compass install

[Birdnest]

I was reading about compass installation the other day and was impressed at the complexity that this publication went into concerning proper placement. Proximity to the boat's centerline was the jest of the arguement.

So what was the determing factor for your install, form or function? The place I want to mount my compass is farther from my centerline than this booked would recomend.

Hey Mr. Pilar(or anybody else)...you briefly mentioned a GPS/Compass exercise to keep ones skills sharp in the event that your plastic Jesus goes down. Could you expand on that and other related exercises? Thanks.

Birdnest

[ 06-14-2003, 07:57 AM: Message edited by: Birdnest ]

[skein]

Birdnest,

As I was taught, go out with the boat before things break and check out your boat's speed at various rpm's, and how drift effects your course at those speeds. Naturally, currents will always change, but you can get an idea of how much you can drift off your compass course.

You also need to know the variation for your area (the difference between true and magnetic north) and the deviation (the effects of the metals in the boat itself). The deal is, 345 degrees on the compass ain't necessarily 345 on the route.

The idea is if you know you're 8 miles from where you want to be and its direction is 34 degrees on your chart, and you make 10 knots at the rpm's you're running, you can deduct a heading and approximate ETA.

Now it's time for the real skippers to step in and explain it better - and tell me where I'm wrong. [img]graemlins/dork.gif[/img]

Skein

[Pilar]

Ok. The compass should be in a line with the pilots eye parallel to the centerline of the boat. Center of the boat is a bit impractical, no?

The exercise is simple. Set your GPS waypoint as the destination and Goto to the waypoint. The GPS will generate a bearing for you to follow. Steer the boat until the course of the boat matches the bearing to get to your destination.

There are several things to do now.

First check out the compass and try to determine how closely it matches your course on the GPS. For example when I steer for Newport from the rockpile the GPS usually gives a bearing of around 45. If I steer 45 by the C.O.G. (course on ground) given by the box and look at the compass it usually says 55 to 60 degrees. I have adjusted the compass and that is as good as it gets. Oddly enough it reads almost dead on if I am going anywhere between due north and due west.

Second thing is to practice steering with the compass. Try to steer, using the offset you figured out and get where you are going with just the compass. You can do a couple of things to learn about how wind and current affect your course. Try steering a steady course. Just follow say 45 degrees. After a few miles check the gps plotter to see how far off you are from the straight line track shown by the GPS. Often wind and current will push you a significant distance off track. The GPS plotter will show you this.

You can also go dead in the water and drift. the gps will report your S.O.G. (speed on ground) and the plotter will record the direction and speed of the drift. This will show you what influnce there is on you boat as you drive along.

Now try to use the info to 'correct' as you drive. If you know that the drift due to current and wind is due south at 1 mph then try steering more to the left to correct for it. Instead of 45 try 40. Drive that for 10 minutes and look at your plotter again. This is where the 'Art' in the old navigation methods comes in. Practice makes it easier but guessing corrections to your course takes a bit of experience.

Anyway this is a practice for when your GPS stops working. You can memorize the bearing to your destination, IE: Chicken Ranch = 242 or just set up the GPS at the start of your transit and get a bearing. Then use the compass to make the run, switch the GPS over to sonar. If you do this as a habit, doing it for real because the GPS really is on vacation will be much easier. It won't be your first rodeo.

In the bad old days people would actually use charts and get bearings with a straight edge and a rose. Then they would follow thier compass to interesting places like America, Columbus, 1492..

Thanks, Jim, good points about speed. I just read your post. If the GPS goes away then so does an accurate speed indicator. I guess you could use the analog gage on the dash or know rpm vs. speed. I have studied how speed changes on ocean runs as you go along. It changes alot. So speed can give you an approximate time to go but not much more than that. To get the full idea on speed changes, watch the time to go on your GPS when you are running in a big swell or into a chop and wind. For speed I estimate based on rpm, 3200 is planed at about 18 mph. It can vary between 17 and 22 at that same rpm depending on climbing a swell or surfing the front of it.

[ 06-14-2003, 08:44 AM: Message edited by: Pilar ]

[Pitch Pocket]

I've learned quite a few things about the compass in the piloting class I'm taking with the Portland Power Squadron.

The compass has to be lined up parrallel to the centerline of the boat in line with the pilots eye. Centerline is the best place to put your compass, but not many small boats steer from that line.

Anything you have near your compass that has magnetic properties (magnets, radar units, large blocks of steel) will create a deviation. That deviation is different for each compass point (hence the dead on north and south and the large deviation at 45 degrees).

If you have a good compass, it should have compensating magnets built in to it to correct the deviation to a managable number. Correcting a compass is a complicated procedure, but is doable.

If you rely on the "plastic Jesus" (your gps) entirely for navigation offshore, you will eventually find yourself in trouble. If you can't read and operate a compass, plot a course, create a line of position and dead reckon, it might be a good idea to learn how.

The Portland Power Squadron is an excellent resource for piloting and seamanship. I've got the phone numbers to join if anyone is interested.

[crabbait]

When navigating off a chart, it is important to know the magnetic variation (20 degrees east) and how to calculate from true to magnetic and back. Everything is referenced from True North. If the magnetic variation is 20 degrees east (which it is) then Magnetic North is 20 degrees east of True North. As long as everything you are doing is in either all True or all Magnetic, none of this matters (GPS is usually set up to read Magnetic but can be set for True). Know that magnetic varriation...uh...varies by location and is constantly but slowly changing for any one location. Twenty years ago the PDX mag variation was 19 degrees, now twenty, soon 21.

