Perspective refocus

That big moon we've been seeing the last few days?  The one that shines in the hatch all nite and keeps you from sleeping soundly?

This morning as I was leaving the boat in bright moonlight (not yet dawn here in Seattle), I briefly wondered if the moon was full for our friends in the Caribbean or Mexico...

And then that perspective refocus happened.

Of course they have it, just like we do.  The moon is not weather - it is a world-wide phenomenon.  Every human on Earth is experiencing the exact same full moon.

Talk about stretching your perspective from local to global...

Perspective refocus

Cold toothpaste

When toothpaste is cold, it is stiff.  I mean so stiff that you fear rupturing the tube when you lean on it to try to squeeze some out.

One of the objectives that I set for myself in writing this blog was to convey to those of you who are not, what it is like to live on a boat.  The toothpaste is part of that.

Eolian's interior is designed so that almost universally there is cabinetry against the hull in the living spaces.  Beyond being just a well-designed use of space, this has the further advantage of providing some insulation against the outside temperatures.  So, your clothes help keep the interior of the boat warm when they are hanging in the hanging locker, just like they keep you warm when they are hanging on your body.

And when you open the medicine cabinet behind the sink in the aft head, you find it full of cold air.

And stiff toothpaste.

Cold toothpaste

The bells

The bells of St. Mary's, they are not.

Every boat with a mast has lines that run up that mast to hoist sails.  What to do with those lines when there are no sails hoisted?

You can tie them off to cleats thoughtfully provided on the mast down at deck level for just that purpose.  But if you do that then the length of that line, running up the mast and inches, nay fractions of an inch, away from the mast will come alive when the wind starts.  It is a kind of aeolian harp, but with a very low frequency due to the long length of the halyard and the lack of tension in it.  And unfortunately when it gets to vibrating, it slaps against the mast.  With irritating regularity.

Whang whang whang whang
Whang whang whang whang

I have written before how living on a boat is like living inside a guitar.  Well this is like living inside a guitar that has the action set too low, making for string buzz, but again at a very low frequency.

And not only is it irritating down below, but it can be heard by your neighbors, and their neighbors.  One of the least neighborly things you can do is to leave your halyards rigged in such a way that they will begin to rhythmically ring the bell.  Your neighbors will grow to despise you...

And yet...

And yet, after a long day of sailing, when the anchor is finally down and the boat is rocking gently to and fro, the gentle clank of the halyards, still rigged to hoist the sails and ready for the morrow, is somehow soothing.

Like the bells of St. Mary's.

{listening to Gillian Welch: Tennessee}

The bells

Project ST5000: Report #3

My testing workbench

I have made some progress.

• The MOV's have arrived (part no. V33ZA70)
• I have determined that feeding the control inputs to a SSR with reverse voltage results in a 76 mA current flow - probably unacceptable for long-term use.  I got a couple of diodes (Radio Shack, $1.19) which will prevent back-feeding the inputs.
• The MOV's are temporarily installed - they're the two red disks attached to the terminal strip in the drive... one for the clutch relay, and one for the motor.
• I have successfully actuated the clutch in the drive unit, using the ST5000+
• I have successfully activated the drive motor from the ST5000+, using the one SSR that I have. 
• Irony: reviewing the schematic shows that the output circuit of the ST5000+ is actually an H-bridge.  Which I will be using to feed another H-bridge (albeit one that can handle a lot more current).

Now some thoughts about specifying solid state relays (SSR's) for the H-bridge... One of the figures of merit for solid state relays is its resistance in the "on" state - indirectly, this is used to rate the current carrying capacity of the units.  Using a SSR from Crydom as an example, the D1D40 (40 amps capacity) has an internal resistance of 0.05 ohms.  Using Ohm's law, the D1D40 will be generating heat at a rate of 80 watts (that's a lot - think about how hot a 75 watt light bulb gets) when carrying full load.  Obviously, this would require a heat sink.  The Benmar drive unit has a 10-amp rated motor in it - that's what it says on the motor.  Actual tests with my installed Benmar show that when the motor is stalled (I'm holding the wheel and preventing it from turning), the load is 15 amps.  But under normal operations, the load is something like 5 amps.  So with the D1D40, I could expect to see heat production at something like 1.25 watts under normal operation, and 11 watts in worst-case, locked rotor conditions. 

A Crydom D1D20 (20 amps rated capacity; 0.10 ohms internal resistance) would produce twice the heat.  Both Crydom units far exceed the needed current rating, but considering heat output, I am leaning toward the 40-amp unit.  My plan is to bolt the SSR's to the inside of the cast aluminum drive case cover, using it as a heat sink.  If I stick with 40-amp units, and given the intermittent nature of the motor's duty cycle when the autopilot is in use, heat should not be a problem.

There are a large number of Chinese SSR's on eBay with suitable advertised current carrying capacity.  However, none of these units show internal resistance in their specifications (on purpose?), and many of the ads show the SSR's bolted to large exotic heat sinks.  Tho they are more expensive, I am strongly tempted by the Crydom units.

Project ST5000: Report #3