compass

[FF]

Our method ( if off the AP) is to use a QUALITY hand held compass to obtain the course, and then use the "main compass" to steer with .

It is frequently easier to check the hand bearing unit than the main as it can be read on a mark much closer.

 

[Larry M]

markpierce wrote:
*...both my boat's regular magnetic compass as well as the binocular compasses read only between S and SW regardless of the boat's direction.* (The engine is behind and to the left of the compass.)* The boat's builder said not to bother compensating the compass before a year has passed.

__________________________________________________

Mark:* Did you*get your compass to read correctly?** What was the problem?

 

[RickB]

Welding and hammering on a steel hull during construction aligns the magnetic domains in the steel with the local lines of magnetic flux. If you wait long enough normal stresses and strains will realign them to the area you normally operate. That is why the builders said to wait a year. That and the fact that they would have had to pay the compass adjuster.

If you are in a hurry, do it like the V-1 buzz bomb crews did and line the boat up on a known heading and beat the hull with rubber hammers for a while.

There is no reason not to swing the compass now. It shouldn't be done at the dock and it really should be done by a compass adjuster. As the boat ages and gets knocked around a bit the compass error will increase again and it will need to be swung again if accuracy matters, as it should and certainly will when you lose power to all those nice displays in the fog while cruising the Bay. At the very least make a correction card try to keep it up to date.

 

[yachtbrokerguy]

Since we have a great compass discussion I thought I should mention auto pilot compasses. I go out on so many different boats each year and I often see that the auto pilot compass does not agree with the main compass, and that often does not agree with the GPS. The GPS is of course actually a history but it will be the most accurate. Often when on a sea trial for a survey the surveyor will report metal or magnetic items have been stowed near the auto pilot compass and often they will suggest to re-calibrate the auto pilot compass every few years. You can check your owners manual to see what settings to use while doing a simple circle.
It is not that often that I see a compass deviation card on board now except for older boats and long distance trawlers.

 

[skipperdude]

If you have an Auto pilot it is a great way to save on the fuel bill.

Shortest distance between two points is a straight line.

Ever watch your wake after hand steering for a while. It's hard to keep her straight what with wind and current.

If turned on the auto is working like a son of a gun making minute corrections.

I love it.

SD

 

[Nomad Willy]

I thought "swinging the compass" was to compensate for all the magnetic things on the boat. All boats have their magnetic things (disturbances) in different places and there'fore the compass needs the compensating magnets to be adjusted so as to cancel out the disturbances such as the engine, stove and all other magnetic things in the boat. The older compass's had the compensating magnets outside the compass usually in the form of 2 balls about 1.5 to 2" in diameter on each side of the compass. So I do'nt see how one could take a compass to a shop and get it compensated. Perhaps I'm not understanding this correctly.

 

[skipperdude]

Swinging the Compass

1) Locate two transit objects (objects in line) and determine their true bearing on the chart
(2) Secure the boat at anchor so that these transits are exactly aligned. Swing the boat's head until it is pointing due north.
(3) Read off the transit bearing on the compass and apply the variation.
(4) The difference between this result and the true transit bearing is the deviation on this heading. If the true bearing is greater, the deviation is named east; if it is the lesser, the deviation is named west.
(5) Repeat the procedure taking transit bearings on each of the cardinal points.
(6) From the results, make up a deviation card
Further errors are caused by boat heel, and the difficulty of reading when on a moving platform. An electronic compass can be set up to remove the effect of deviation, but it is still a magnetic compass.
Hand bearing compasses usually show very small deviation effects which can be ignored if you use them away from a magnetic area.
Deviation can also be East or West, but the same logic applies:
Sounds simple but try it once.
SD

 

[Codger2]

nomadwilly wrote:
I So I do'nt see how one could take a compass to a shop and get it compensated. Perhaps I'm not understanding this correctly.

* * * ** The shop person comes to your boat....you take the boat out in calm water and initiate a circular turn (either direction) while he/she adjusts the ship's compass. This is done several times until the person doing the adjusting is satisfied with the process. They then give you a correction card to store by your compass for navigating.

 

[skipperdude]

SeaHorse II wrote:
The shop person comes to your boat....

*I would truly love that but I can't find a compass guy that will travel to where my boat is.

As a matter of fact there isn't anybody in South Central Alaska that even works on compasses.

I have tried google and all the phonebooks. One guy in Kodiak but he doesn't leave the Island

So Gotta figure it out myself.

SD


-- Edited by skipperdude on Thursday 10th of November 2011 01:42:20 PM

 

[Codger2]

skipperdude wrote:
*I would truly love that but I can't find a compass guy that will travel to where my boat is.

* * * * Sorry SD...point taken.

 

[skipperdude]

Yeah. You would think with as many commercial fishing boats as there are in Alaska I could find someone that knew how to do this. Nada.

It is kind of funny trying to*explain to*someone what I am looking for. I made about a dozen phone calls It was suprising. I called boat builders, Boat repair, boat equipment and any body else I could think of.

You fellas in the lower 48 have it easy.

As with most things in Alaska You have to be a jack of all trades to own a boat up here.

Population base is to small for anyone to make a living by specializing

SD*

 

[RickB]

You can do it yourself SD. Take a look at this site then get someone to help drive the boat.

