Navigation is not about just set the autopilot and forget it. It requires constant monitoring and adjusting the course. If steering by compass bearing, the bearing will change based on conditions. Even, perhaps especially, on very long distances, like crossing an ocean, the bearing will change because the shortest distances, the great circle routes, are not along a line of latitude. (http://www.gcmap.com/mapui?P=fco-jfk &MS=wls&DU=mi) Rome and NYC are on about the same line of latitude, but to fly to Rome, first head north toward Newfoundland.
To @capta point's point it is important to know if there are large deviations between the actual bearing and the compass bearing, the question I raise is how much difference is a difference to worry about?
Given the lack of precision in our steering compasses, using the GPS and COG bearing is probably adequate. Even with current, heading directly downstream or upstream shouldn't induce leeway and again be reasonably accurate. Even in tidal waters, there is always a short period of slack water during which the deviation can be determined.
Navigation is not about just set the autopilot and forget it. It requires constant monitoring and adjusting the course. If steering by compass bearing, the bearing will change based on conditions. Even, perhaps especially, on very long distances, like crossing an ocean, the bearing will change because the shortest distances, the great circle routes, are not along a line of latitude. (http://www.gcmap.com/mapui?P=fco-jfk &MS=wls&DU=mi) Rome and NYC are on about the same line of latitude, but to fly to Rome, first head north toward Newfoundland.
To @capta point's point it is important to know if there are large deviations between the actual bearing and the compass bearing, the question I raise is how much difference is a difference to worry about?
When I first set up my AP the motor was close to the compass. The difference was as much as 180 degrees give or take. North always pointed towards the AP Motor. Then I solved it by moving the motor and the difference is now 16 degrees and North is North again. "VAR 16.00 degrees E (2017) Annual Decrease 9' " is the magnetic deviation as printed on our local chart. Then there are some special areas of the Sound that warn greater deviation may occur.
I think some of the need is in knowing your equipment and being able to find your port or any port in as storm. I follow the suggestion by the CG on the NOAA chart. to to rely on any one navigational aide. If I have maintained a decent record of where I have sailed, then I have a good start point. I sail in the general direction of shore by compass and watch the depth sounder. As I approach shore I now have 2 points of reference. I can follow the depth contour till I find a known location and then duck into shelter till the situation changes for the better. Sure it is old school and no laser driven directional trip, but is is a great fall back to known successful skills.
Last edited:
I'd say that meets the criterion for a difference worth worrying about.
Normal compass location are at the helm, now if you have any metal that could effect the compass nearby no matter how well you swing your compass it will be wrong. To swing a compass on a sailboat you have to have motor on then off and the same with your electronics and doing it for the 4 main points 360,90/180/270 deg taking note the offset. Doing a compass swing on a Aircraft takes hours so on a boat +- 5 deg would be good remember a plane travels at 120 - 550 kt we travel a 6-8 5deg off in 1 hour is not the end of the world but at 120 kt is..
That would be variation, not deviation. Variation is the difference between true north and magnetic north in your location. Deviation is compass error.
Believe it or not, it can be hard to tell when it goes slack where we would have adequate room to swing the compass, and figure out some kind of range. There are several currents mixing, from Buzzards Bay, Vineyard Sound, Rhode Island Sound, and the Westport River. Eldridge's predictions are often wrong, too. The only sure way, I guess, would be to clean off my paddle wheel and compare GPS SOG with Boat Speed (speed through the water, via paddle wheel sensor).
In addition, one could sail reciprocal courses and note SOG.
So does the AP get you where you want to go? If so, it's working properly. Seems like they are all fairly close.
It occurs to me that if I sail on several compass headings - say, 24, for every 15º; and once I establish the boat on that compass heading, note the boat speed, SOG, and COG, that I can later deconvolve both the current and the compass deviation. (I'm not saying it will be easy.) (Predictive deconvolution is a mathematical technique commonly applied in the geophysical exploration field. Basically, it is that if you know something of the underlying system, you can factor out distortions, for example, going from sonic time delays and reflections to distances because you know the speed of sound in various constituents of the system.) It could be that I just have too many unknowns and not enough "equations" to solve this, but I'm going to start trying to figure it out. Of course, in the no-current scenario, the raw data would be your compass deviation. Repeat for motoring.
