Thursday, April 19, 2012

Project ST5000: Locating the compass

New autopilot => new compass. Now where to put it? The compass installation instructions advise that the compass should be mounted:
  1. At least 3 feet from major ferrous metal (eg, the engine)
  2. At least 3 feet from radios (because of the magnet in the speakers)
  3. At least 3 feet from other compasses (magnets, again)
  4. On the forward side of a bulkhead
  5. As close to the roll center of the boat as possible
  6. As close to the pitch center of the boat as possible
  7. As close to the yaw center of the boat as possible.
As it turns out, these are conflicting requirements aboard Eolian, given that the roll/pitch center is just about at the top of the engine.

Since this was going to be an experiment no matter where I chose, I first (temporarily... no cutting of cable yet) chose the most convenient location: the bulkhead directly beneath the drive unit, right below the wheel in the cockpit.  This took care of numbers 2, 3, and 4, and wasn't bad for 5-7.

When testing tho, a strange thing happened.  As soon as I engaged the autopilot, it dove wildly to port.  This perplexed me for about a beer.  Then I questioned whether the electromagnetic clutch in the drive unit might be causing the problem.  So I pulled the clutch wire (blue) off of the back of the ST5000 and tested again...  it happily maintained our "course", sitting there in our slip.  so now I have another constraint to add:
  • Mount the compass at least 4 feet away from the drive unit
In fact, in studying this I noticed that the drive unit clutch was also causing a 2° change in the heading shown by the main navigational compass at the wheel!  Worse, I can see no reason that this has not been present since the drive unit was installed, decades ago.  Gonna have to remember to take that into account.

Things are getting more difficult now.  In the "engine room" space beneath the cockpit, I could not find any place for the compass where the drive unit did not interfere. 

So now I could either go forward, or aft of the engine room.  But going forward, I'd have to string wire all the way to forward of the mast to get the compass away from both the engine and the generator.  Going aft, there is a much closer convenient location beneath the berth, near the compass unit for the radar.  Actually, this is pretty close to where the compass for the old Benmar had been mounted (although that unit had been mounted quite a bit off the centerline of the boat, the source, I believe, of that autopilot's inability to track well in a seaway).

I mounted it in the aft location and cut the cable.   We'll do sea trials and try it there for a while.  If it is unsatisfactory, I'll relocate the compass forward of the mast as a second choice location.

Now all we need is some decent weather that coincides with when we have time to get off the dock...



Chuck and Jackie on SV Aria said...

I have a spare Raytheon Fluxgate in my garage if you need an extra for testing.

bob said...

Chuck/Jackie -

Thanks for your kind offer. Based on what I saw in the sea trials, the compass seems to be in an acceptable location.

But I haven't cut the wire yet...


Anonymous said...

It never ceases to amaze me - 90 percent of electrical mysteries are related to corrosion and poor contact. I've solved numerous electrical quirks just like you described.

I have had NO problems with electrical connections that I made correctly: well crimped connectors, shrink-wrap tubing insulation, proper wire sizing. But when I cut corners early on in my yachting career, the problems always returned. I hate redoing something that I should have done right the first time!


bob said...

Rick -

I am not sure what your comment refers to in this post... perhaps it was intended for another post?


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