Ours seems to be in one of the worst locations aside from on the engine block. It's touching that 1/2 stainless backing plate for the aft most keel bolt that also serves as part of the grounding system. Autohelm exhibits signs of intoxication. I'm thinking of relocating aft and slightly starboard of centerline. Possibly starboard lazerette.
This is the first time lifting floorboards, going to look better after some soap in here. Bilge pump is also about to get replaced.
When I upgraded electronics to all B&G, I put the Precision 9 compass at the location where the Raymarine fluxgate was. I put the B&G motion sensor on the other side of the bulkhead in the main cabin under the stbd setee. Either of these locations would be good. I may move the B&G compass on the other side of the bulkhead next to the motion sensor, just so the locker is clear for cruising use. There is a space forward of the bladder tank to fit the compass there, and it will be out of the way.
I installed a B&G system on J/109 Leading Edge and put the compass under the v-berth bolted to the bulkhead. I did a similar installation with J/109 Marquise. See pictures below
Motion Sensor location
Compass Location on J/109 Leading Edge
Compass location on J/109 Marquise
Bill provided pictures of the perfect spot to put your compass. There is a challenge, however. Keep in mind, with a fluxgate compass the signal in the wire is not digital and can be interfered with by radio's etc. So for example if you are tempted to extend the Raymarine wire to that perfect location Bill photographed in the forward birth, you don't want to run the wire past large DC wires, your DC panel, VHF radio, stereo radio, refrigerator, etc. on the way to the forward birth.
Battery wires are particularly bad as they are not "twisted pair", a configuration useful for canceling out magnetic fields that can turn back into electrical fields on wires that run along their length.
I experienced my prior Beneteau with Raymarine autopilot changing course 3+ degrees every time the refrigerator cycled, just because the fluxgate sensor wire ran past a one foot length of battery wire, then having the compass heading being affected by electric current by the refrigerator on the other side of the boat. Every other electrical device on the boat having the same effect.
The higher precision compasses are a joy to have, they can help your instruments do a better job of providing you situational awareness, as I wrote here on Panbo.
https://www.panbo.com/airmar-h2183-impr ... raymarine/
Until you choose to purchase one, check out your corrected deviation that appears in the compass calibration menu, and if under 8%, choose another improvement project for your J/109. And if over 8%, maybe you can find another location within a couple of feet that is much easier to move it too then the bow. As bottom line, the Raymarine electronics are pretty good at taking what it learns from turning your boat in a circle, and applying it to the imperfect measurements of your fluxgate compass.
This gives me enough motivation to try another spot without making a big production out of it. Suggestions in the raymarine installation .pdf for finding sources of interference are far from scientific and involve a handheld compass.
Thanks again, Todd