Best Location for an Autopilot Compass?

[Shoalcove]

I'm in the process of installing my new Simrad autopilot and am trying to choose a good spot for the compass. The instructions say "not near motors, electrical cable, chunks of metal etc" but low and near the pivot point. My problem of course is finding such a place. My boat has an iron keel to complicate the issue. It seems that there are two reasonable choices: low on the centerline in the bilge but only a few inches above the iron keel or in the back of the chart table. The chart table is near the centerline and only about 30" above the waterline. It is fairly far from the engine and keel but may be subject to interference from a laptop or ipods. The bilge location would seem ideal except for the chunk of iron. I could also locate it about halfway between the keel and motor down low but then it is within a foot of 00 battery cables.
Any thoughts of best choice or should I just try one and see how it works and be prepared to move it?
This is turning into a bit of a project so I'm open to enlightenment to save a step or two.
Thanks,
Shoal

[Ex-Calif]

Originally Posted by Shoalcove

I'm in the process of installing my new Simrad autopilot and am trying to choose a good spot for the compass. The instructions say "not near motors, electrical cable, chunks of metal etc" but low and near the pivot point. My problem of course is finding such a place. My boat has an iron keel to complicate the issue. It seems that there are two reasonable choices: low on the centerline in the bilge but only a few inches above the iron keel or in the back of the chart table. The chart table is near the centerline and only about 30" above the waterline. It is fairly far from the engine and keel but may be subject to interference from a laptop or ipods. The bilge location would seem ideal except for the chunk of iron. I could also locate it about halfway between the keel and motor down low but then it is within a foot of 00 battery cables.
Any thoughts of best choice or should I just try one and see how it works and be prepared to move it?
This is turning into a bit of a project so I'm open to enlightenment to save a step or two.
Thanks,
Shoal

The location is to avoid pitching or yawing moments of the boat affecting the compass. The vertical, lateral and longitudinal "center" of the boat is ideal but I wouldn't stress about a few feet.

The caution to avoid metal and motors is obvious.

I'd get it close and logical and not sweat it too much. I'd avoid the masthead and tip of the bowsprit, for example - LOL

[Capt Phil]

Amidships, low and easy to get to for service if necessary. One other caution, mount it away from SSB, VHF and radar installations by at least 4 feet. My buddy had his mounted by a marine technical installer who mounted the fluxgate less than 2 feet from his VHF under his chart table and couldn't understand why the boat went in circles everytime he engaged the autopilot. As a result, he didn't use the thing for about 2 years and was complaining to me about having to steer from Catalina Island back to San Diego.
It took about 5 minutes to figure it out, 15 minutes to rerun the wires and connect them up and 30 minutes to take the boat out and check the autopilots performance.
I think it was noted in the installation instructions to avoid electrical interference sources when mounting. Too bad the installer couldn't read... Capt Phil

[Shoalcove]

Thanks for the feedback. The compass has about 18' of cable so that does limit my location choices somewhat. I am very concerned about having interference from electronics and electrical devices so may have to rethink a couple things. On the bright side, I don't have other simnet or NMEA2000 devices to consider so may be able to set up the system to optimize the compass location. Thanks again!

[Pblais]

Thanks for the feedback. The compass has about 18' of cable so that does limit my location choices somewhat

OK, so you put it some place and try it out. It's not uncommon for the "perfect" place not to work. Even though it is supposed to be a good spot it often is not. The compass will know if it is happy.

One quick dockside check is to locate the compass and have someone hold it unattached and slowly rotate it both clockwise and counter clockwise while you watch the reading at the control head. It should display in smooth increments watch the 90 degree boundaries too. This is just a quick way to tell if the spot really is perfect. One of the pro installers here made a remote control turn table so he could do this test without an assitant. It's a good way to avoid a hassle and have to move the compass. You can do it dockside too and save even more hassle and time.

In our last boat it was installed in the perfect place and ddin't work. We could find no metal or interference but there was some there any way. It loved the spot under the nav staion near the electrical panel. I'm not saying the electrical panel is a good spot but the compass was happy. Low and in the center is a good place to start looking.

The cable can be extended too it's just wires.

[svHyLyte]

The Fluxgate for our Simrad is mounted under the settee on the bulkhead between the salon and the forward cabin, about as far away from our engine, electronics et al as possible and yet closer to the center of roll, pitch and yaw than it would be farther forward and more remote from our Iron keel. (It is/would be very difficult to mount the thing entirely outside the influence of a big chunk of iron.)

Overall, our system works very well in terms of maintaining the yacht's heading once set although the fluxgate's indicated heading rarely comports with our actual (Compass) heading, erring moreso as one's heading has more east/west components. (As such, it would be difficult to safely interconnect one's autopilot to a plotting system to follow a route unless the plotting system's route headings were first filtered through a correction table calibrated to the Fluxgate.) In our case we have made up a correction card for the Fluxgate--just as one might for a Compass--and have had no difficulty.

FWIW...

