DSK wrote:
Terry Spragg wrote:


I can use a bicycle pump or similar to evacuate my holding tank
offshore. I close both air vents, open the sea valve, and pump up
the holding tank with air.


This is a really bad idea. The question is not *if* the tank will blow up, but
when.

Woudn't that depend on system parameters and design? If one pound
pressure won't do 'it' in my system, then there is something else to
look at.


The highest standard for marine tanks (including fuel BTW) and associated
fittings is .5 psi overpressure. Most are not built to withstand even that
much.

Who's fault or design is that?


If you really have been doing this Terry, you've been very lucky (and so have
your neighbors).

Fresh Breezes- Doug King


Amen to that;-)

I would likely not try to shoot PWC's with poo from the deck cannon,
but the thought is so entertaining, I just might be tempted after
dinner and drinks on the hook. Keep clear, PWC fools! You ought to
spread the word that this is a viable system, maybe it will keep
them away if a story, even a bogus story should be told...

I built the tank out of 1/4" thick polyester and glass, except for
the top, which is more like 3/16" thick, with an epoxy lining. I do
not do this deck cannon blow regularly, but did do it to see if it
would work when it was new and clean. It works. After it has aged a
few more years, when the joker is a little older, I intend to clean
it out as well as can be done, and then I will see where the weak
link is, just for fun, up to about 5 pounds just for the sake of
curiosity. I am quit certain that the joker or a rubber bellows hose
between the head and the Seacock will be the part that blows up
eventually, even without 'dive pressure' testing. The tank itself is
not rated, and has not been tested to destruction, so I can't say,
yet. It can take about a 10 foot head of pressure.

I do check for leaks under moderate pressure before launch, and
would expect to detect seepage before total failure in the joker,
which I will not change until I must. Moderate pressure is about 10
pounds force on the head pump handle, which, by thumb, equates with
about 2 pounds system pressure. It would be possible for a normal
physique to apply perhaps 50 pounds force, or 10 lbs system
overpressure. That would be very scary, but I doubt anybody could be
so stupid as to actually lean that hard on the handle, as someone
suggests must have been done, since he said it blew up.

When I got the boat, the joker was the wrong one, and leaked. That
is where I started. I wonder how many others are co-existing with a
dodgy setup, improperly maintained?

Usually I 'blow tanks' via the head seacock which is only about an
inch below the waterline, so the pressure required in too low to
register on any gague that I have. The top quarter of the holding
tank runs out without assistance, anyway, on starboard tack

It does work, and that logic stands in opposition, apparently, only
to fear on the part of the operator who prefers to ignore head
maintenance.

After some of the messes I have cleaned up, a little poo from folks
I know is of small consequence. They are all healthy, and I have
developed a plumber's stomach, having raised a few kids from diapers
to college, looking after an old relative or two, and having fixed a
few heads, at sea and ashore, in the last 50 years.

Not to wax too political, but so many people seem to be so
unreasonably squeamish about such little things, akin to so many who
seem so concerned with miniscule calculated risks in both physical
safety and so called 'security.' There is, and can be, no perfect
safety or security system. I cannot understand how such people ever
get into thier boats for a sail, or thier cars to drive in traffic,
or even get to sleep at night, given that it is so easy to blow up
an entire city block with just a gallon or two of gasoline. It seems
the stock market must respond too easily to minor rumours of
'terror' attacks on stock values, could this be a popular tool
amongst market manipulators?

Ask any farmer about animal husbandry, now there's a messy business.

Do we all forget how recently is was that most folks a generation or
two ago worked in shall we say, primitive conditions? How soon our
expectations have risen, and how far. How quickly too, our modern
conveniences have spoiled so many waterways for those who appreciate
nature's way of sun, wind and water. Could it be that those who must
have power boats are afraid to see close up and slowly the water
over which they now travel so quickly? We evolved in natural healthy
'filth' unimaginable to modern youth, and it did not stop us from
prospering, perhaps too much. A sterile upbringing gives no
excercise to our immune system, weakening it.

Sorry about that.

Terry K