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Old 02-07-2020, 19:06   #1
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Placement of B&G Precision-9 Compass

I am in the middle of a complete electronics refit on our 2006 Beneteau 473 and am about to mount the Precision-9 compass from B&G. The instructions are to mount the compass "as close as to the vessel's centre of roll and pitch as possible, i.e., close to the water line". It also states that it should be placed "as far as possible away from disturbing magnetic interferences such as engines (min. 2 meters), ignition cables and other large metal objects".

This leaves me with no obvious options as the centre of roll and pitch would be somewhere on the centerline, low inside the cabin approximately over the keel, which in the case of a Beneteau 473 is cast iron! It has been suggested to mount it on the bulkhead wall next to the mast (aluminum so that should be OK) but that position, if near the waterline would still be within 2 meters of the leading edge of the keel. I could put it up high to get away from the keel but then it obviously would be far from the waterline.

Am I being overly concerned with the placement? Will this compass compensate for the proximity of the keel when calibrated? Any advice would be welcome!

Cheers,
Bill
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Old 02-07-2020, 19:31   #2
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Re: Placement of B&G Precision-9 Compass

I
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Originally Posted by BEB_B473 View Post
I am in the middle of a complete electronics refit on our 2006 Beneteau 473 and am about to mount the Precision-9 compass from B&G. The instructions are to mount the compass "as close as to the vessel's centre of roll and pitch as possible, i.e., close to the water line". It also states that it should be placed "as far as possible away from disturbing magnetic interferences such as engines (min. 2 meters), ignition cables and other large metal objects".

This leaves me with no obvious options as the centre of roll and pitch would be somewhere on the centerline, low inside the cabin approximately over the keel, which in the case of a Beneteau 473 is cast iron! It has been suggested to mount it on the bulkhead wall next to the mast (aluminum so that should be OK) but that position, if near the waterline would still be within 2 meters of the leading edge of the keel. I could put it up high to get away from the keel but then it obviously would be far from the waterline.

Am I being overly concerned with the placement? Will this compass compensate for the proximity of the keel when calibrated? Any advice would be welcome!

Cheers,
Bill
All boats are different

I often see them mounted at deck level on the forward side of the main bulkhead ... close to centerline

I’ve seen them mounted near deck level , inside of hanging lockers

I have seen them mounted inside of saloon tables

I have seen them mounted on the front side of the mast , inside the boat, near deck level

I have seen them mounted outside , on deck ...inside of a dorade box


I have never seen one mounted at or below waterline level
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Old 02-07-2020, 19:49   #3
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Re: Placement of B&G Precision-9 Compass

Thanks Slug. Mounting it high on the bulkhead near deck level makes sense to me as it will get the unit about 7' above the keel.
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Old 02-07-2020, 20:12   #4
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Re: Placement of B&G Precision-9 Compass

I tried various locations and settled on 3/4 aft 0.75 mt from the centerline at the waterline.

The compass seems to be self adjusting as every location (except 0.5 mt from engine) gave a 180# change on running reciprocal transits.

So how is the effectiveness of the location measured?

I used auto pilot performance.
Ability to hold course and speed as a % polars over time.

Hight above waterline seemed to have a bigger (negative) impact that distance from roll center.

Distance from centerline had less effect.

There is probably a better way to measure the effects of placement.
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Old 02-07-2020, 20:24   #5
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Re: Placement of B&G Precision-9 Compass

Thanks Rucksta - more to think about!
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Old 03-07-2020, 10:05   #6
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Re: Placement of B&G Precision-9 Compass

My cat had a dedicated spot for the compass as originally built. I needed to put my compass in a slightly different position and following guidelines seems to have worked fine.

I used my original hydraulic ram to steer the boat and had to replace the brain which had fried hence the new compass.

In order to get the new AP to work we had to calibrate the compass 1st. The diretions were terrible and B&G couldn't help us. Took us 3 days to figure out how to find the page in the menu to allow us to do this.

Doing some electrical work all power was disconnected and all that info disappeared. Now I have to recalibrate again and neither of us remember how to find that stupid page! Just coming off major back surgery so am not able to do anything for 3 months but when you program your unit, and find out how to program the compass would you let me know LOL?
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Old 03-07-2020, 10:13   #7
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Re: Placement of B&G Precision-9 Compass

Thanks - if I figure it out before you do, I'll let you know!
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Old 03-07-2020, 10:34   #8
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Re: Placement of B&G Precision-9 Compass

Use a handbearing compass in the proposed location to see if there is any magnetic anomaly. If you have any wires or devices nearby turn on the equipment to check there is no undesirable difference.
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Old 03-07-2020, 10:55   #9
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Re: Placement of B&G Precision-9 Compass

We found under the heads sink free of electrical items other than the shower sump pump, but that isn't used for very long. In a straight line the Raymarine gyro compass is about 1m from one of our steel bilge keels and the steel fuel tank. This has proven to be a better location than previously high up in the cockpit locker towards the stern.

My local RM dealer suggested that since yachts turn around the keel a modern gyro compass rather than the older fluxgate type, may detect the stern moving initially in the wrong direction. Therefore we should move it forwards to the middle of the yacht (bow to stern) even if we can't mount it on the centre line (port to starboard).

That change and fitting a rudder reference unit has made a significant improvement and noticeably reduced the amount of work the connected autopilot has to do.

There probably isn't a perfect location, particularly somewhere that a temporary object can't be placed like a mobile phone. Been there and got the tea shirt after the autopilot crash gybed when I sat next to it with my phone in pocket
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Old 03-07-2020, 11:03   #10
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Re: Placement of B&G Precision-9 Compass

I have the same compass on a 42' center cockpit. The only easy to access location (for mounting and connecting to the NMEA2000 network) that was also far from large metal objects was in the aft cabin about 10-11' from the transom all the way port under a drawer.

It's maybe a foot over the waterline at most and less than a foot from the side of the hull.

Have only used it for about 2300-2400 nm but sail and power from calms up to 6-7' swells it holds a very good course on all points of sail and power usage very moderate.
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