[Ex-Calif]

Originally Posted by svHyLyte

The Fluxgate for our Simrad is mounted under the settee on the bulkhead between the salon and the forward cabin, about as far away from our engine, electronics et al as possible and yet closer to the center of roll, pitch and yaw than it would be farther forward and more remote from our Iron keel. (It is/would be very difficult to mount the thing entirely outside the influence of a big chunk of iron.)

Overall, our system works very well in terms of maintaining the yacht's heading once set although the fluxgate's indicated heading rarely comports with our actual (Compass) heading, erring moreso as one's heading has more east/west components. (As such, it would be difficult to safely interconnect one's autopilot to a plotting system to follow a route unless the plotting system's route headings were first filtered through a correction table calibrated to the Fluxgate.) In our case we have made up a correction card for the Fluxgate--just as one might for a Compass--and have had no difficulty.

FWIW...

How do your spirit and fluxgate compasses compare to gps heading?

Steering a plotter course, even with fluxgate error, is not a problem. Steering commands come from the gps to the autopilot not the fluxgate in track mode. The only time it is a factor is if the plotter course is 090 and you need to select, say 087, in heading mode to get the boat to drive 090 mag. In other words you arr compensating for a 3 degree mismatch in compass headings.

Of course in track mode with set the system would not know if it is being set by current and leeway or if the compasses disagreed. In heading mode I am usually steering way off the plotted course heading due to cross currents.

It is not uncommon for the numbers to all be a few degrees off from each other.

[Shoalcove]

Thanks for the advice everyone. I was out to the boat all day today getting ready for tomorrows launch so never got too far with the compass. I've been glassing in a platform for the drive unit and attempting (unsuccessfully so far) to drill my rudder shaft to pin my AP tiller. It is difficult to get enough force on the drill and it's resisting my good bits and cutting oil! I'll try the compass mounting tips and see how I make out. I figured someone here would have been through this.
Best regards,Shoal

[Cheechako]

not in the bilge!

[xymotic]

Mine's mounted in underneath the dining table.

[macpowell]

On my cat. The absolute center is out of question, so in the nav station it lives, but it is easy to take a regular compass and check the deviation at the location with things off and on and I found my box of stainless clamps, non magnetic? Affected my flux gate.

[Shoalcove]

I have a couple locker areas that may work as long as I remember not to shove something beside it that is magnetic. I was worried about the effect of motion and therefore was keen to keep it low, but in re-reading the instructions I notice that they recommend mounting it on a mast 3 or 4 feet above the wheelhouse on a steel vessel. Free from interference seems to trump motion then. A nice dry locker does seem better than the bilge.

[svHyLyte]

Originally Posted by Ex-Calif

How do your spirit and fluxgate compasses compare to gps heading?

Our binnacle compass and our GPS bearings seem to agree quite well although our Compass is only graduated to 5º increments so it is difficult to make assessments. I do use GPS bearings to a mark data as a reference rather than heading as, depending upon tides/currents/leeway et al, one's heading/cog may have little to do with the direction one's bow/enter-line is pointing.

Steering a plotter course, even with fluxgate error, is not a problem. Steering commands come from the gps to the autopilot not the fluxgate in track mode. The only time it is a factor is if the plotter course is 090 and you need to select, say 087, in heading mode to get the boat to drive 090 mag. In other words you are compensating for a 3 degree mismatch in compass headings.

Of course in track mode with set the system would not know if it is being set by current and leeway or if the compasses disagreed. In heading mode I am usually steering way off the plotted course heading due to cross currents.

It is not uncommon for the numbers to all be a few degrees off from each other.

Quite right. I was thinking of heading data being sent from the fluxgate but that is not how most systems work. In any case, aboard our yacht the plotting system and autopilot system are not integrated. Frankly, I don't like these systems talking to one another. One never knows what they might cook up on a long night watch. I prefer to set my AP courses manually and I do update my paper plot hourly which helps me to spot set and estimate drift reasonably early on during a passage.

FWIW...

[Ex-Calif]

Originally Posted by svHyLyte

Our binnacle compass and our GPS bearings seem to agree quite well although our Compass is only graduated to 5º increments so it is difficult to make assessments. I do use GPS bearings to a mark data as a reference rather than heading as, depending upon tides/currents/leeway et al, one's heading/cog may have little to do with the direction one's bow/enter-line is pointing.

Quite right. I was thinking of heading data being sent from the fluxgate but that is not how most systems work. In any case, aboard our yacht the plotting system and autopilot system are not integrated. Frankly, I don't like these systems talking to one another. One never knows what they might cook up on a long night watch. I prefer to set my AP courses manually and I do update my paper plot hourly which helps me to spot set and estimate drift reasonably early on during a passage.

FWIW...

+1. My auto steers exclusively in heading mode.

I read with interest reports of the autopilot going "haywire.". Mine has never done that but a charter boat I rented did. I found that the clutch on the wheel was no fully engaging, the clutch would slip and then the system would get confused. I let it steer a 360 to see how crazy it would get. Once I figured out the clutch thing it settled down for the next 3 days.

Weird those electrons are...

[Kijit]

Our compass is in a lower locker in the aft cabin. It works just fine there most of the time. When it doesn't, I just say "Honey, where did you put your purse?" She goes below and magically it starts working again!