Shower sump; alternator; etc.

Thursday night last week I got to the boat kind of late with all sorts of grand plans for things I could get done, and I was quickly derailed (what’s new?) by the shower sump pump.  As soon as I flipped the breaker, I heard it start running, and it never stopped.  Something wrong with the pressure switch, which I spent some time debugging.  To make a long story short, I found that removing the tube from the base of the pressure switch and then re-attaching it resolved the issue, and then the pressure switch worked better than ever before.  Of course, Friday morning I forgot to switch off the breaker like I usually do when I leave the boat, and in the evening I returned and found the pump running dry.  Luckily it didn’t burn out; some more fiddling with the tubing resolved it and I haven’t seen a recurrence.  Still kind of annoying.

After a hike and a swim Saturday Charlene and I returned to the boat for a BBQ and I ran the engine for a while, and in particular I played a bit with the alternator, adjusting the engine RPM and watching the alternator’s ammeter.  I was a bit surprised to find the amperage topping out around 20A, reaching it around 1300 RPM and staying around 20A even up to 2200 RPM, which is as high as I went.  My alternator is 105A-rated.  I was also disturbed by the voltage drop from the alternator to the batteries (~14.3V at the alternator posts, but only ~13.2V at the batteries).  Talking to Lee this morning, he suggested my alternator-to-battery wiring may be undersized, which I’ll check.  Brian felt that my numbers didn’t actually sound so bad and  that the alternator regulator was probably just in the float phase, since when I was running the engine the batteries were already around 12.5V (~85% charged).  After some more pondering today I’ve decided I have better things to worry about for now and will revisit this after doing some rewiring.

I did some rough measurements of cabin volume to verify my guesstimate of ~11k BTU for air conditioning.  (I was right.)  However, after quoting various air conditioning options I’m thinking I may have to set that aside for now, in favor of other priorities.  The cheapest options are around US$3k, sans installation, which will require a haulout.

On the upside, having a gas grill is really pretty awesome.  Now I can’t wait to get a battery charger in place so I can leave my fridge on at all times at the dock, and do more cooking.

2 Comments

Filed under electrical, liveaboard, maintenance, mechanical, planning

2 Responses to Shower sump; alternator; etc.

  1. Lee

    Yeah, a 1 V drop between the alternator and the batteries at only 20 A is bad. If you do have 40′ of 10 gauge wire (20′ to and 20′ from), you would come up with about this resistance. You’re looking for, at worst, a 10% voltage drop at full load. So to see a 10% voltage drop at 1/5 load is bad. It seems to me that if your battery was fully discharged, you might be able to put more current through it and start a fire (though probably not). You could never really put more than 40 A through it, unless your batteries were below 11.8 A.

    Float phase might be a reason for producing only 20 A, but not for the 1 V drop. As a side note, on my boat I did trace down about a 1 V drop at low current between the house battery and the breaker panel. It turned out to be a loose screw – I was surprised that it’s possible to get such a drop just from a loose bus bar screw. Maybe check the grounding strap on your motor?

    • Kris

      Thanks for this, I’ll check around and see if I can figure out where the voltage drop is coming from. At worst, I’ll get an electrician to look at it as part of all the other stuff I need to get done. The most obvious candidates are all the terminals around the alternator, which are newly attached; and maybe the terminals on the starting bank, since I just changed those batteries. Possibly also at the battery switch which is pretty old (and which I’m going to replace).

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