Compass basics... the lubber line.

[The Chairman]

I believe that's backwards. The drift is amplified when you're swimming with the current, and it's diminished when you're swimming against the current.

Actually it's not. When you are swimming with a slight bias you are exposed to it for the duration of your swim. It will take you LONGER to swim into the current, so the bias will be affecting you longer. IOW if it took me 30 minutes to swim X with a westerly drift of 1 foot/minute I will have been driven off course 30 feet. Now when I swim back, the current allows me to traverse the same distance in only 10 minutes. I now only been driven off course by 10 feet. Unless I corrected for this "bias" I will be 40 ft to the west! I hope this helps!

 

[Mako Mark]

did you know that you get north and southern hemisphere compasses.

I recently changed hemispheres and all my compasses now have excessive dip.

 

[The Chairman]

I find that using the window just complicates things. I like to just look down from above and use the number that intersects with the lubber line. It just seems like an unnecessary step to me.

Everyone is different. I find that there are many divers who don't even know that the window exists or why. When first exposed to a mariner's compass, most of us who grew up with compasses on land (yay Boy Scouts) felt like it's backwards. If we are traveling North it is towards us, rather than away from us. That's because a mariner's compass shows us the direction of travel.

I found on my Cobra, that the easiest way to keep the compass level enough so the needle moved was almost dead in front of me. I could also read the depth at the same time! So over time I learned how to justify my direction with the use of the window. I am equally at ease using either method.

 

[The Chairman]

I recently changed hemispheres and all my compasses now have excessive dip.

Dip? I am unfamiliar with this term. Is it nautical in nature? I know that I have used a compass below the equator and found no difference in it's use.

 

[The Kraken]

The magnet pulls in a straight line to the pole. The pole in the southern hemisphere is, more or less, directly through the center of the earth, so the compass will "dip" down toward the ground.

the K

 

[BKP]

It's called magnetic declination. It's the angle of difference between true north, and magnetic north, and it *does* change depending on your location.

Suunto does sell their compasses based on location. The adjustment would be different if you were to buy a Suunto in Europe, as opposed to N. America.

 

[Centerius]

Actually it's not. When you are swimming with a slight bias you are exposed to it for the duration of your swim. It will take you LONGER to swim into the current, so the bias will be affecting you longer. IOW if it took me 30 minutes to swim X with a westerly drift of 1 foot/minute I will have been driven off course 30 feet. Now when I swim back, the current allows me to traverse the same distance in only 10 minutes. I now only been driven off course by 10 feet. Unless I corrected for this "bias" I will be 40 ft to the west! I hope this helps!

You are saying it's amplified because when you swim against the current, it takes longer to get somewhere, whereas I'm saying it's amplified when you swim with the current because you traverse a much greater distance. Just differences in how we see things I guess. :14:

 

[The Chairman]

For those of you who are lost as to what we are talking about. The true north pole is not the same as our magnetic north. Ergo when our compass points north it is pointing to the magnetic pole and not necessarily to true north. The deviation between the two is referred to as our declination and it varies with both longitude and latitude. All maps use magnetic north as their orientation. So, those using land maps will find the declination in their legend and those using nautical charts will find the declination included in the various rosettas that can be found on them.

Most land compasses have a settable "declination" while most nautical compasses do not! Nautical charts, with their rosettas, have the declination on them in various spots so that a ship does not have to CONTINUALLY adjust their compass to the right declination. I have yet to see a diver's compass with an adjustable declination. For our purpose of underwater navigation, you do not need to fully understand declinations nor will you have to be able to set one. All of our subsequent discussions will be centered around using magnetic north (the one your compass points to) as our orientation.

I hope this helps!

 

[The Chairman]

You are saying it's amplified because when you swim against the current, it takes longer to get somewhere, whereas I'm saying it's amplified when you swim with the current because you traverse a much greater distance. Just differences in how we see things I guess. :14:

But drift is almost always a function of time. We see things more as a function of distance. We traveled "X" distance and yet our drift while swimming INTO the current was three times as great as when we returned (refering back to our example of course). There is no mechanism to measure or adjust for such a drift underwater, but you have to make allowances in order to get where you want to go.

 

[The Chairman]

The magnet pulls in a straight line to the pole. The pole in the southern hemisphere is, more or less, directly through the center of the earth, so the compass will "dip" down toward the ground.

Check your physics. You are assigning gravity to a polar direction. Better shake that glow stick!