Now, compass correction - that's another story!
Now, compass correction - that's another story!
You all need is a chart with the Deviation table on it, a chronometer , sextant, , straight edge, and pencil.
Navy taught a simple way...
"Can Dead Men Vote Twice At Elections?"
or
C D M V T A E
Compass +/- Deviation = Magnetic +/- Variation = TRUE...clue: ADD "East"...OR... SUBTRACT "West"
__________
If you are in the Western Hemisphere, mostly, and just want to ADD and do a bit of algebra.
"True Virgins Make Dull Companions At Weddings"
T V M D C A W
TRUE +/- Variation = Magnetic +/- Deviation = Compass ...clue: ADD "West"....OR... SUBTRACT "East"
Buy and learn how to use..
https://www.amazon.com/United-Scien...=UTF8&qid=1532279131&sr=8-22&keywords=sextant
Fairly simple to use.
I had aboard, from the first owner, a set with a teak drawing board for easy chart plotting..
http://www.weems-plath.com/Products/Sextants-Scopes-1/Navigation-Kit.html
Still in canvas carrying case and used only 2 times.... By me.
Jim...
Navy taught a simple way...
"Can Dead Men Vote Twice At Elections?"
or
C D M V T A E
Compass +/- Deviation = Magnetic +/- Variation = TRUE...clue: ADD "East"...OR... SUBTRACT "West"
__________
If you are in the Western Hemisphere, mostly, and just want to ADD and do a bit of algebra.
"True Virgins Make Dull Companions At Weddings"
T V M D C A W
TRUE +/- Variation = Magnetic +/- Deviation = Compass ...clue: ADD "West"....OR... SUBTRACT "East"
Buy and learn how to use..
https://www.amazon.com/United-Scien...=UTF8&qid=1532279131&sr=8-22&keywords=sextant
Fairly simple to use.
I had aboard, from the first owner, a set with a teak drawing board for easy chart plotting..
http://www.weems-plath.com/Products/Sextants-Scopes-1/Navigation-Kit.html
Still in canvas carrying case and used only 2 times.... By me.
Jim...
To you coastal sailors, perhaps, you would think so. Use your GPS.
______
To those Captains, who lose sight of land for a few days, NOPE!
You better understand the Stars, without a sextant, and How to Dead Reckon plot a course!
I never joke about the Safety of my Crew and boat.
Jim...
PS: I know how, do you?
This thread started simple and drifted to Deviation tables.
Those tables are on every Navigational Chart and if you use my post #36, you can correct your True Position by those Deviation Tables.
Celestial Navigational is Required of a Naval Line Officer and is taught at the United States Naval Academy still.
You may wonder why? I don't.
Captain of my boat...
Jim...
Those tables are on every Navigational Chart and if you use my post #36, you can correct your True Position by those Deviation Tables.
Celestial Navigational is Required of a Naval Line Officer and is taught at the United States Naval Academy still.
You may wonder why? I don't.
Captain of my boat...
Jim...
The link you posted was for a $59 decorative, fake sextant.
Oh, really? I'm glad you think so.
I happen to know celestial navigation, and have several good sextants that cost new more than 10 times what that fake one does. And, I wouldn't say a sextant is "fairly simple to use." One must know what they are doing, have sight reduction tables and a current almanac, or have a smartphone or computer based celestial nav app. Plus, you have to be able to take a sight, requiring the horizon.
I don't consider a sextant to be necessary safety equipment; I doubt any reasonable person does. Certainly a $59 decoration is not. Unless you are concerned about the GPS system going down, it's cheaper and easier to just carry spare, battery powered GPS receivers.