10 June 2007:  The Only Good Holding Tank is a Big Enough Holding Tank.   

Its been a while since I wrote anything and a lot has been accomplished.  Lets start with one of the current projects: the holding tank and the head.  Our Islander 28 came with a rubber holding bladder, which was pretty fragrantly obnoxious after 30 years.  In addition, the vent line was plugged, so when the bladder filled up, waste would begin to leak back into the head.  Better than the bladder bursting, I guess.  The Wilcox-Crittendon bladder probably held five gallons, certainly not more than 10.  I think "big enough" holding-tank-wise, is more on the order of a gallon per foot of boat, minimum.  I won't claim to follow all the rules, but I do think discharging sewage into a marina is particularly gross.  I also think a lot more people do it than will admit to it.  If marinas really cared about the problem they would all regularly put dye tablets in boater's heads, as they do in Avalon harbor.  

Anyway, with careful attention and using minimal flush water, Julie and I could go several days on the boat between pump-out runs to the open ocean.  A pump-out run is also an excuse to go sailing, so that isn't all bad.  But a boat that's a little smelly is an excuse for the wife to want to stay home, so fixes were in order.   In mid-April I carefully measured the space available.  The 18 gallon fresh water tank is aft under the starboard settee and the bladder and macerator were forward.  I calculated the largest Ronco standard tank that would fit, did drawings and everything.  At the end of May I took delivery of a B-170 28 gallon rotary-molded polyethylene tank.  The tank is roughly a right triangle in cross section to fit the curve of the hull and it narrows down a little at the forward end as the hull curves in toward the pointy end of the boat.  The tank turned out to be just a teeny bit too big at the forward outside corner.  Oops.  

Well, the tank is polyethylene, right.  So I got out a heat gun and figured I'd soften up the corner of the tank until I could collapse it enough to fit, which meant reducing the height of the tank by about an inch over a six inch square area which was originally about two to three inches high.  Having done it, this is not a project I would recommend to anyone.  The poly is thick, does not conduct heat very well and transitions from solid to liquid over a very narrow temperature range.  You want the tank soft enough to deform but not hot enough melt.  Because the material is about 3/8 inch thick, it melts on the outside (or inside) where you apply the heat long before it gets soft enough all the way through.  You can see what is going on because the material turns transparent when it melts and is translucent while its solid.  As soon as it gets hot enough all the way through gravity wants to take over.  And it's not like it cools down right away when you take the heat away.  As carefully and slowly as I worked, I still melted a hole in the tank.  Fortunately, I was working on a corner and could reposition the tank and flow the molten material back into the hole, using pieces of wood to push and/or support the plastic. 

I tested for leaks by pressurizing the tank to some fraction of a PSI for several hours.  The flat walls want to bulge out under very little pressure so this is a delicate business.  Thankfully, the pressure test was successful.  No leaks.  Installed, the tank looks like it was poured in place, which it was, sort of.  There isn't a quarter inch to spare anywhere.  It's not at first apparent that the tank in the photo fully fills the width under the settee because the bottom of the picture is a piece of plywood attached to the edge of the settee by a piano hinge.  Normally this lays on top of the (missing) plywood covering the tank and is flipped out and has hinged legs to transform the settee into a double berth on the Islander 28.

There were about four inches to spare on the forward end of the compartment for plumbing connections.  Just enough.  The head is on the other side of the bulkhead and the lines run along the hull and are almost all concealed by the existing cabinetry.  As is my normal practice, there is no Y valve.  The head discharges into the top of the tank. The bottom of the tank goes to a tee which branches to the deck pump out port and to the macerator.  This works because the deck port is normally capped so the macerator can pull a vacuum to empty the tank and the macerator seals well enough that the tank can be suctioned out through the deck fitting as well.

Because the ports are all on the end of the tank, it is not possible to either fully fill or fully empty the tank.  I placed the vent line (3/4" white hose in the photo above) so that it is uncovered at equal starboard or port angles of heel.  I installed an external capacitance level sensor but am not very happy with it.  Apparently because the foil strips are almost touching a painted bulkhead as well, the capacitance does not drop enough with an empty tank to register "empty" on the scale.  I put this down to poor design on the part of the device maker.  As an old instrumentation hand, I know that every instrument should have both a zero and a span adjustment.  These folks only provide a span adjustment and assume the zero will take care of itself, which clearly it does not.

Here, ten or so inches down the page, is where the story gets interesting, or at least beyond cramming 28 gallons of tank into an 18 gallon space.  Because La Mouette is my little experimental boat, I get to try offbeat solutions to problems.  Three problems: How to empty a tank where the outlet starts sucking air two or three inches before the tank is empty? - and - How to figure out how full the tank is?  A corollary problem involves the water tank level.  The water tank is also poly, but its encased in wood on all sides and just too much of a pain to apply capacitance strips which don't work that well anyway.  Finally, how to stop sanitation related odors?

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