In order to set a compass on an aircraft, a Compass Rose is painted on the ground (just like a compass face only with N, E, S, W, painted and verified to be correct). For a boat, you would have to try to determine an accurate magnetic compass rose, then rotate the boat to each of the cardinal directions and adjust the compass to align with the cardinal direction. I guess you could do it with the boat on the trailer in a big parking lot. You would need an accurate compass (or several to compare with and maybe a GPS as a cross reference) paint your compass rose, then starting with North, adjust your compass to align as nearly as possible with the cardinal directions. Several corrections may be needed as correcting one affects the others.

Other electic circuits that are near the compass should be checked for interference by energizing on a motionless boat and observing what if any affect they have on the compass until all are energized. Wiring may have to be relocated or a piece of equipment may have to be moved. At he very least, you will know what your compasses deviation is and you can adjust for it (if it only reads correctly with the fishfinder on then ensure that the fishfinder is on).

'Nuf magnetic rambling for now.

[Pitch Pocket]

I imagine the aircraft carrier compass has all the deviation expertly corrected where you can use magnetic headings to pilot the boat (ship). If your compass has any deviation (and it does), you have to convert true direction to compass direction subtracting the variation and adding or subracting compass deviation.

The math and process of paper piloting are tedious, but it is another repetitive system onboard that is not reliant on electrical equipment to operate.

If something goes wrong, (25 miles out, fog bank sets in, chart plotter blinks out, hand held's batteries turn out dead) your compass and chart should not look like foreign objects to you.
One degree of latitude = 1 nautical mile. If you are off your course by 4 degrees, you can miss your target by miles.

But then, that's why God created color chart plotters isn't it?

[crabbait]

Here is a method for compensating a compass, from the Ritchie Compass site:

Step one. While at sea, with the compass in its intended position, but not finally secured, obtain the Loran/GPS bearing to a visual buoy or landmark that is within 10o of a North/South line.

Step two. Position your boat along that line and steer your boat directly at that mark. Turn the port/starboard compensator until the compass heading matches the Loran/GPS bearing.
Step three. Check the Southerly course by steering away from the mark, to a bearing 180° from Step 2. The compass heading should be bearing from or bearing to +180 degrees. If any error is present, it is an alignment error. Rotate the compass itself to correct for one half of this error. Repeat Steps 1 & 2 and then recheck this Step 3.

Step four. Simply repeat the procedures of Steps 1, 2 and 3 for the East/West course, using the fore/aft compensator, although, at this time, any alignment error should have been eliminated, and no alignment correction is required.

Step five. Upon completing the procedure, secure the compass in its final position.
Note: When performing this method, always use the bearing "To or From" mode on the Loran/GPS. Do not use the Heading Information because it is inaccurate in real time.

To assure accuracy on all headings, check for deviation every thirty degrees and record any deviation on a deviation card. We recommend checking at the start of each boating season for changes in deviation.
If you feel that the deviation on your boat is of an unusual nature, the services of a professional compass adjuster will be a wise investment.

[kenai]

From the CG auxillary navigation book: Even a skilled compass adjuster will be left with some residual deviation. This deviation can be from 2 to 4 degrees or even more. Problem is the deviation isnt consistent around the rose. For example at compass heading 000 there may be a 2 degrees east deviation and at 180 it may be 4 degrees west deviation.
To compensate for this problem a deviation table can be set up listing the deviations for various compass headings. This can be accomplished by "swinging ship", or holding steady on a known range (say due N of Mt Hood) and recording deviation at multiple points around the rose.
Whew,I've gained a lot of respect for them old navigators before electronics, Loran, GPS etc.

[Pitch Pocket]

Yes, you have to interpolate the deviation for your bearing or course unless you have all 360 degrees calculated. It's not just a simple average, but a ratio of 2 different sequences.

We learned that correcting a compass in the Portland area is relatively easy because of Mt Hood's presence almost due east from anywhere in the Columbia River around Portland. You can also use channel ranges (known bearings).

If you are interested in the Power Squadron and the classes, you can call Bev or Kevin Kennedy at (503) 663-7406 for the Portland Chapter. Classes are free to members except for the materials (books and supplies you need). You can see what the Power Squadron has to offer at US Power Squadron Web Page

[ 06-14-2003, 11:28 AM: Message edited by: Pitch Pocket ]

[Birdnest]

Thanks for all the info, now I just need to go lay down in the shade and start to digest all this.

Birdnest

[Mello-Yello]

Ah hah! I had what I thought was a pretty good compass on my old boat (I used it in Puget Sound a lot). Last year, upon buying my new boat (old boat glass, new boat aluminum)I went to install my compass and found that it read WAY different from my GPS (didn't have a decent GPS in WA). I had assumed that either I had somehow damaged the compass or for some reason, the aluminum hull was throwing my compass off.

After reading this stuff (haven't absorbed it all yet though), it looks like my problem might not be a defunct compass after all, but rather a defunct boat owner... [img]graemlins/dork.gif[/img]

My solution had been, I always carry my hand-held GPS as a backup...along w/an extra set of AA batteries. That and I've yet to travel in conditions where I have needed any navigational aids to find my way home. Heaven help me if they ever take the GPS satellites down...I gotta get that compass back out and start learning new stuff......ugh...

Thanks for this post people!

PS...if you use your GPS all the time, why wouldn't you just adjust your compass so that it reads identical to your gps, even if it looks like it's misaligned w/your boat? I am now prepared to finish my meal w/a little humble pie....

M-Y

[crabbait]

Mello Yello - make sure you have your GPS default set to Magnetic, not True, or you will spend forever trying to get them to correspond.

If your compass has several little brass screwheads sticking out of the sides near the bottom, these are the compensators. They will provide up to 20 degrees of adjustment. For Ritchie compasses, make sure they are turned so that the slot is horizontal prior to beginning your installation. Look for installation guidance on the Ritchie Compass home page to walk you through the process.