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post #1 of 18 Old 08-29-2007 Thread Starter
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Compass Question

I have a Danforth Saturn Compass (dual read), flush mount as I have tiller steering.

When I first took the boat out, all my courses were magnetic, and my hand held GPS also reads for Magnetic North. I noticed immediately on my first course, that my compass was off (roughly 15 degrees). It seems that the previous owner adjusted the compass to read for true north. Not sure why...?

Two questions:

1.) How does one adjust these compasses so I can go back to magnetic?

2.) Would it just be easier to use True North when setting courses on the charts, and reprogram my handheld to read for true?

Any suggestions?

Chris
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post #2 of 18 Old 08-29-2007
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Rotate it 15 degrees.
jef
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post #3 of 18 Old 08-29-2007 Thread Starter
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Originally Posted by SanderO View Post
Rotate it 15 degrees.
Hi Jef!

How would I do that? It is done with a screwdriver? Do I need to loosen it from its housing, or am I making this out to be harder than it actually is?

Chris
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post #4 of 18 Old 08-29-2007
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If you have one of these:



and it is adjusted to read true north, he must have used the compensators to do this. However, I'm not sure if the compensators could alter the compass readings 15˚ accurately over the entire 360˚ range of readings. I would bet that the compass isn't accurate for true bearings all the way around. What you need to do is swing the compass and have it re-adjusted to read properly.

The reason I think it is very important that you do this, is that if you get used to this compass the way it is, and get on another boat... the chance of you making a huge navigation error is rather high... Likewise, if you have someone else at the helm of your boat, and they're not used to a magnetic compass reading true... you're much more likely to have a problem. Navigation errors can result in very, very, expensive repairs.

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post #5 of 18 Old 08-29-2007
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Scurvy- Is your compass off 15 degrees on all headings, or does it read differently on different headings? While it may need to be adjusted, I would guess that it is just deviation. If you want the compass to be as close to perfect as possible I would recommend hiring someone to adjust it and record a deviation card for it.

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post #6 of 18 Old 08-29-2007
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Scurvy...I think the compensating screw is at the bottom of the Saturn..you can try it and see. Before attempting compensation...
1. check to see whether it is a compensation issue or if the compass is being affected by nearby metal or wiring
2. Is the 15 degrees off the SAME at all points of the compass or does it vary if you point the boat due south or due west or due east. The problem may be either compass alignment itself OR a need to compensate the compass.

Check this link from Ritchie to see HOW to adjust your compass in general terms.
http://www.ritchienavigation.com/ser...pensation.html
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post #7 of 18 Old 08-29-2007 Thread Starter
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Quote:
Originally Posted by sailingdog View Post
The reason I think it is very important that you do this, is that if you get used to this compass the way it is, and get on another boat... the chance of you making a huge navigation error is rather high... Likewise, if you have someone else at the helm of your boat, and they're not used to a magnetic compass reading true... you're much more likely to have a problem. Navigation errors can result in very, very, expensive repairs.
Sailingdog, I have the older model of that, but essentially, that is the one, and your point is well made and taken, and is the chief reason that I want this issue corrected. I caught myself a number of times heading off on the wrong course after a tack and had to catch myself. Adding 15-16 degrees everytime I used the GPS was a nightmare as I am of left brain and can't count to 21 unless I am naked!!!

Chris
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post #8 of 18 Old 08-29-2007 Thread Starter
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Originally Posted by camaraderie View Post
Is the 15 degrees off the SAME at all points of the compass or does it vary if you point the boat due south or due west or due east. The problem may be either compass alignment itself OR a need to compensate the compass.
Hi Camaraderie!

From what I can remember, it seems to be off on all points. your first point is a good one, as I do have a knotmeter roughly 8-10" away from the compass. I looked into this possibility first, but the folks that looked at it and heard my dilemma didn't think that that was the cause of my problem. What do you think...?

I had wheel steering on the Sabre, and had the binnicle compass which was so nice! Never had a problem with that unit.

PS-thanks for the Ritchie Link!

Chris

Last edited by scurvy; 08-29-2007 at 10:09 AM.
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post #9 of 18 Old 08-29-2007
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I have one of these. There are two compensation screws at the bottom. One for North/South and one for East/West. From SD's pic above you can see one of the adjusting screws just below the zero on the inclination scale. The silver dot to the right is a label telling you which one it is. Good chance that's gone since the compass is a few years old. The screw slots would be horizontal for no compensation.
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post #10 of 18 Old 08-29-2007
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Geessh! 15 degrees off from the GPS? Well the GPS adjusted to Magnetic will only have and show what it will be with variation. You still have to adjust for deviation.
Deviation changes as you change course, the variation does not, it changes as you change positions around the world.
you may or may not need compensating magnetics to adjust your compass to the least deviation error possible. And then develop a Deviation card for all of the cardinal and inter-cardinal points. Then you can use the compass as it should be used.
Have fun.

Notes: No magnetic compass can be adjusted to show true north on all points of steering. Due to variation and deviation, so don't even bother to try to do so. It may show Magnetic North if variation and deviation cancel each other out. But then again read Bowditch, Dutton's or Chapman's on compass adjusting. You really need to do this in order to understand what is happening. Then call in a professional compass adjuster.

But a gyro-compass can and does show true north. Any errors on the Gyro can be removed by mechanical adjustments.

Plot everything on the chart in True degrees, then convert to magnetic for the headings to steer by.

I've swung vessels and have developed deviation cards for them. Have done some compass adusting also. But then I hold a 1600 Ton Master / 2nd Mate Unlimited tonnage, Ocean license and have the knowledge on how to do this.

Last edited by Boasun; 08-29-2007 at 10:44 AM.
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