Any steel regardless of how long its been there will affect your Mag Compass. No one is going to recommend that you go ahead and blindly mount the cabinet, but you can at least do a simple test to start with.
How much does the cabinet affect the compass . . .
Hold the cabinet near the compass and watch for swing as you bring it in close or pull away. Then put the cabinet in position while you or someone else watches the compass for swing. I'm betting that you will in fact see some swing.
Prepare for a bunch of advice on this thread, but here are four course of action to address interferrence.
1. Don't put that cabinet in there. Swith to a non-ferrous material. (aka wood/palstic)
2. Move the cabinet to a new location.
3. Move the compass to a new location.
4. Install the cabinet and have the compass deviation recalibrated. (Sometimes called a "compass swing" and often done on dry land).
- a. Point the boat to magnetic North and note the actual reading on the compass. That reading now becomes your "new" Mag North heading.
- b. Repeat this for all cardinal headings (N,S,E,W) as well as for any intermediate intervals. (recommend 10 degree intervals).
- c. Write these numbers down on a peice of paper, and that becomes your "Compass Calibration Card". Keep it handy for navigation. Some even mount this card next to their mag compass. Interpolate when using for headings between calibration points. (i.e. Heading 127 deg Mag)
- d. I'm sure a boat yard can pull your boat out of the water and charge you a hefty $$$$ to do a compass swing for you.
Don't forget to apply reasonable-man theory and consider your require accuracy. Will you need this compass for ocean navigation, coastal nav, or are you in a small body of water and rely mainly on your GPS or the big day time star.