Wednesday, May 9, 2012

Project ST5000: My kingdom for a block of foam

My next step in the ST5000 project is to mount the rudder reference transducer - it tells the autopilot where the rudder is positioned. To do so, I wanted a block of foam to prototype the awkward shape that would be required to provide a surface parallel to the rudder quadrant while mounting to the hull.  This is a difficult problem because there are no benchmarks, and nothing is straight or vertical.  The hull is curved in two planes there, and the rudder shaft is far from vertical - it is canted significantly aft and so therefore is the plane of the rudder quadrant.

So where could I get a block of foam, say a 4" or 5" cube? I asked everyone I knew, and even begged a piece of polystyrene packing foam from a computer monitor, that might work... in a pinch.

And then a miracle occurred: someone discarded two giant blocks of urethane foam at the marina recycling center. I grabbed one, and cut what I needed from it.
Big block 'o foam
This is truly a boon because first, there is more than enough there for several trials, and second it is urethane foam - it will be unaffected by polyester resin. I had originally intended to make a foam block and then recreate it in wood for final mounting.  But with the urethane foam I can just laminate a piece of plywood to the top to provide a platform for the mount screws to bite into and stick it all in place with fiberglass and resin. Yes, I can hear the purists out there saying that I should use epoxy.  Well, epoxy takes hours to cure (and I expect to need multiple applications), and this is, after all, not a structural application.  Polyester resin is perfectly adequate to the task.  So polyester resin it is, and glory be, it goes directly on the foam.

Foam is already shaped here

Now, to shape the foam... how to do it?  I devised this plan:
  • Hold a block of foam in place where I think it should be (mark the hull so that I can locate it there again).  This has to be a place that is close enough to the quadrant to avoid interference problems with the nearby exhaust hose, and far enough away from the quadrant that it won't be hit by it when the rudder is hard over.
  • Using a straightedge laid across the rudder quadrant, mark where the plane of the rudder quadrant crosses the foam
  • Remove the foam and cut on the line I had marked

It took two tries, but here is the result (the black dot is where the center of the transducer goes):

Weird shape, empirically derived

It wasn't easy working in the small access hatch.

Awkward access

(By the way, that nasty rust stain is from a standard plumbing elbow that one of Eolian's previous owners had installed on the exhaust hose to mate up with the bronze discharge fitting. I replaced that rotting mess with a fiberglass elbow a long time ago.)

Now all I have to do is glass it into place.  But that's why there's next weekend...


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