 

[skipperdude]

Thank you Mr.Rick. This makes it a whole lot easier than what I have been trying to do. I'm gonna give it a try.

One of my problems is that there are no fixed objects like bouys or markers to calibrate on.* I mean objects with a known GPS cordinate. The harbor has one small green light and you can't even see that untill you are right up on it.****What I would like to do is go paint a big mark on a rock on shore. but the greenies wouldn't like that

If I can get this figured out perhaps I can hire myself out as a compass adjuster. I would be the only one in South Central Alaska.

Man I would love to just hire someone.

Swing the compass thats one in a row.

SD

*

 

[RickB]

You're in PWS right? Go to the nearest outdoor outfitters, get a good topo map and use the peaks and creeks.

 

[skipperdude]

That could work the Alaska Pacific University has a USGS office where they sell TOPO maps. Thanks for the idea.

I may have to wait till next summer as the mountains around Whittier doesn't let the sun hit the harbor from oct 21 thru april 15 th.

In the winter up here the sun doesn't really come up it just sort of skims around the horizon.

SD


-- Edited by skipperdude on Thursday 10th of November 2011 03:10:43 PM

 

[psneeld]

With gps now it's REALLY easy to swing a compass...in fact most new compasses have it right in their directions.

All you have to do is set the GPS to read in magnetic and align the compass to it.* You have to be in an area with little or no wind/current and you're all set.* Just drive the boat in a straight line at 5 knots or so (a little faster is better than slower).* If you have autopilot GREAT!* When the gps heading is steady (it's also the same as compass course if no set or leeway)...adjust the screws that move the compass using the brass tool.

Once you have it as good as you can get on a bunch of different headings...then you can make up a deviation card for what you couldn't compensate out.

Paying someone to do it these days (except for a steel boat that may get tricky) is a kind gesture but a waste of money.

 

[Marin]

nomadwilly wrote:
I thought "swinging the compass" was to compensate for all the magnetic things on the boat.

*Swinging the compass is what you do to compensate for everything, including the way the compass was set when it left the factory.* So it compensates for any magnetic influences in the boat's structure, influences that might occur from electrical systems that are on when the boat is being navigated, variations in the earth's magnetic field where you are boating, etc.

Depending on where one lives, GPS is not always the best guide to use for swinging the compass.* The best way to do it in our experience--- albeit not necessarily the easiest way--- is to select easily-spotted, charted*objects that don't move around like buildings and other structures on shore and plot lines between them on the chart.* Ideally you want at least a triangular set of lines although they don't need to form an actual triangle.* Then you plot and jot down the magnetic bearing of each line in each direction.

Then you take the boat out, line it up between the two points that create one of the lines and hold the boat on that line while adjusting the compass to read the plotted magnetic direction of the line.* Then you turn the boat around and set the compass (if it needs setting) to match the bearing in the other direction.* Then you do the same for your other lines.

If you're lucky, you'll get the compass to read accurately in all directions on all the lines.* But the reality is that you'll get it close but not right on in very direction.* So then you make up a compass deviation chart that says if the compass reads this, you're boat is really doing this.

 

[FF]

Not at all hard to learn to use a Sun line . Purchase an almanac .Good for 2 years at least.

Advantage , it can be done almost anytime you simply need a compass check.

Almost no cost, and as accurate as can be done.

 

[Nomad Willy]

I could see that on Lake Washington w no wind but anywhere in sea water there is going to be tidal currents and the boat will be required to be at an angle (crab) so only rough approximations could be established. One could minimize the error by doing it at slack neap tide and along the usual current path. Or possibly it could be better to do it w some tidal flow so as to know the current that did exist would be in a known direction and not meandering around as it does at so called slack. And of course running 180 degrees would eliminate some more variation. But rough approximations could be quite close and that's probably the best that can be done however some very accurate navigation is regularly done in predicted log races and one would think "rough approximations" would'nt work for that.

*I remember making plans to go into Lake Washington to make speed runs at various engine speeds to determine a fairly accurate speed/rpm graph. We had too much on our plate (time wise) and lost the opportunity. Can't do anything like that in Thorne Bay.

 

[Marin]

nomadwilly wrote:..anywhere in sea water there is going to be tidal currents and the boat will be required to be at an angle (crab) so only rough approximations could be established.

*Apparently not.* The GBs in the charter fleet in Bellingham have their compasses professionally swung out in Bellingham Bay, and while I can only speak for the specific boat we chartered, its compass was right on.* You are correct in that a current or wind can cause a boat to crab to maintain a specific course, but in swinging a compass you don't need but a moment to be aligned fore and aft between the two points forming the line to see the error and correct it out.

I did our compass using a handbook on how to do it and while I didn't get it as accurate and the pros ours is very, very close.* It's not necessary to hold the boat aligned between the two physical points for very long, only long enough to see how much the compass is off and to dial in the correction.* The boat can be realigned to the points if necessary for the compass alignment to be checked or readjusted.

The process itself is very easy.* The tricky bit is the actual adjustment of the compass, and this will vary with the type of compass, the sensitivit of he adjustment controls, and how easy or difficult the physical placement of the compass makes it to get the adjustment tool onto the adjustment screws .* Ours is in a less-than ideal location to access the adjustment screws.* Make sure you use a brass screwdriver